[PATCH 0/5] hook API: support stdin, convert post-rewrite

[PATCH 0/5] hook API: support stdin, convert post-rewrite

[Ævar Arnfjörð Bjarmason]

The greater "config-based-hooks" topic has been stalled for a while.

In the last couple of cycles it was held up by run-command.c API
changes (which will make the rest of this much simpler), and lack of
time on my part.

But let's get the ball rolling again. The last time this was
on-list[1] it was part of a 36 patch series, these 5 patches only
convert one hook (implemented by both sequencer & git-am) to the hook
API, but it's an important step: To do so we need so support reading
from stdin, and passing that to the hooks.

The (passing) CI & topic branch for this is at[2].

The immediate motivation for this is to supply a "stdin" interface
that git-send-email.perl can use, see [3].

Changes since the [1] v5:

 * A new 1/5 here, picked from
   https://lore.kernel.org/git/patch-v2-07.22-b90961ae76d-20221012T084850Z-avarab@gmail.com/;
   A small while-at-it cleanup of a related API.

 * Updates for the aforementioned run-command API changes.

 * Commit message updates & clarifications.

 * The previous version of the "sequencer.c" change changed a variable
   name while at it, now it doesn't, making the diff much easier to
   read.

 * Updates for the *.txt and -h usage, so that we'll pass the
   since-added t0450 test.

1. https://lore.kernel.org/git/cover-v5-00.36-00000000000-20210902T125110Z-avarab@gmail.com/
2. https://github.com/avar/git/tree/es-avar/config-based-hooks-the-beginning
3. https://lore.kernel.org/git/230123.86wn5ds602.gmgdl@evledraar.gmail.com/

Emily Shaffer (4):
  run-command: allow stdin for run_processes_parallel
  hook API: support passing stdin to hooks, convert am's 'post-rewrite'
  sequencer: use the new hook API for the simpler "post-rewrite" call
  hook: support a --to-stdin=<path> option for testing

Ævar Arnfjörð Bjarmason (1):
  run-command.c: remove dead assignment in while-loop

 Documentation/git-hook.txt |  7 ++++++-
 builtin/am.c               | 20 ++++----------------
 builtin/hook.c             |  4 +++-
 hook.c                     |  8 +++++++-
 hook.h                     |  5 +++++
 run-command.c              | 13 +++++++++----
 sequencer.c                | 18 ++++--------------
 t/t1800-hook.sh            | 18 ++++++++++++++++++
 8 files changed, 56 insertions(+), 37 deletions(-)

Range-diff:
 1:  ac419613fdc <  -:  ----------- Makefile: mark "check" target as .PHONY
 2:  a161b7f0a5c <  -:  ----------- Makefile: stop hardcoding {command,config}-list.h
 3:  ffef1d3257e <  -:  ----------- Makefile: remove an out-of-date comment
 4:  545e16c6f04 <  -:  ----------- hook.[ch]: move find_hook() from run-command.c to hook.c
 5:  a9bc4519e9a <  -:  ----------- hook.c: add a hook_exists() wrapper and use it in bugreport.c
 6:  e99ec2e6f8f <  -:  ----------- hook.c users: use "hook_exists()" instead of "find_hook()"
 7:  2ffb2332c8a <  -:  ----------- hook-list.h: add a generated list of hooks, like config-list.h
 8:  72dd1010f5b <  -:  ----------- hook: add 'run' subcommand
 9:  821cc9bf11e <  -:  ----------- gc: use hook library for pre-auto-gc hook
10:  d71c90254ea <  -:  ----------- rebase: convert pre-rebase to use hook.h
11:  ea3af2ccc4d <  -:  ----------- am: convert applypatch to use hook.h
12:  fed0b52f88f <  -:  ----------- hooks: convert 'post-checkout' hook to hook library
13:  53d8721a0e3 <  -:  ----------- merge: convert post-merge to use hook.h
14:  d60827a2856 <  -:  ----------- git hook run: add an --ignore-missing flag
15:  d4976a0821f <  -:  ----------- send-email: use 'git hook run' for 'sendemail-validate'
16:  99f3dcd1945 <  -:  ----------- git-p4: use 'git hook' to run hooks
17:  509761454e6 <  -:  ----------- commit: convert {pre-commit,prepare-commit-msg} hook to hook.h
18:  e2c94d95427 <  -:  ----------- read-cache: convert post-index-change to use hook.h
19:  fa7d0d24ea2 <  -:  ----------- receive-pack: convert push-to-checkout hook to hook.h
20:  428bb5a6792 <  -:  ----------- run-command: remove old run_hook_{le,ve}() hook API
 -:  ----------- >  1:  351c6a55a41 run-command.c: remove dead assignment in while-loop
21:  994f6ad8602 !  2:  81eef2f60a0 run-command: allow stdin for run_processes_parallel
    @@ Commit message
         Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
     
      ## run-command.c ##
    -@@ run-command.c: static int pp_start_one(struct parallel_processes *pp)
    - 	if (i == pp->max_processes)
    +@@ run-command.c: static int pp_start_one(struct parallel_processes *pp,
    + 	if (i == opts->processes)
      		BUG("bookkeeping is hard");
      
     +	/*
    @@ run-command.c: static int pp_start_one(struct parallel_processes *pp)
     +	 */
     +	pp->children[i].process.no_stdin = 1;
     +
    - 	code = pp->get_next_task(&pp->children[i].process,
    - 				 &pp->children[i].err,
    - 				 pp->data,
    -@@ run-command.c: static int pp_start_one(struct parallel_processes *pp)
    + 	code = opts->get_next_task(&pp->children[i].process,
    + 				   opts->ungroup ? NULL : &pp->children[i].err,
    + 				   opts->data,
    +@@ run-command.c: static int pp_start_one(struct parallel_processes *pp,
    + 		pp->children[i].process.err = -1;
    + 		pp->children[i].process.stdout_to_stderr = 1;
      	}
    - 	pp->children[i].process.err = -1;
    - 	pp->children[i].process.stdout_to_stderr = 1;
     -	pp->children[i].process.no_stdin = 1;
      
      	if (start_command(&pp->children[i].process)) {
    - 		code = pp->start_failure(&pp->children[i].err,
    + 		if (opts->start_failure)
23:  f548e3d15e7 !  3:  c6b9b69c516 am: convert 'post-rewrite' hook to hook.h
    @@ Metadata
     Author: Emily Shaffer <emilyshaffer@google.com>
     
      ## Commit message ##
    -    am: convert 'post-rewrite' hook to hook.h
    +    hook API: support passing stdin to hooks, convert am's 'post-rewrite'
    +
    +    Convert the invocation of the 'post-rewrite' hook run by 'git am' to
    +    use the hook.h library. To do this we need to add a "path_to_stdin"
    +    member to "struct run_hooks_opt".
    +
    +    In our API this is supported by asking for a file path, rather
    +    than by reading stdin. Reading directly from stdin would involve caching
    +    the entire stdin (to memory or to disk) once the hook API is made to
    +    support "jobs" larger than 1, along with support for executing N hooks
    +    at a time (i.e. the upcoming config-based hooks).
     
         Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
         Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
    @@ builtin/am.c: static int run_applypatch_msg_hook(struct am_state *state)
     -
     -	if (!hook)
     -		return 0;
    --
    ++	struct run_hooks_opt opt = RUN_HOOKS_OPT_INIT;
    + 
     -	strvec_push(&cp.args, hook);
     -	strvec_push(&cp.args, "rebase");
    --
    ++	strvec_push(&opt.args, "rebase");
    ++	opt.path_to_stdin = am_path(state, "rewritten");
    + 
     -	cp.in = xopen(am_path(state, "rewritten"), O_RDONLY);
     -	cp.stdout_to_stderr = 1;
     -	cp.trace2_hook_name = "post-rewrite";
    -+	struct run_hooks_opt opt = RUN_HOOKS_OPT_INIT;
    - 
    +-
     -	ret = run_command(&cp);
    -+	strvec_push(&opt.args, "rebase");
    -+	opt.path_to_stdin = am_path(state, "rewritten");
    - 
    +-
     -	close(cp.in);
     -	return ret;
    -+	return run_hooks_oneshot("post-rewrite", &opt);
    ++	return run_hooks_opt("post-rewrite", &opt);
      }
      
      /**
    +
    + ## hook.c ##
    +@@ hook.c: static int pick_next_hook(struct child_process *cp,
    + 	if (!hook_path)
    + 		return 0;
    + 
    +-	cp->no_stdin = 1;
    + 	strvec_pushv(&cp->env, hook_cb->options->env.v);
    ++	/* reopen the file for stdin; run_command closes it. */
    ++	if (hook_cb->options->path_to_stdin) {
    ++		cp->no_stdin = 0;
    ++		cp->in = xopen(hook_cb->options->path_to_stdin, O_RDONLY);
    ++	} else {
    ++		cp->no_stdin = 1;
    ++	}
    + 	cp->stdout_to_stderr = 1;
    + 	cp->trace2_hook_name = hook_cb->hook_name;
    + 	cp->dir = hook_cb->options->dir;
    +
    + ## hook.h ##
    +@@ hook.h: struct run_hooks_opt
    + 	 * was invoked.
    + 	 */
    + 	int *invoked_hook;
    ++
    ++	/**
    ++	 * Path to file which should be piped to stdin for each hook.
    ++	 */
    ++	const char *path_to_stdin;
    + };
    + 
    + #define RUN_HOOKS_OPT_INIT { \
 -:  ----------- >  4:  7a55c95f60f sequencer: use the new hook API for the simpler "post-rewrite" call
22:  3ccc654a664 !  5:  cb9ef7a89c4 hook: support passing stdin to hooks
    @@ Metadata
     Author: Emily Shaffer <emilyshaffer@google.com>
     
      ## Commit message ##
    -    hook: support passing stdin to hooks
    +    hook: support a --to-stdin=<path> option for testing
     
    -    Some hooks (such as post-rewrite) need to take input via stdin.
    -    Previously, callers provided stdin to hooks by setting
    -    run-command.h:child_process.in, which takes a FD. Callers would open the
    -    file in question themselves before calling run-command(). However, since
    -    we will now need to seek to the front of the file and read it again for
    -    every hook which runs, hook.h:run_command() takes a path and handles FD
    -    management itself. Since this file is opened for read only, it should
    -    not prevent later parallel execution support.
    -
    -    On the frontend, this is supported by asking for a file path, rather
    -    than by reading stdin. Reading directly from stdin would involve caching
    -    the entire stdin (to memory or to disk) and reading it back from the
    -    beginning to each hook. We'd want to support cases like insufficient
    -    memory or storage for the file. While this may prove useful later, for
    -    now the path of least resistance is to just ask the user to make this
    -    interim file themselves.
    +    Expose the "path_to_stdin" API added in the preceding commit in the
    +    "git hook run" command. For now we won't be using this command
    +    interface outside of the tests, but exposing this functionality makes
    +    it easier to test the hook API.
     
         Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
         Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
     
      ## Documentation/git-hook.txt ##
    -@@ Documentation/git-hook.txt: git-hook - run git hooks
    +@@ Documentation/git-hook.txt: git-hook - Run git hooks
      SYNOPSIS
      --------
      [verse]
     -'git hook' run [--ignore-missing] <hook-name> [-- <hook-args>]
    -+'git hook' run [--to-stdin=<path>] [--ignore-missing] <hook-name> [-- <hook-args>]
    ++'git hook' run [--ignore-missing] [--to-stdin=<path>] <hook-name> [-- <hook-args>]
      
      DESCRIPTION
      -----------
    -@@ Documentation/git-hook.txt: what those are.
    +@@ Documentation/git-hook.txt: linkgit:githooks[5] for arguments hooks might expect (if any).
      OPTIONS
      -------
      
    @@ builtin/hook.c
     @@ builtin/hook.c: static int run(int argc, const char **argv, const char *prefix)
      	struct option run_options[] = {
      		OPT_BOOL(0, "ignore-missing", &ignore_missing,
    - 			 N_("exit quietly with a zero exit code if the requested hook cannot be found")),
    + 			 N_("silently ignore missing requested <hook-name>")),
     +		OPT_STRING(0, "to-stdin", &opt.path_to_stdin, N_("path"),
     +			   N_("file to read into hooks' stdin")),
      		OPT_END(),
      	};
      	int ret;
     
    - ## hook.c ##
    -@@ hook.c: static int pick_next_hook(struct child_process *cp,
    - 	if (!run_me)
    - 		return 0;
    - 
    --	cp->no_stdin = 1;
    -+	/* reopen the file for stdin; run_command closes it. */
    -+	if (hook_cb->options->path_to_stdin) {
    -+		cp->no_stdin = 0;
    -+		cp->in = xopen(hook_cb->options->path_to_stdin, O_RDONLY);
    -+	} else {
    -+		cp->no_stdin = 1;
    -+	}
    - 	cp->env = hook_cb->options->env.v;
    - 	cp->stdout_to_stderr = 1;
    - 	cp->trace2_hook_name = hook_cb->hook_name;
    -
    - ## hook.h ##
    -@@ hook.h: struct run_hooks_opt
    - 
    - 	/* Path to initial working directory for subprocess */
    - 	const char *dir;
    -+
    -+	/* Path to file which should be piped to stdin for each hook */
    -+	const char *path_to_stdin;
    - };
    - 
    - #define RUN_HOOKS_OPT_INIT { \
    -
      ## t/t1800-hook.sh ##
    -@@ t/t1800-hook.sh: test_expect_success 'git -c core.hooksPath=<PATH> hook run' '
    +@@ t/t1800-hook.sh: test_expect_success 'git hook run a hook with a bad shebang' '
      	test_cmp expect actual
      '
      
24:  bb119fa7cc0 <  -:  ----------- run-command: add stdin callback for parallelization
25:  2439f7752b8 <  -:  ----------- hook: provide stdin by string_list or callback
26:  48a380b3a91 <  -:  ----------- hook: convert 'post-rewrite' hook in sequencer.c to hook.h
27:  af6b9292aaa <  -:  ----------- transport: convert pre-push hook to hook.h
28:  957691f0b6d <  -:  ----------- hook tests: test for exact "pre-push" hook input
29:  88fe2621549 <  -:  ----------- hook tests: use a modern style for "pre-push" tests
30:  1d905e81779 <  -:  ----------- reference-transaction: use hook.h to run hooks
31:  fac56a9d8af <  -:  ----------- run-command: allow capturing of collated output
32:  7d185cdf9d1 <  -:  ----------- hooks: allow callers to capture output
33:  c8150e1239f <  -:  ----------- receive-pack: convert 'update' hook to hook.h
34:  a20ad847c14 <  -:  ----------- post-update: use hook.h library
35:  79c380be6ed <  -:  ----------- receive-pack: convert receive hooks to hook.h
36:  fe056098534 <  -:  ----------- hooks: fix a TOCTOU in "did we run a hook?" heuristic
-- 
2.39.1.1301.gffb37c08dee

[PATCH 1/5] run-command.c: remove dead assignment in while-loop

[Ævar Arnfjörð Bjarmason]

Remove code that's been unused since it was added in
c553c72eed6 (run-command: add an asynchronous parallel child
processor, 2015-12-15), the next use of "i" in this function is:

	for (i = 0; ...

So we'll always clobber the "i" that's set here. Presumably the "i"
assignment is an artifact of WIP code that made it into our tree.

A subsequent commit will need to adjust the type of the "i" variable
in the otherwise unrelated for-loop, which is why this is being
removed now.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 run-command.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/run-command.c b/run-command.c
index 50cc011654e..b439c7974ca 100644
--- a/run-command.c
+++ b/run-command.c
@@ -1632,9 +1632,7 @@ static void pp_buffer_stderr(struct parallel_processes *pp,
 			     const struct run_process_parallel_opts *opts,
 			     int output_timeout)
 {
-	int i;
-
-	while ((i = poll(pp->pfd, opts->processes, output_timeout) < 0)) {
+	while (poll(pp->pfd, opts->processes, output_timeout) < 0) {
 		if (errno == EINTR)
 			continue;
 		pp_cleanup(pp, opts);
-- 
2.39.1.1301.gffb37c08dee

[PATCH 2/5] run-command: allow stdin for run_processes_parallel

[Ævar Arnfjörð Bjarmason]

From: Emily Shaffer <emilyshaffer@google.com>

While it makes sense not to inherit stdin from the parent process to
avoid deadlocking, it's not necessary to completely ban stdin to
children. An informed user should be able to configure stdin safely. By
setting `some_child.process.no_stdin=1` before calling `get_next_task()`
we provide a reasonable default behavior but enable users to set up
stdin streaming for themselves during the callback.

`some_child.process.stdout_to_stderr`, however, remains unmodifiable by
`get_next_task()` - the rest of the run_processes_parallel() API depends
on child output in stderr.

Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 run-command.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/run-command.c b/run-command.c
index b439c7974ca..6bd16acb060 100644
--- a/run-command.c
+++ b/run-command.c
@@ -1586,6 +1586,14 @@ static int pp_start_one(struct parallel_processes *pp,
 	if (i == opts->processes)
 		BUG("bookkeeping is hard");
 
+	/*
+	 * By default, do not inherit stdin from the parent process - otherwise,
+	 * all children would share stdin! Users may overwrite this to provide
+	 * something to the child's stdin by having their 'get_next_task'
+	 * callback assign 0 to .no_stdin and an appropriate integer to .in.
+	 */
+	pp->children[i].process.no_stdin = 1;
+
 	code = opts->get_next_task(&pp->children[i].process,
 				   opts->ungroup ? NULL : &pp->children[i].err,
 				   opts->data,
@@ -1601,7 +1609,6 @@ static int pp_start_one(struct parallel_processes *pp,
 		pp->children[i].process.err = -1;
 		pp->children[i].process.stdout_to_stderr = 1;
 	}
-	pp->children[i].process.no_stdin = 1;
 
 	if (start_command(&pp->children[i].process)) {
 		if (opts->start_failure)
-- 
2.39.1.1301.gffb37c08dee

[PATCH 3/5] hook API: support passing stdin to hooks, convert am's 'post-rewrite'

[Ævar Arnfjörð Bjarmason]

From: Emily Shaffer <emilyshaffer@google.com>

Convert the invocation of the 'post-rewrite' hook run by 'git am' to
use the hook.h library. To do this we need to add a "path_to_stdin"
member to "struct run_hooks_opt".

In our API this is supported by asking for a file path, rather
than by reading stdin. Reading directly from stdin would involve caching
the entire stdin (to memory or to disk) once the hook API is made to
support "jobs" larger than 1, along with support for executing N hooks
at a time (i.e. the upcoming config-based hooks).

Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 builtin/am.c | 20 ++++----------------
 hook.c       |  8 +++++++-
 hook.h       |  5 +++++
 3 files changed, 16 insertions(+), 17 deletions(-)

diff --git a/builtin/am.c b/builtin/am.c
index 82a41cbfc4e..8be91617fef 100644
--- a/builtin/am.c
+++ b/builtin/am.c
@@ -495,24 +495,12 @@ static int run_applypatch_msg_hook(struct am_state *state)
  */
 static int run_post_rewrite_hook(const struct am_state *state)
 {
-	struct child_process cp = CHILD_PROCESS_INIT;
-	const char *hook = find_hook("post-rewrite");
-	int ret;
-
-	if (!hook)
-		return 0;
+	struct run_hooks_opt opt = RUN_HOOKS_OPT_INIT;
 
-	strvec_push(&cp.args, hook);
-	strvec_push(&cp.args, "rebase");
+	strvec_push(&opt.args, "rebase");
+	opt.path_to_stdin = am_path(state, "rewritten");
 
-	cp.in = xopen(am_path(state, "rewritten"), O_RDONLY);
-	cp.stdout_to_stderr = 1;
-	cp.trace2_hook_name = "post-rewrite";
-
-	ret = run_command(&cp);
-
-	close(cp.in);
-	return ret;
+	return run_hooks_opt("post-rewrite", &opt);
 }
 
 /**
diff --git a/hook.c b/hook.c
index a4fa1031f28..86c6dc1fe70 100644
--- a/hook.c
+++ b/hook.c
@@ -53,8 +53,14 @@ static int pick_next_hook(struct child_process *cp,
 	if (!hook_path)
 		return 0;
 
-	cp->no_stdin = 1;
 	strvec_pushv(&cp->env, hook_cb->options->env.v);
+	/* reopen the file for stdin; run_command closes it. */
+	if (hook_cb->options->path_to_stdin) {
+		cp->no_stdin = 0;
+		cp->in = xopen(hook_cb->options->path_to_stdin, O_RDONLY);
+	} else {
+		cp->no_stdin = 1;
+	}
 	cp->stdout_to_stderr = 1;
 	cp->trace2_hook_name = hook_cb->hook_name;
 	cp->dir = hook_cb->options->dir;
diff --git a/hook.h b/hook.h
index 4258b13da0d..19ab9a5806e 100644
--- a/hook.h
+++ b/hook.h
@@ -30,6 +30,11 @@ struct run_hooks_opt
 	 * was invoked.
 	 */
 	int *invoked_hook;
+
+	/**
+	 * Path to file which should be piped to stdin for each hook.
+	 */
+	const char *path_to_stdin;
 };
 
 #define RUN_HOOKS_OPT_INIT { \
-- 
2.39.1.1301.gffb37c08dee

[PATCH 4/5] sequencer: use the new hook API for the simpler "post-rewrite" call

[Ævar Arnfjörð Bjarmason]

From: Emily Shaffer <emilyshaffer@google.com>

Change the invocation of the "post-rewrite" hook added in
795160457db (sequencer (rebase -i): run the post-rewrite hook, if
needed, 2017-01-02) to use the new hook API.

This leaves the more complex "post-rewrite" invocation added in
a87a6f3c98e (commit: move post-rewrite code to libgit, 2017-11-17)
here in sequencer.c unconverted. That'll be done in a subsequent
commit.

Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 sequencer.c | 18 ++++--------------
 1 file changed, 4 insertions(+), 14 deletions(-)

diff --git a/sequencer.c b/sequencer.c
index 3e4a1972897..d8d59d05dd4 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -4834,8 +4834,7 @@ static int pick_commits(struct repository *r,
 		if (!stat(rebase_path_rewritten_list(), &st) &&
 				st.st_size > 0) {
 			struct child_process child = CHILD_PROCESS_INIT;
-			const char *post_rewrite_hook =
-				find_hook("post-rewrite");
+			struct run_hooks_opt hook_opt = RUN_HOOKS_OPT_INIT;
 
 			child.in = open(rebase_path_rewritten_list(), O_RDONLY);
 			child.git_cmd = 1;
@@ -4845,18 +4844,9 @@ static int pick_commits(struct repository *r,
 			/* we don't care if this copying failed */
 			run_command(&child);
 
-			if (post_rewrite_hook) {
-				struct child_process hook = CHILD_PROCESS_INIT;
-
-				hook.in = open(rebase_path_rewritten_list(),
-					O_RDONLY);
-				hook.stdout_to_stderr = 1;
-				hook.trace2_hook_name = "post-rewrite";
-				strvec_push(&hook.args, post_rewrite_hook);
-				strvec_push(&hook.args, "rebase");
-				/* we don't care if this hook failed */
-				run_command(&hook);
-			}
+			hook_opt.path_to_stdin = rebase_path_rewritten_list();
+			strvec_push(&hook_opt.args, "rebase");
+			run_hooks_opt("post-rewrite", &hook_opt);
 		}
 		apply_autostash(rebase_path_autostash());
 
-- 
2.39.1.1301.gffb37c08dee

[PATCH 5/5] hook: support a --to-stdin=<path> option for testing

[Ævar Arnfjörð Bjarmason]

From: Emily Shaffer <emilyshaffer@google.com>

Expose the "path_to_stdin" API added in the preceding commit in the
"git hook run" command. For now we won't be using this command
interface outside of the tests, but exposing this functionality makes
it easier to test the hook API.

Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 Documentation/git-hook.txt |  7 ++++++-
 builtin/hook.c             |  4 +++-
 t/t1800-hook.sh            | 18 ++++++++++++++++++
 3 files changed, 27 insertions(+), 2 deletions(-)

diff --git a/Documentation/git-hook.txt b/Documentation/git-hook.txt
index 77c3a8ad909..3407f3c2c07 100644
--- a/Documentation/git-hook.txt
+++ b/Documentation/git-hook.txt
@@ -8,7 +8,7 @@ git-hook - Run git hooks
 SYNOPSIS
 --------
 [verse]
-'git hook' run [--ignore-missing] <hook-name> [-- <hook-args>]
+'git hook' run [--ignore-missing] [--to-stdin=<path>] <hook-name> [-- <hook-args>]
 
 DESCRIPTION
 -----------
@@ -31,6 +31,11 @@ linkgit:githooks[5] for arguments hooks might expect (if any).
 OPTIONS
 -------
 
+--to-stdin::
+	For "run"; Specify a file which will be streamed into the
+	hook's stdin. The hook will receive the entire file from
+	beginning to EOF.
+
 --ignore-missing::
 	Ignore any missing hook by quietly returning zero. Used for
 	tools that want to do a blind one-shot run of a hook that may
diff --git a/builtin/hook.c b/builtin/hook.c
index b6530d189ad..f95b7965c58 100644
--- a/builtin/hook.c
+++ b/builtin/hook.c
@@ -7,7 +7,7 @@
 #include "strvec.h"
 
 #define BUILTIN_HOOK_RUN_USAGE \
-	N_("git hook run [--ignore-missing] <hook-name> [-- <hook-args>]")
+	N_("git hook run [--ignore-missing] [--to-stdin=<path>] <hook-name> [-- <hook-args>]")
 
 static const char * const builtin_hook_usage[] = {
 	BUILTIN_HOOK_RUN_USAGE,
@@ -28,6 +28,8 @@ static int run(int argc, const char **argv, const char *prefix)
 	struct option run_options[] = {
 		OPT_BOOL(0, "ignore-missing", &ignore_missing,
 			 N_("silently ignore missing requested <hook-name>")),
+		OPT_STRING(0, "to-stdin", &opt.path_to_stdin, N_("path"),
+			   N_("file to read into hooks' stdin")),
 		OPT_END(),
 	};
 	int ret;
diff --git a/t/t1800-hook.sh b/t/t1800-hook.sh
index 2ef3579fa7c..3506f627b6c 100755
--- a/t/t1800-hook.sh
+++ b/t/t1800-hook.sh
@@ -177,4 +177,22 @@ test_expect_success 'git hook run a hook with a bad shebang' '
 	test_cmp expect actual
 '
 
+test_expect_success 'stdin to hooks' '
+	write_script .git/hooks/test-hook <<-\EOF &&
+	echo BEGIN stdin
+	cat
+	echo END stdin
+	EOF
+
+	cat >expect <<-EOF &&
+	BEGIN stdin
+	hello
+	END stdin
+	EOF
+
+	echo hello >input &&
+	git hook run --to-stdin=input test-hook 2>actual &&
+	test_cmp expect actual
+'
+
 test_done
-- 
2.39.1.1301.gffb37c08dee

Re: [PATCH 1/5] run-command.c: remove dead assignment in while-loop

[Junio C Hamano]

Ævar Arnfjörð Bjarmason  <avarab@gmail.com> writes:

> Remove code that's been unused since it was added in
> c553c72eed6 (run-command: add an asynchronous parallel child
> processor, 2015-12-15), the next use of "i" in this function is:
>
> 	for (i = 0; ...

And it has been updated to a different type, i.e.

	for (size_t i = 0; ...

so it doubly makes sense to kill that unused variable.

Makes sense.

> So we'll always clobber the "i" that's set here. Presumably the "i"
> assignment is an artifact of WIP code that made it into our tree.
>
> A subsequent commit will need to adjust the type of the "i" variable
> in the otherwise unrelated for-loop, which is why this is being
> removed now.

That, together with the earlier mention of the other i (which I
think came from the same source---perhaps the original topic this
was taken from had int->size_t change in it) are both stale.

Please proofread what you send out before submitting.

Thanks.

Re: [PATCH 2/5] run-command: allow stdin for run_processes_parallel

[Junio C Hamano]

Ævar Arnfjörð Bjarmason  <avarab@gmail.com> writes:

> +	/*
> +	 * By default, do not inherit stdin from the parent process - otherwise,
> +	 * all children would share stdin! Users may overwrite this to provide
> +	 * something to the child's stdin by having their 'get_next_task'
> +	 * callback assign 0 to .no_stdin and an appropriate integer to .in.
> +	 */
> +	pp->children[i].process.no_stdin = 1;
> +
>  	code = opts->get_next_task(&pp->children[i].process,
>  				   opts->ungroup ? NULL : &pp->children[i].err,
>  				   opts->data,
> @@ -1601,7 +1609,6 @@ static int pp_start_one(struct parallel_processes *pp,
>  		pp->children[i].process.err = -1;
>  		pp->children[i].process.stdout_to_stderr = 1;
>  	}
> -	pp->children[i].process.no_stdin = 1;

For this single process, by default .no_stdin is set before it is
passed to start_command(), so the default behaviour does not change.

This needs a new safety to ensure that processes that have .no_stdin
turned off do not share the same value in their .in member, doesn't
it?  Hopefully that will be added in a later step in the series?

Provided that such a safety will appear in the end result, this
conversion does make sense to me.  Let's read on.

>  	if (start_command(&pp->children[i].process)) {
>  		if (opts->start_failure)

Re: [PATCH 3/5] hook API: support passing stdin to hooks, convert am's 'post-rewrite'

[Junio C Hamano]

Ævar Arnfjörð Bjarmason  <avarab@gmail.com> writes:

> diff --git a/hook.c b/hook.c
> index a4fa1031f28..86c6dc1fe70 100644
> --- a/hook.c
> +++ b/hook.c
> @@ -53,8 +53,14 @@ static int pick_next_hook(struct child_process *cp,
>  	if (!hook_path)
>  		return 0;
>  
> -	cp->no_stdin = 1;
>  	strvec_pushv(&cp->env, hook_cb->options->env.v);
> +	/* reopen the file for stdin; run_command closes it. */
> +	if (hook_cb->options->path_to_stdin) {
> +		cp->no_stdin = 0;
> +		cp->in = xopen(hook_cb->options->path_to_stdin, O_RDONLY);
> +	} else {
> +		cp->no_stdin = 1;
> +	}

Do we need this else clause?  I thought that we've made sure
no_stdin is the default.  Is it just being explicit?

> diff --git a/hook.h b/hook.h
> index 4258b13da0d..19ab9a5806e 100644
> --- a/hook.h
> +++ b/hook.h
> @@ -30,6 +30,11 @@ struct run_hooks_opt
>  	 * was invoked.
>  	 */
>  	int *invoked_hook;
> +
> +	/**
> +	 * Path to file which should be piped to stdin for each hook.
> +	 */
> +	const char *path_to_stdin;
>  };
>  
>  #define RUN_HOOKS_OPT_INIT { \

Re: [PATCH 3/5] hook API: support passing stdin to hooks, convert am's 'post-rewrite'

[Junio C Hamano]

Ævar Arnfjörð Bjarmason  <avarab@gmail.com> writes:

> @@ -53,8 +53,14 @@ static int pick_next_hook(struct child_process *cp,
>  	if (!hook_path)
>  		return 0;
>  
> -	cp->no_stdin = 1;
>  	strvec_pushv(&cp->env, hook_cb->options->env.v);
> +	/* reopen the file for stdin; run_command closes it. */
> +	if (hook_cb->options->path_to_stdin) {
> +		cp->no_stdin = 0;
> +		cp->in = xopen(hook_cb->options->path_to_stdin, O_RDONLY);
> +	} else {
> +		cp->no_stdin = 1;
> +	}

By the way, using the path_to_stdin as the customization machinery
for the API users, and keeping it to the API implementation to
actually open the file and stuff .in member with it, is a good way
to make sure that multiple processes do not compete for the same
standard input stream.  IOW, what I was worried about in my review
of [2/5] is addressed by this mechanism.

Thanks.

Re: [PATCH 5/5] hook: support a --to-stdin=<path> option for testing

[Junio C Hamano]

Ævar Arnfjörð Bjarmason  <avarab@gmail.com> writes:

> From: Emily Shaffer <emilyshaffer@google.com>
>
> Expose the "path_to_stdin" API added in the preceding commit in the
> "git hook run" command. For now we won't be using this command
> interface outside of the tests, but exposing this functionality makes
> it easier to test the hook API.

Presumably, the send-email validation topic would be using this
immediately once it becomes available, no?

When "git hook" finds and runs more than one hook script, do they
get the same input?  What I am wondering is if "to-stdin" should be
exposed like this interface (which may be sufficient for testing
purposes).  I imagine that scripters (e.g. send-email developers)
would find it more convenient if they do not have to come up with a
temporary file and they can just run "git hook" and feed whatever
they want to give to the hook from its standard input.  "git hook"
command, upon startup, should do the reading of its standard input
and spooling it to a temporary file it uses to pass the contents to
the hook scripts, in other words.

Other than that, it surely looks not just handy for tests, but has
immediate uses.

Thanks.

Re: [PATCH 5/5] hook: support a --to-stdin=<path> option for testing

[Michael Strawbridge]

On 2023-01-23 19:55, Junio C Hamano wrote:
> Ævar Arnfjörð Bjarmason  <avarab@gmail.com> writes:
>
>> From: Emily Shaffer <emilyshaffer@google.com>
>>
>> Expose the "path_to_stdin" API added in the preceding commit in the
>> "git hook run" command. For now we won't be using this command
>> interface outside of the tests, but exposing this functionality makes
>> it easier to test the hook API.
> Presumably, the send-email validation topic would be using this
> immediately once it becomes available, no?

Yes I would be trying to use it for my send-email header patch right away.

>
> When "git hook" finds and runs more than one hook script, do they
> get the same input?  What I am wondering is if "to-stdin" should be
> exposed like this interface (which may be sufficient for testing
> purposes).  I imagine that scripters (e.g. send-email developers)
> would find it more convenient if they do not have to come up with a
> temporary file and they can just run "git hook" and feed whatever
> they want to give to the hook from its standard input.  "git hook"
> command, upon startup, should do the reading of its standard input
> and spooling it to a temporary file it uses to pass the contents to
> the hook scripts, in other words.
>
> Other than that, it surely looks not just handy for tests, but has
> immediate uses.
>
> Thanks.

Re: [PATCH 4/5] sequencer: use the new hook API for the simpler "post-rewrite" call

[Phillip Wood]

Hi Ævar

On 23/01/2023 17:15, Ævar Arnfjörð Bjarmason wrote:
> From: Emily Shaffer <emilyshaffer@google.com>
> 
> Change the invocation of the "post-rewrite" hook added in
> 795160457db (sequencer (rebase -i): run the post-rewrite hook, if
> needed, 2017-01-02) to use the new hook API.
> 
> This leaves the more complex "post-rewrite" invocation added in
> a87a6f3c98e (commit: move post-rewrite code to libgit, 2017-11-17)
> here in sequencer.c unconverted. That'll be done in a subsequent
> commit.

As a reader I'd find it more helpful to explain why the conversion isn't 
done here rather than leaving be to run "git show" to figure it out. If 
you re-roll perhaps we could replace the commit citation with something like

sequencer.c also contains an invocation of the "post-rewrite" hook in 
run_rewrite_hook() that is not converted as the hook API does not allow 
us to pass the hook input as a string yet.

Best Wishes

Phillip

> Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
> Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
> ---
>   sequencer.c | 18 ++++--------------
>   1 file changed, 4 insertions(+), 14 deletions(-)
> 
> diff --git a/sequencer.c b/sequencer.c
> index 3e4a1972897..d8d59d05dd4 100644
> --- a/sequencer.c
> +++ b/sequencer.c
> @@ -4834,8 +4834,7 @@ static int pick_commits(struct repository *r,
>   		if (!stat(rebase_path_rewritten_list(), &st) &&
>   				st.st_size > 0) {
>   			struct child_process child = CHILD_PROCESS_INIT;
> -			const char *post_rewrite_hook =
> -				find_hook("post-rewrite");
> +			struct run_hooks_opt hook_opt = RUN_HOOKS_OPT_INIT;
>   
>   			child.in = open(rebase_path_rewritten_list(), O_RDONLY);
>   			child.git_cmd = 1;
> @@ -4845,18 +4844,9 @@ static int pick_commits(struct repository *r,
>   			/* we don't care if this copying failed */
>   			run_command(&child);
>   
> -			if (post_rewrite_hook) {
> -				struct child_process hook = CHILD_PROCESS_INIT;
> -
> -				hook.in = open(rebase_path_rewritten_list(),
> -					O_RDONLY);
> -				hook.stdout_to_stderr = 1;
> -				hook.trace2_hook_name = "post-rewrite";
> -				strvec_push(&hook.args, post_rewrite_hook);
> -				strvec_push(&hook.args, "rebase");
> -				/* we don't care if this hook failed */
> -				run_command(&hook);
> -			}
> +			hook_opt.path_to_stdin = rebase_path_rewritten_list();
> +			strvec_push(&hook_opt.args, "rebase");
> +			run_hooks_opt("post-rewrite", &hook_opt);
>   		}
>   		apply_autostash(rebase_path_autostash());
>   

Re: [PATCH 4/5] sequencer: use the new hook API for the simpler "post-rewrite" call

[Phillip Wood]

Hi Ævar

On 24/01/2023 14:46, Phillip Wood wrote:
> Hi Ævar
> 
> On 23/01/2023 17:15, Ævar Arnfjörð Bjarmason wrote:
>> From: Emily Shaffer <emilyshaffer@google.com>
>>
>> Change the invocation of the "post-rewrite" hook added in
>> 795160457db (sequencer (rebase -i): run the post-rewrite hook, if
>> needed, 2017-01-02) to use the new hook API.
>>
>> This leaves the more complex "post-rewrite" invocation added in
>> a87a6f3c98e (commit: move post-rewrite code to libgit, 2017-11-17)
>> here in sequencer.c unconverted. That'll be done in a subsequent
>> commit.
> 
> As a reader I'd find it more helpful to explain why the conversion isn't 
> done here rather than leaving be to run "git show" to figure it out. If 
> you re-roll perhaps we could replace the commit citation with something 
> like
> 
> sequencer.c also contains an invocation of the "post-rewrite" hook in 
> run_rewrite_hook() that is not converted as the hook API does not allow 
> us to pass the hook input as a string yet.

Sorry, I forgot to say in my previous reply that I like the code change 
here - it is a nice simplification for callers. builtin/am.c has a 
similar function to the one that is converted here.

Best Wishes

Phillip

> Best Wishes
> 
> Phillip
> 
>> Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
>> Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
>> ---
>>   sequencer.c | 18 ++++--------------
>>   1 file changed, 4 insertions(+), 14 deletions(-)
>>
>> diff --git a/sequencer.c b/sequencer.c
>> index 3e4a1972897..d8d59d05dd4 100644
>> --- a/sequencer.c
>> +++ b/sequencer.c
>> @@ -4834,8 +4834,7 @@ static int pick_commits(struct repository *r,
>>           if (!stat(rebase_path_rewritten_list(), &st) &&
>>                   st.st_size > 0) {
>>               struct child_process child = CHILD_PROCESS_INIT;
>> -            const char *post_rewrite_hook =
>> -                find_hook("post-rewrite");
>> +            struct run_hooks_opt hook_opt = RUN_HOOKS_OPT_INIT;
>>               child.in = open(rebase_path_rewritten_list(), O_RDONLY);
>>               child.git_cmd = 1;
>> @@ -4845,18 +4844,9 @@ static int pick_commits(struct repository *r,
>>               /* we don't care if this copying failed */
>>               run_command(&child);
>> -            if (post_rewrite_hook) {
>> -                struct child_process hook = CHILD_PROCESS_INIT;
>> -
>> -                hook.in = open(rebase_path_rewritten_list(),
>> -                    O_RDONLY);
>> -                hook.stdout_to_stderr = 1;
>> -                hook.trace2_hook_name = "post-rewrite";
>> -                strvec_push(&hook.args, post_rewrite_hook);
>> -                strvec_push(&hook.args, "rebase");
>> -                /* we don't care if this hook failed */
>> -                run_command(&hook);
>> -            }
>> +            hook_opt.path_to_stdin = rebase_path_rewritten_list();
>> +            strvec_push(&hook_opt.args, "rebase");
>> +            run_hooks_opt("post-rewrite", &hook_opt);
>>           }
>>           apply_autostash(rebase_path_autostash());

[PATCH v2 4/5] sequencer: use the new hook API for the simpler "post-rewrite" call

[Ævar Arnfjörð Bjarmason]

From: Emily Shaffer <emilyshaffer@google.com>

Change the invocation of the "post-rewrite" hook added in
795160457db (sequencer (rebase -i): run the post-rewrite hook, if
needed, 2017-01-02) to use the new hook API.

This leaves the more complex "post-rewrite" invocation added in
a87a6f3c98e (commit: move post-rewrite code to libgit, 2017-11-17)
here in sequencer.c unconverted.

Here we can pass in a file's via the "in" file descriptor, in that
case we don't have a file, but will need to write_in_full() to an "in"
provide by the API. Support for that will be added to the hook API in
the future, but we're not there yet.

Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 sequencer.c | 18 ++++--------------
 1 file changed, 4 insertions(+), 14 deletions(-)

diff --git a/sequencer.c b/sequencer.c
index 3e4a1972897..d8d59d05dd4 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -4834,8 +4834,7 @@ static int pick_commits(struct repository *r,
 		if (!stat(rebase_path_rewritten_list(), &st) &&
 				st.st_size > 0) {
 			struct child_process child = CHILD_PROCESS_INIT;
-			const char *post_rewrite_hook =
-				find_hook("post-rewrite");
+			struct run_hooks_opt hook_opt = RUN_HOOKS_OPT_INIT;
 
 			child.in = open(rebase_path_rewritten_list(), O_RDONLY);
 			child.git_cmd = 1;
@@ -4845,18 +4844,9 @@ static int pick_commits(struct repository *r,
 			/* we don't care if this copying failed */
 			run_command(&child);
 
-			if (post_rewrite_hook) {
-				struct child_process hook = CHILD_PROCESS_INIT;
-
-				hook.in = open(rebase_path_rewritten_list(),
-					O_RDONLY);
-				hook.stdout_to_stderr = 1;
-				hook.trace2_hook_name = "post-rewrite";
-				strvec_push(&hook.args, post_rewrite_hook);
-				strvec_push(&hook.args, "rebase");
-				/* we don't care if this hook failed */
-				run_command(&hook);
-			}
+			hook_opt.path_to_stdin = rebase_path_rewritten_list();
+			strvec_push(&hook_opt.args, "rebase");
+			run_hooks_opt("post-rewrite", &hook_opt);
 		}
 		apply_autostash(rebase_path_autostash());
 
-- 
2.39.1.1397.g8c8c074958d

[PATCH v2 0/5] hook API: support stdin, convert post-rewrite

[Ævar Arnfjörð Bjarmason]

BEGIN UPDATE

I sent this v2 in already as [0]; but didn't fill in the
In-Reply-Header, consequently it was disconnected from the v1 thread,
and per https://lore.kernel.org/git/xmqqr0v7o0pp.fsf@gitster.g/ (and
in "seen") hasn't been picked up. Sorry about that.

END UPDATE

As noted in the v1[1] this is the initial part of the greater
"config-based hooks" topic. I believe this iteration addresses all
comments on v1. Changes since then:

* Remove a couple of paragraphs in 1/4 that aren't relevant anymore,
  an already-landed topic addressed those.

* Don't needlessly change "cp->no_stdin = 1" and introduce an
  "else". This refactoring was there because that code eventually
  changes in the full "config-based hooks" topic, but going through
  those future changes I found that it wasn't for a good reason there
  either. We can just keep the "no_stdin = 1" by default, and have
  specific cases override that.

* Elaborate on why we're not converting the last "post-rewrite" hook
  here.

* Mention the future expected use for sendemail-validate in 5/5

The (passing) CI & topic branch for this is at[2].

0. https://lore.kernel.org/git/cover-v2-0.5-00000000000-20230203T104319Z-avarab@gmail.com/
1. https://lore.kernel.org/git/cover-0.5-00000000000-20230123T170550Z-avarab@gmail.com/
2. https://github.com/avar/git/tree/es-avar/config-based-hooks-the-beginning-2

Emily Shaffer (4):
  run-command: allow stdin for run_processes_parallel
  hook API: support passing stdin to hooks, convert am's 'post-rewrite'
  sequencer: use the new hook API for the simpler "post-rewrite" call
  hook: support a --to-stdin=<path> option

Ævar Arnfjörð Bjarmason (1):
  run-command.c: remove dead assignment in while-loop

 Documentation/git-hook.txt |  7 ++++++-
 builtin/am.c               | 20 ++++----------------
 builtin/hook.c             |  4 +++-
 hook.c                     |  5 +++++
 hook.h                     |  5 +++++
 run-command.c              | 13 +++++++++----
 sequencer.c                | 18 ++++--------------
 t/t1800-hook.sh            | 18 ++++++++++++++++++
 8 files changed, 54 insertions(+), 36 deletions(-)

Range-diff against v1:
1:  351c6a55a41 ! 1:  488b24e1c98 run-command.c: remove dead assignment in while-loop
    @@ Commit message
     
         Remove code that's been unused since it was added in
         c553c72eed6 (run-command: add an asynchronous parallel child
    -    processor, 2015-12-15), the next use of "i" in this function is:
    -
    -            for (i = 0; ...
    -
    -    So we'll always clobber the "i" that's set here. Presumably the "i"
    -    assignment is an artifact of WIP code that made it into our tree.
    -
    -    A subsequent commit will need to adjust the type of the "i" variable
    -    in the otherwise unrelated for-loop, which is why this is being
    -    removed now.
    +    processor, 2015-12-15).
     
         Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
     
2:  81eef2f60a0 = 2:  9a178577dcc run-command: allow stdin for run_processes_parallel
3:  c6b9b69c516 ! 3:  3d3dd6b900a hook API: support passing stdin to hooks, convert am's 'post-rewrite'
    @@ builtin/am.c: static int run_applypatch_msg_hook(struct am_state *state)
     
      ## hook.c ##
     @@ hook.c: static int pick_next_hook(struct child_process *cp,
    - 	if (!hook_path)
    - 		return 0;
      
    --	cp->no_stdin = 1;
    + 	cp->no_stdin = 1;
      	strvec_pushv(&cp->env, hook_cb->options->env.v);
     +	/* reopen the file for stdin; run_command closes it. */
     +	if (hook_cb->options->path_to_stdin) {
     +		cp->no_stdin = 0;
     +		cp->in = xopen(hook_cb->options->path_to_stdin, O_RDONLY);
    -+	} else {
    -+		cp->no_stdin = 1;
     +	}
      	cp->stdout_to_stderr = 1;
      	cp->trace2_hook_name = hook_cb->hook_name;
4:  7a55c95f60f ! 4:  b96522d593f sequencer: use the new hook API for the simpler "post-rewrite" call
    @@ Commit message
     
         This leaves the more complex "post-rewrite" invocation added in
         a87a6f3c98e (commit: move post-rewrite code to libgit, 2017-11-17)
    -    here in sequencer.c unconverted. That'll be done in a subsequent
    -    commit.
    +    here in sequencer.c unconverted.
    +
    +    Here we can pass in a file's via the "in" file descriptor, in that
    +    case we don't have a file, but will need to write_in_full() to an "in"
    +    provide by the API. Support for that will be added to the hook API in
    +    the future, but we're not there yet.
     
         Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
         Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
5:  cb9ef7a89c4 ! 5:  b4e02f41194 hook: support a --to-stdin=<path> option for testing
    @@ Metadata
     Author: Emily Shaffer <emilyshaffer@google.com>
     
      ## Commit message ##
    -    hook: support a --to-stdin=<path> option for testing
    +    hook: support a --to-stdin=<path> option
     
         Expose the "path_to_stdin" API added in the preceding commit in the
    -    "git hook run" command. For now we won't be using this command
    -    interface outside of the tests, but exposing this functionality makes
    -    it easier to test the hook API.
    +    "git hook run" command.
    +
    +    For now we won't be using this command interface outside of the tests,
    +    but exposing this functionality makes it easier to test the hook
    +    API. The plan is to use this to extend the "sendemail-validate"
    +    hook[1][2].
    +
    +    1. https://lore.kernel.org/git/ad152e25-4061-9955-d3e6-a2c8b1bd24e7@amd.com
    +    2. https://lore.kernel.org/git/20230120012459.920932-1-michael.strawbridge@amd.com
     
         Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
         Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
-- 
2.39.1.1475.gc2542cdc5ef

[PATCH v2 1/5] run-command.c: remove dead assignment in while-loop

[Ævar Arnfjörð Bjarmason]

Remove code that's been unused since it was added in
c553c72eed6 (run-command: add an asynchronous parallel child
processor, 2015-12-15).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 run-command.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/run-command.c b/run-command.c
index 50cc011654e..b439c7974ca 100644
--- a/run-command.c
+++ b/run-command.c
@@ -1632,9 +1632,7 @@ static void pp_buffer_stderr(struct parallel_processes *pp,
 			     const struct run_process_parallel_opts *opts,
 			     int output_timeout)
 {
-	int i;
-
-	while ((i = poll(pp->pfd, opts->processes, output_timeout) < 0)) {
+	while (poll(pp->pfd, opts->processes, output_timeout) < 0) {
 		if (errno == EINTR)
 			continue;
 		pp_cleanup(pp, opts);
-- 
2.39.1.1475.gc2542cdc5ef

[PATCH v2 2/5] run-command: allow stdin for run_processes_parallel

[Ævar Arnfjörð Bjarmason]

From: Emily Shaffer <emilyshaffer@google.com>

While it makes sense not to inherit stdin from the parent process to
avoid deadlocking, it's not necessary to completely ban stdin to
children. An informed user should be able to configure stdin safely. By
setting `some_child.process.no_stdin=1` before calling `get_next_task()`
we provide a reasonable default behavior but enable users to set up
stdin streaming for themselves during the callback.

`some_child.process.stdout_to_stderr`, however, remains unmodifiable by
`get_next_task()` - the rest of the run_processes_parallel() API depends
on child output in stderr.

Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 run-command.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/run-command.c b/run-command.c
index b439c7974ca..6bd16acb060 100644
--- a/run-command.c
+++ b/run-command.c
@@ -1586,6 +1586,14 @@ static int pp_start_one(struct parallel_processes *pp,
 	if (i == opts->processes)
 		BUG("bookkeeping is hard");
 
+	/*
+	 * By default, do not inherit stdin from the parent process - otherwise,
+	 * all children would share stdin! Users may overwrite this to provide
+	 * something to the child's stdin by having their 'get_next_task'
+	 * callback assign 0 to .no_stdin and an appropriate integer to .in.
+	 */
+	pp->children[i].process.no_stdin = 1;
+
 	code = opts->get_next_task(&pp->children[i].process,
 				   opts->ungroup ? NULL : &pp->children[i].err,
 				   opts->data,
@@ -1601,7 +1609,6 @@ static int pp_start_one(struct parallel_processes *pp,
 		pp->children[i].process.err = -1;
 		pp->children[i].process.stdout_to_stderr = 1;
 	}
-	pp->children[i].process.no_stdin = 1;
 
 	if (start_command(&pp->children[i].process)) {
 		if (opts->start_failure)
-- 
2.39.1.1475.gc2542cdc5ef

[PATCH v2 3/5] hook API: support passing stdin to hooks, convert am's 'post-rewrite'

[Ævar Arnfjörð Bjarmason]

From: Emily Shaffer <emilyshaffer@google.com>

Convert the invocation of the 'post-rewrite' hook run by 'git am' to
use the hook.h library. To do this we need to add a "path_to_stdin"
member to "struct run_hooks_opt".

In our API this is supported by asking for a file path, rather
than by reading stdin. Reading directly from stdin would involve caching
the entire stdin (to memory or to disk) once the hook API is made to
support "jobs" larger than 1, along with support for executing N hooks
at a time (i.e. the upcoming config-based hooks).

Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 builtin/am.c | 20 ++++----------------
 hook.c       |  5 +++++
 hook.h       |  5 +++++
 3 files changed, 14 insertions(+), 16 deletions(-)

diff --git a/builtin/am.c b/builtin/am.c
index 82a41cbfc4e..8be91617fef 100644
--- a/builtin/am.c
+++ b/builtin/am.c
@@ -495,24 +495,12 @@ static int run_applypatch_msg_hook(struct am_state *state)
  */
 static int run_post_rewrite_hook(const struct am_state *state)
 {
-	struct child_process cp = CHILD_PROCESS_INIT;
-	const char *hook = find_hook("post-rewrite");
-	int ret;
-
-	if (!hook)
-		return 0;
+	struct run_hooks_opt opt = RUN_HOOKS_OPT_INIT;
 
-	strvec_push(&cp.args, hook);
-	strvec_push(&cp.args, "rebase");
+	strvec_push(&opt.args, "rebase");
+	opt.path_to_stdin = am_path(state, "rewritten");
 
-	cp.in = xopen(am_path(state, "rewritten"), O_RDONLY);
-	cp.stdout_to_stderr = 1;
-	cp.trace2_hook_name = "post-rewrite";
-
-	ret = run_command(&cp);
-
-	close(cp.in);
-	return ret;
+	return run_hooks_opt("post-rewrite", &opt);
 }
 
 /**
diff --git a/hook.c b/hook.c
index a4fa1031f28..1a848318634 100644
--- a/hook.c
+++ b/hook.c
@@ -55,6 +55,11 @@ static int pick_next_hook(struct child_process *cp,
 
 	cp->no_stdin = 1;
 	strvec_pushv(&cp->env, hook_cb->options->env.v);
+	/* reopen the file for stdin; run_command closes it. */
+	if (hook_cb->options->path_to_stdin) {
+		cp->no_stdin = 0;
+		cp->in = xopen(hook_cb->options->path_to_stdin, O_RDONLY);
+	}
 	cp->stdout_to_stderr = 1;
 	cp->trace2_hook_name = hook_cb->hook_name;
 	cp->dir = hook_cb->options->dir;
diff --git a/hook.h b/hook.h
index 4258b13da0d..19ab9a5806e 100644
--- a/hook.h
+++ b/hook.h
@@ -30,6 +30,11 @@ struct run_hooks_opt
 	 * was invoked.
 	 */
 	int *invoked_hook;
+
+	/**
+	 * Path to file which should be piped to stdin for each hook.
+	 */
+	const char *path_to_stdin;
 };
 
 #define RUN_HOOKS_OPT_INIT { \
-- 
2.39.1.1475.gc2542cdc5ef

[PATCH v2 4/5] sequencer: use the new hook API for the simpler "post-rewrite" call

[Ævar Arnfjörð Bjarmason]

From: Emily Shaffer <emilyshaffer@google.com>

Change the invocation of the "post-rewrite" hook added in
795160457db (sequencer (rebase -i): run the post-rewrite hook, if
needed, 2017-01-02) to use the new hook API.

This leaves the more complex "post-rewrite" invocation added in
a87a6f3c98e (commit: move post-rewrite code to libgit, 2017-11-17)
here in sequencer.c unconverted.

Here we can pass in a file's via the "in" file descriptor, in that
case we don't have a file, but will need to write_in_full() to an "in"
provide by the API. Support for that will be added to the hook API in
the future, but we're not there yet.

Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 sequencer.c | 18 ++++--------------
 1 file changed, 4 insertions(+), 14 deletions(-)

diff --git a/sequencer.c b/sequencer.c
index 3e4a1972897..d8d59d05dd4 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -4834,8 +4834,7 @@ static int pick_commits(struct repository *r,
 		if (!stat(rebase_path_rewritten_list(), &st) &&
 				st.st_size > 0) {
 			struct child_process child = CHILD_PROCESS_INIT;
-			const char *post_rewrite_hook =
-				find_hook("post-rewrite");
+			struct run_hooks_opt hook_opt = RUN_HOOKS_OPT_INIT;
 
 			child.in = open(rebase_path_rewritten_list(), O_RDONLY);
 			child.git_cmd = 1;
@@ -4845,18 +4844,9 @@ static int pick_commits(struct repository *r,
 			/* we don't care if this copying failed */
 			run_command(&child);
 
-			if (post_rewrite_hook) {
-				struct child_process hook = CHILD_PROCESS_INIT;
-
-				hook.in = open(rebase_path_rewritten_list(),
-					O_RDONLY);
-				hook.stdout_to_stderr = 1;
-				hook.trace2_hook_name = "post-rewrite";
-				strvec_push(&hook.args, post_rewrite_hook);
-				strvec_push(&hook.args, "rebase");
-				/* we don't care if this hook failed */
-				run_command(&hook);
-			}
+			hook_opt.path_to_stdin = rebase_path_rewritten_list();
+			strvec_push(&hook_opt.args, "rebase");
+			run_hooks_opt("post-rewrite", &hook_opt);
 		}
 		apply_autostash(rebase_path_autostash());
 
-- 
2.39.1.1475.gc2542cdc5ef

[PATCH v2 5/5] hook: support a --to-stdin=<path> option

[Ævar Arnfjörð Bjarmason]

From: Emily Shaffer <emilyshaffer@google.com>

Expose the "path_to_stdin" API added in the preceding commit in the
"git hook run" command.

For now we won't be using this command interface outside of the tests,
but exposing this functionality makes it easier to test the hook
API. The plan is to use this to extend the "sendemail-validate"
hook[1][2].

1. https://lore.kernel.org/git/ad152e25-4061-9955-d3e6-a2c8b1bd24e7@amd.com
2. https://lore.kernel.org/git/20230120012459.920932-1-michael.strawbridge@amd.com

Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 Documentation/git-hook.txt |  7 ++++++-
 builtin/hook.c             |  4 +++-
 t/t1800-hook.sh            | 18 ++++++++++++++++++
 3 files changed, 27 insertions(+), 2 deletions(-)

diff --git a/Documentation/git-hook.txt b/Documentation/git-hook.txt
index 77c3a8ad909..3407f3c2c07 100644
--- a/Documentation/git-hook.txt
+++ b/Documentation/git-hook.txt
@@ -8,7 +8,7 @@ git-hook - Run git hooks
 SYNOPSIS
 --------
 [verse]
-'git hook' run [--ignore-missing] <hook-name> [-- <hook-args>]
+'git hook' run [--ignore-missing] [--to-stdin=<path>] <hook-name> [-- <hook-args>]
 
 DESCRIPTION
 -----------
@@ -31,6 +31,11 @@ linkgit:githooks[5] for arguments hooks might expect (if any).
 OPTIONS
 -------
 
+--to-stdin::
+	For "run"; Specify a file which will be streamed into the
+	hook's stdin. The hook will receive the entire file from
+	beginning to EOF.
+
 --ignore-missing::
 	Ignore any missing hook by quietly returning zero. Used for
 	tools that want to do a blind one-shot run of a hook that may
diff --git a/builtin/hook.c b/builtin/hook.c
index b6530d189ad..f95b7965c58 100644
--- a/builtin/hook.c
+++ b/builtin/hook.c
@@ -7,7 +7,7 @@
 #include "strvec.h"
 
 #define BUILTIN_HOOK_RUN_USAGE \
-	N_("git hook run [--ignore-missing] <hook-name> [-- <hook-args>]")
+	N_("git hook run [--ignore-missing] [--to-stdin=<path>] <hook-name> [-- <hook-args>]")
 
 static const char * const builtin_hook_usage[] = {
 	BUILTIN_HOOK_RUN_USAGE,
@@ -28,6 +28,8 @@ static int run(int argc, const char **argv, const char *prefix)
 	struct option run_options[] = {
 		OPT_BOOL(0, "ignore-missing", &ignore_missing,
 			 N_("silently ignore missing requested <hook-name>")),
+		OPT_STRING(0, "to-stdin", &opt.path_to_stdin, N_("path"),
+			   N_("file to read into hooks' stdin")),
 		OPT_END(),
 	};
 	int ret;
diff --git a/t/t1800-hook.sh b/t/t1800-hook.sh
index 2ef3579fa7c..3506f627b6c 100755
--- a/t/t1800-hook.sh
+++ b/t/t1800-hook.sh
@@ -177,4 +177,22 @@ test_expect_success 'git hook run a hook with a bad shebang' '
 	test_cmp expect actual
 '
 
+test_expect_success 'stdin to hooks' '
+	write_script .git/hooks/test-hook <<-\EOF &&
+	echo BEGIN stdin
+	cat
+	echo END stdin
+	EOF
+
+	cat >expect <<-EOF &&
+	BEGIN stdin
+	hello
+	END stdin
+	EOF
+
+	echo hello >input &&
+	git hook run --to-stdin=input test-hook 2>actual &&
+	test_cmp expect actual
+'
+
 test_done
-- 
2.39.1.1475.gc2542cdc5ef

Re: [PATCH v2 1/5] run-command.c: remove dead assignment in while-loop

[Junio C Hamano]

Ævar Arnfjörð Bjarmason  <avarab@gmail.com> writes:

> Remove code that's been unused since it was added in
> c553c72eed6 (run-command: add an asynchronous parallel child
> processor, 2015-12-15).

Obviously correct.  Thanks, will queue.

>
> Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
> ---
>  run-command.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
>
> diff --git a/run-command.c b/run-command.c
> index 50cc011654e..b439c7974ca 100644
> --- a/run-command.c
> +++ b/run-command.c
> @@ -1632,9 +1632,7 @@ static void pp_buffer_stderr(struct parallel_processes *pp,
>  			     const struct run_process_parallel_opts *opts,
>  			     int output_timeout)
>  {
> -	int i;
> -
> -	while ((i = poll(pp->pfd, opts->processes, output_timeout) < 0)) {
> +	while (poll(pp->pfd, opts->processes, output_timeout) < 0) {
>  		if (errno == EINTR)
>  			continue;
>  		pp_cleanup(pp, opts);

Re: [PATCH v2 2/5] run-command: allow stdin for run_processes_parallel

[Junio C Hamano]

Ævar Arnfjörð Bjarmason  <avarab@gmail.com> writes:

> From: Emily Shaffer <emilyshaffer@google.com>
>
> While it makes sense not to inherit stdin from the parent process to
> avoid deadlocking, it's not necessary to completely ban stdin to
> children.

I do not think deadlock avoidance is an issue.  Unpredictable
feeding of pieces of input into multiple children is.  One possible
semantics is to grab the input and dup/tee into all the children, so 
that each child gets its own copy to process.  A hook like "pre-receive",
when it has more than one scripts listening to the event, may want
such a semantics.  Another possible semantics is to give priority
among the children running simultaneously and feed only one child,
while starving others.

> An informed user should be able to configure stdin safely. By
> setting `some_child.process.no_stdin=1` before calling `get_next_task()`
> we provide a reasonable default behavior but enable users to set up
> stdin streaming for themselves during the callback.

I _think_ this alludes to the latter, e.g. "only one child is
allowed, and the one that controls what children are spawned sets
no_stdin for everybody but the chosen one".  We may want to be a bit
more explicit in the proposed log message and definitely in the
documentation.

The implementation is "nice".

> +	/*
> +	 * By default, do not inherit stdin from the parent process - otherwise,
> +	 * all children would share stdin! Users may overwrite this to provide
> +	 * something to the child's stdin by having their 'get_next_task'
> +	 * callback assign 0 to .no_stdin and an appropriate integer to .in.
> +	 */
> +	pp->children[i].process.no_stdin = 1;
> +
>  	code = opts->get_next_task(&pp->children[i].process,
>  				   opts->ungroup ? NULL : &pp->children[i].err,
>  				   opts->data,
> @@ -1601,7 +1609,6 @@ static int pp_start_one(struct parallel_processes *pp,
>  		pp->children[i].process.err = -1;
>  		pp->children[i].process.stdout_to_stderr = 1;
>  	}
> -	pp->children[i].process.no_stdin = 1;
>  
>  	if (start_command(&pp->children[i].process)) {
>  		if (opts->start_failure)

Re: [PATCH v2 3/5] hook API: support passing stdin to hooks, convert am's 'post-rewrite'

[Junio C Hamano]

Ævar Arnfjörð Bjarmason  <avarab@gmail.com> writes:

> From: Emily Shaffer <emilyshaffer@google.com>
>
> Convert the invocation of the 'post-rewrite' hook run by 'git am' to
> use the hook.h library. To do this we need to add a "path_to_stdin"
> member to "struct run_hooks_opt".
>
> In our API this is supported by asking for a file path, rather
> than by reading stdin. Reading directly from stdin would involve caching
> the entire stdin (to memory or to disk) once the hook API is made to
> support "jobs" larger than 1, along with support for executing N hooks
> at a time (i.e. the upcoming config-based hooks).

OK, that is a sensible plan to spool and dup/tee the input to
children.  It may not be necessary yet at this step, but it is very
good to be thinking ahead.

Looking good.

Re: [PATCH v2 4/5] sequencer: use the new hook API for the simpler "post-rewrite" call

[Junio C Hamano]

Ævar Arnfjörð Bjarmason  <avarab@gmail.com> writes:

> From: Emily Shaffer <emilyshaffer@google.com>
>
> Change the invocation of the "post-rewrite" hook added in
> 795160457db (sequencer (rebase -i): run the post-rewrite hook, if
> needed, 2017-01-02) to use the new hook API.

Very straight-forward, thanks to the previous step (i.e. addition of
the "path_to_stdin" support).  Nicely done.

Re: [PATCH v2 5/5] hook: support a --to-stdin=<path> option

[Junio C Hamano]

Ævar Arnfjörð Bjarmason  <avarab@gmail.com> writes:

> From: Emily Shaffer <emilyshaffer@google.com>
>
> Expose the "path_to_stdin" API added in the preceding commit in the
> "git hook run" command.
>
> For now we won't be using this command interface outside of the tests,
> but exposing this functionality makes it easier to test the hook
> API. The plan is to use this to extend the "sendemail-validate"
> hook[1][2].

OK.

What does it take to tackle the obvious leftover bits of [4/5]?  Use
tempfile API to allocate a temporary file, slurp the input and close
it, and then use the "path_to_stdin" feature to spawn the hook?

Re: [PATCH v2 0/5] hook API: support stdin, convert post-rewrite

[Junio C Hamano]

Ævar Arnfjörð Bjarmason  <avarab@gmail.com> writes:

> As noted in the v1[1] this is the initial part of the greater
> "config-based hooks" topic. I believe this iteration addresses all
> comments on v1. Changes since then:
>
> * Remove a couple of paragraphs in 1/4 that aren't relevant anymore,
>   an already-landed topic addressed those.
>
> * Don't needlessly change "cp->no_stdin = 1" and introduce an
>   "else". This refactoring was there because that code eventually
>   changes in the full "config-based hooks" topic, but going through
>   those future changes I found that it wasn't for a good reason there
>   either. We can just keep the "no_stdin = 1" by default, and have
>   specific cases override that.
>
> * Elaborate on why we're not converting the last "post-rewrite" hook
>   here.
>
> * Mention the future expected use for sendemail-validate in 5/5

All read well.  Will queue.  Thanks.

Re: [PATCH v2 5/5] hook: support a --to-stdin=<path> option

[Ævar Arnfjörð Bjarmason]

On Wed, Feb 08 2023, Junio C Hamano wrote:

> Ævar Arnfjörð Bjarmason  <avarab@gmail.com> writes:
>
>> From: Emily Shaffer <emilyshaffer@google.com>
>>
>> Expose the "path_to_stdin" API added in the preceding commit in the
>> "git hook run" command.
>>
>> For now we won't be using this command interface outside of the tests,
>> but exposing this functionality makes it easier to test the hook
>> API. The plan is to use this to extend the "sendemail-validate"
>> hook[1][2].
>
> OK.
>
> What does it take to tackle the obvious leftover bits of [4/5]?  Use
> tempfile API to allocate a temporary file, slurp the input and close
> it, and then use the "path_to_stdin" feature to spawn the hook?

You did ask for it :)

The below is something I wrote for the end of the initial v2 CL, but
then decided it was way too long and dropped it.

The tl;dr is that no, that would be the shortest way forward, and
arguably what we should do now.

But those hooks currently print straight to the pipe, so having the API
force them to use a tempfile would suck.

But since this is all going for supporting N hooks in parallel the next
step requires an API that's future-proofing the feeding of the same
content to multiple hooks.

So, without further adieu, that dropped part of the v2 CL: 

The rest of this is a large digression about the future API design of
this topic, please don't read ahead unless you're very curious about
that (mainly I wanted to brain-dump this somewhere).

I considered expanding this series to include the rest of the
remaining "post-rewrite" hook in sequencer.c, but as that needs at
least a couple of prep patches to expand the API I've left it out for
now.

The main reason I didn't include (aside from the "let's start small"
of this topic) it is that I still don't like the API we'll eventually
need for the parallel hooks that take "stdin", and would like to mull
it over a bit.

What comes after this series eventually wants to convert these hooks
from (pseudocode):

	struct child_process proc = CHILD_PROCESS_INIT;
	[...]
	start_command(&proc);
	[...]
	for item in transaction:
		write_in_full(proc.in, item, strlen(item));

To e.g.:

	struct run_hooks_opt opt = RUN_HOOKS_OPT_INIT_PARALLEL;
	struct string_list to_stdin = STRING_LIST_INIT_DUP;

	[...]

        opt.feed_pipe = pipe_from_string_list;
        opt.feed_pipe_ctx = &to_stdin;

	[...]

	for item in transaction:
		string_list_append(&to_stdin, item);

	run_hooks_opt("some-hook", &opt);
	string_list_clear(&to_stdin, 0);

The reason is that if we're producing data for stdin we'll need to
give it to N hooks. For this topic we neatly side-step that with a
"to-stdin", as Junio notes in [3] and [4].

I think even that is arguably a bit ugly, but it's all internally
changable uglyness, i.e. for the "sendemail-validate" whether we feed
"git hook" with content on stdin or are forced to create a file and
feed it with "--to-stdin" is something we can change later.

But the answer to the question raised in [4] is that we'll eventually
need something like the above. But I don't like it, because:

A) The currently proposed API[5] wants to represent lines to the hooks
   are a "struct string_list", so you add "\n"-less items to it, and
   we'll always print "%s\n" for each item.

   This is an arbitrary limitation over the write_in_full() we do now,
   and will be a hassle e.g. if you have already prepared content,
   you'll first need to line-split it.

   Maybe I'm missing some reason for why the hook interface needs to
   intrinsically promise that it'll be doing write()'s to the hooks
   ending in \n's, but right now I don't see that, we could leave that
   up to each hook, and if they're spewing content larger than that
   the hook itself should just be line-buffering if they care about
   the distinction.

B) Even if we internally line-split a "struct string_list *" is just a
   bad fit as we lose the string size, we should pass something that
   give us a "size_t len" (which we could stuff in the "util" field,
   but...)

C) We need to buffer up the stdin in full before we feed the first
   line to the hook, which seems to me to be an API design that
   creates the problem the second paragraph of [5] claims to be trying
   to avoid. I.e. "simply taking a string_list or strbuf is not as
   scalable as using a callback".

   That would be true if the result of the callback were streamed to
   the N hooks we have, but it's not the case. It's a callback
   mechanism that amounts to just handing off a big "struct
   string_list", which we need to fully populate before we start the
   hook.

D) Most importantly, the API seems to be structured around a problem
   we don't actually have.

   The more general problem *could be* that you'd want to feed N hooks
   with the same content, we ourselves get that content streamed into
   us on "stdin", and therefore need to either buffer it in full
   up-front, or as we read it re-spew it into the N hooks we're
   executing.

   But e.g. for the "reference-transaction" hook all we need to
   support N hooks is to allocate a single "size_t pos" for them, as
   when we're executing them we have a "struct ref_transaction
   *transaction" that doesn't change for the duration of the hook. The
   same goes for the "pre-push.

   Actually, the only eventual API users that really could have used
   buffering to ensure consistent results won't use the buffering
   API. E.g. for the "post-rewrite" and "rebase" hooks we consume a
   file in ".git/" to give to the hooks on stdin. Currently (and still
   with this topic) we'll only have one hook, so the file's content
   will always be the same.

   But if we were being paranoid we'd buffer it up, so that we could
   ensure that our N hooks all get the same input, but for the API
   users that could get a benefit from that we don't use it, but only
   for those that are guaranteed not to need it.

So before the next iteration I'll try to find some time to play with
that. I.e. I think we can rip out the whole "struct string_list" feeder,
and just have an eventual mechanism for each of the N hooks to
init/release their "feed stdin" state with callbacks, and to save away
their state in their own "void *".

Then e.g. for the reference-transaction we'd just allocate a "size_t
pos" per hook, and then just spew content at them on the fly from the
transaction struct, no pre-generation necessary.

Such an interface will nicely support streaming without pre-buffering
delay, and could even run lock-less while multi-threaded (the source
data being const, and (almost) all state per-thread.

The interface would then be general enough to support
pre-slurping/buffering content at the start, and then streaming to N
hooks, e.g. for the "read a file, but guarantee that everyone gets the
same version". We won't need a string list for that, at most a strbuf,
but maybe we can just mmap() those...

3. https://lore.kernel.org/git/xmqqy1pskfo6.fsf@gitster.g/
4. https://lore.kernel.org/git/xmqqtu0gkaye.fsf@gitster.g/
5. https://lore.kernel.org/git/patch-v5-24.36-bb119fa7cc0-20210902T125110Z-avarab@gmail.com/