Re: [PATCH] rev-list: make empty --stdin not an error

[Jeff King <peff@peff.net>]

On Wed, Aug 22, 2018 at 01:42:26PM -0700, Junio C Hamano wrote:

> Jeff King <peff@peff.net> writes:
> 
> > Instead, let's keep two flags: one to denote when we got
> > actual input (which triggers both roles) and one for when we
> > read stdin (which triggers only the first).
> >
> > This does mean a caller interested in the first role has to
> > check both flags, but there's only one such caller. And any
> > future callers might want to make the distinction anyway
> > (e.g., if they care less about erroring out, and more about
> > whether revision.c soaked up our stdin).
> >
> > [1] https://public-inbox.org/git/20170802223416.gwiezhbuxbdmbjzx@sigill.intra.peff.net/
> >
> > Helped-by: Junio C Hamano <gitster@pobox.com>
> > Signed-off-by: Jeff King <peff@peff.net>
> > ---
> >  builtin/rev-list.c       | 2 +-
> >  revision.c               | 1 +
> >  revision.h               | 5 +++++
> >  t/t6018-rev-list-glob.sh | 2 +-
> >  4 files changed, 8 insertions(+), 2 deletions(-)
> 
> I think this makes sense, but if we were to give a dedicated field
> in the revs structure, can we lose the local variable at the same
> time, I wonder?

Yes. I was thinking it had more purpose than this, but it really is just
a flag to check "did we do this already?". Which is one of the main
purposes I claimed for the new flag in my commit message. :)

Here it is with that squashed in (this is the whole patch, since I
updated the commit message to mention it).

-- >8 --
Subject: [PATCH] rev-list: make empty --stdin not an error

When we originally did the series that contains 7ba826290a
(revision: add rev_input_given flag, 2017-08-02) the intent
was that "git rev-list --stdin </dev/null" would similarly
become a successful noop. However, an attempt at the time to
do that did not work[1]. The problem is that rev_input_given
serves two roles:

 - it tells rev-list.c that it should not error out

 - it tells revision.c that it should not have the "default"
   ref kick (e.g., "HEAD" in "git log")

We want to trigger the former, but not the latter. This is
technically possible with a single flag, if we set the flag
only after revision.c's revs->def check. But this introduces
a rather subtle ordering dependency.

Instead, let's keep two flags: one to denote when we got
actual input (which triggers both roles) and one for when we
read stdin (which triggers only the first).

This does mean a caller interested in the first role has to
check both flags, but there's only one such caller. And any
future callers might want to make the distinction anyway
(e.g., if they care less about erroring out, and more about
whether revision.c soaked up our stdin).

In fact, we already keep such a flag internally in
revision.c for this purpose, so this is really just exposing
that to the caller (and the old function-local flag can go
away in favor of our new one).

[1] https://public-inbox.org/git/20170802223416.gwiezhbuxbdmbjzx@sigill.intra.peff.net/

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Jeff King <peff@peff.net>
---
 builtin/rev-list.c       | 2 +-
 revision.c               | 5 ++---
 revision.h               | 5 +++++
 t/t6018-rev-list-glob.sh | 2 +-
 4 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/builtin/rev-list.c b/builtin/rev-list.c
index 5b07f3f4a2..ed0ea7dc5b 100644
--- a/builtin/rev-list.c
+++ b/builtin/rev-list.c
@@ -493,7 +493,7 @@ int cmd_rev_list(int argc, const char **argv, const char *prefix)
 	if ((!revs.commits && reflog_walk_empty(revs.reflog_info) &&
 	     (!(revs.tag_objects || revs.tree_objects || revs.blob_objects) &&
 	      !revs.pending.nr) &&
-	     !revs.rev_input_given) ||
+	     !revs.rev_input_given && !revs.read_from_stdin) ||
 	    revs.diff)
 		usage(rev_list_usage);
 
diff --git a/revision.c b/revision.c
index de4dce600d..46228f82ee 100644
--- a/revision.c
+++ b/revision.c
@@ -2318,7 +2318,7 @@ static void NORETURN diagnose_missing_default(const char *def)
  */
 int setup_revisions(int argc, const char **argv, struct rev_info *revs, struct setup_revision_opt *opt)
 {
-	int i, flags, left, seen_dashdash, read_from_stdin, got_rev_arg = 0, revarg_opt;
+	int i, flags, left, seen_dashdash, got_rev_arg = 0, revarg_opt;
 	struct argv_array prune_data = ARGV_ARRAY_INIT;
 	const char *submodule = NULL;
 
@@ -2348,7 +2348,6 @@ int setup_revisions(int argc, const char **argv, struct rev_info *revs, struct s
 	revarg_opt = opt ? opt->revarg_opt : 0;
 	if (seen_dashdash)
 		revarg_opt |= REVARG_CANNOT_BE_FILENAME;
-	read_from_stdin = 0;
 	for (left = i = 1; i < argc; i++) {
 		const char *arg = argv[i];
 		if (*arg == '-') {
@@ -2367,7 +2366,7 @@ int setup_revisions(int argc, const char **argv, struct rev_info *revs, struct s
 					argv[left++] = arg;
 					continue;
 				}
-				if (read_from_stdin++)
+				if (revs->read_from_stdin++)
 					die("--stdin given twice?");
 				read_revisions_from_stdin(revs, &prune_data);
 				continue;
diff --git a/revision.h b/revision.h
index 007278cc11..1225957927 100644
--- a/revision.h
+++ b/revision.h
@@ -82,6 +82,11 @@ struct rev_info {
 	 */
 	int rev_input_given;
 
+	/*
+	 * Whether we read from stdin due to the --stdin option.
+	 */
+	int read_from_stdin;
+
 	/* topo-sort */
 	enum rev_sort_order sort_order;
 
diff --git a/t/t6018-rev-list-glob.sh b/t/t6018-rev-list-glob.sh
index 0bf10d0686..db8a7834d8 100755
--- a/t/t6018-rev-list-glob.sh
+++ b/t/t6018-rev-list-glob.sh
@@ -255,7 +255,7 @@ test_expect_success 'rev-list accumulates multiple --exclude' '
 	compare rev-list "--exclude=refs/remotes/* --exclude=refs/tags/* --all" --branches
 '
 
-test_expect_failure 'rev-list should succeed with empty output on empty stdin' '
+test_expect_success 'rev-list should succeed with empty output on empty stdin' '
 	git rev-list --stdin </dev/null >actual &&
 	test_must_be_empty actual
 '
-- 
2.19.0.rc0.412.g7005db4e88