[RFC PATCH 2/2] config: add 'config.superproject' file

[Emily Shaffer <emilyshaffer@google.com>]

Some configs, such as wrapper directives like gerrit.createChangeId, or
forthcoming hook configs, should apply to a superproject as well as all
its submodules. It may not be appropriate to apply them globally - for
example, if the user also contributes to many projects which do not use
the configs necessary for one project-with-submodules - and it may be
burdensome to apply them locally to the superproject and each of its
submodules. Even if the user runs 'git submodule foreach "git config
--local foo.bar', if a new submodule is added later on, that config is
not applied to the new submodule.

It is also inappropriate to share the entire superproject config, since
some items - like remote URLs or partial-clone filters - would not apply
to a submodule.

To make life easier for projects with many submodules, then, create a
new "config.superproject" config scope, which is included in the config
parse for the superproject as well as for all the submodules of that
superproject.

For the superproject, this new config file is equally local to the local
config; for the submodule, the new config file is less local than the
local config. So let's include it directly before the local config
during the config parse.

Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
---
 Documentation/git-config.txt   |  21 +++++-
 builtin/config.c               |  10 ++-
 config.c                       |  26 +++++++
 config.h                       |   3 +
 submodule.c                    |  29 ++++++++
 submodule.h                    |   6 ++
 t/t1311-superproject-config.sh | 124 +++++++++++++++++++++++++++++++++
 7 files changed, 216 insertions(+), 3 deletions(-)
 create mode 100755 t/t1311-superproject-config.sh

diff --git a/Documentation/git-config.txt b/Documentation/git-config.txt
index 4b4cc5c5e8..a33136fb08 100644
--- a/Documentation/git-config.txt
+++ b/Documentation/git-config.txt
@@ -48,7 +48,7 @@ unset an existing `--type` specifier with `--no-type`.
 
 When reading, the values are read from the system, global and
 repository local configuration files by default, and options
-`--system`, `--global`, `--local`, `--worktree` and
+`--system`, `--global`, `--superproject`, `--local`, `--worktree` and
 `--file <filename>` can be used to tell the command to read from only
 that location (see <<FILES>>).
 
@@ -127,6 +127,17 @@ rather than from all available files.
 +
 See also <<FILES>>.
 
+--superproject::
+	For writing options: write to the superproject's
+	`.git/config.superproject` file, even if run from a submodule of that
+	superproject.
++
+For reading options: read only from the superproject's
+`.git/config.superproject` file, even if run from a submodule of that
+superproject, rather than from all available files.
++
+See also <<FILES>>.
+
 --local::
 	For writing options: write to the repository `.git/config` file.
 	This is the default behavior.
@@ -283,7 +294,7 @@ The default is to use a pager.
 FILES
 -----
 
-If not set explicitly with `--file`, there are four files where
+If not set explicitly with `--file`, there are five files where
 'git config' will search for configuration options:
 
 $(prefix)/etc/gitconfig::
@@ -301,6 +312,12 @@ $XDG_CONFIG_HOME/git/config::
 	User-specific configuration file. Also called "global"
 	configuration file.
 
+$GIT_DIR/config.superproject::
+	When `git config` is run from a project which is a submodule of another
+	project, that superproject's $GIT_DIR will be used. Use this config file
+	to set configurations which need to be the same across a superproject
+	and all its submodules.
+
 $GIT_DIR/config::
 	Repository specific configuration file.
 
diff --git a/builtin/config.c b/builtin/config.c
index f71fa39b38..f0a57a89ca 100644
--- a/builtin/config.c
+++ b/builtin/config.c
@@ -26,7 +26,7 @@ static char key_delim = ' ';
 static char term = '\n';
 
 static int use_global_config, use_system_config, use_local_config;
-static int use_worktree_config;
+static int use_worktree_config, use_superproject_config;
 static struct git_config_source given_config_source;
 static int actions, type;
 static char *default_value;
@@ -130,6 +130,8 @@ static struct option builtin_config_options[] = {
 	OPT_GROUP(N_("Config file location")),
 	OPT_BOOL(0, "global", &use_global_config, N_("use global config file")),
 	OPT_BOOL(0, "system", &use_system_config, N_("use system config file")),
+	OPT_BOOL(0, "superproject",
+		 &use_superproject_config, N_("use superproject config file")),
 	OPT_BOOL(0, "local", &use_local_config, N_("use repository config file")),
 	OPT_BOOL(0, "worktree", &use_worktree_config, N_("use per-worktree config file")),
 	OPT_STRING('f', "file", &given_config_source.file, N_("file"), N_("use given config file")),
@@ -697,6 +699,12 @@ int cmd_config(int argc, const char **argv, const char *prefix)
 	else if (use_system_config) {
 		given_config_source.file = git_etc_gitconfig();
 		given_config_source.scope = CONFIG_SCOPE_SYSTEM;
+	} else if (use_superproject_config) {
+		struct strbuf superproject_cfg = STRBUF_INIT;
+		git_config_superproject(&superproject_cfg, get_git_dir());
+		given_config_source.file = xstrdup(superproject_cfg.buf);
+		given_config_source.scope = CONFIG_SCOPE_SUPERPROJECT;
+		strbuf_release(&superproject_cfg);
 	} else if (use_local_config) {
 		given_config_source.file = git_pathdup("config");
 		given_config_source.scope = CONFIG_SCOPE_LOCAL;
diff --git a/config.c b/config.c
index 67d9bf2238..28bb80fd0d 100644
--- a/config.c
+++ b/config.c
@@ -21,6 +21,7 @@
 #include "dir.h"
 #include "color.h"
 #include "refs.h"
+#include "submodule.h"
 
 struct config_source {
 	struct config_source *prev;
@@ -1852,6 +1853,17 @@ const char *git_etc_gitconfig(void)
 	return system_wide;
 }
 
+void git_config_superproject(struct strbuf *sb, const char *gitdir)
+{
+	if (!get_superproject_gitdir(sb)) {
+		/* not a submodule */
+		strbuf_reset(sb);
+		strbuf_addstr(sb, gitdir);
+	}
+
+	strbuf_addstr(sb, "/config.superproject");
+}
+
 /*
  * Parse environment variable 'k' as a boolean (in various
  * possible spellings); if missing, use the default value 'def'.
@@ -1909,6 +1921,17 @@ static int do_git_config_sequence(const struct config_options *opts,
 	if (user_config && !access_or_die(user_config, R_OK, ACCESS_EACCES_OK))
 		ret += git_config_from_file(fn, user_config, data);
 
+	current_parsing_scope = CONFIG_SCOPE_SUPERPROJECT;
+	if (opts->git_dir && !opts->ignore_superproject) {
+
+		struct strbuf superproject_gitdir = STRBUF_INIT;
+		git_config_superproject(&superproject_gitdir, opts->git_dir);
+		if (!access_or_die(superproject_gitdir.buf, R_OK, 0))
+			ret += git_config_from_file(fn, superproject_gitdir.buf, data);
+
+		strbuf_release(&superproject_gitdir);
+	}
+
 	current_parsing_scope = CONFIG_SCOPE_LOCAL;
 	if (!opts->ignore_repo && repo_config &&
 	    !access_or_die(repo_config, R_OK, 0))
@@ -2027,6 +2050,7 @@ void read_very_early_config(config_fn_t cb, void *data)
 
 	opts.respect_includes = 1;
 	opts.ignore_repo = 1;
+	opts.ignore_superproject = 1;
 	opts.ignore_worktree = 1;
 	opts.ignore_cmdline = 1;
 	opts.system_gently = 1;
@@ -3515,6 +3539,8 @@ const char *config_scope_name(enum config_scope scope)
 		return "command";
 	case CONFIG_SCOPE_GITMODULES:
 		return "gitmodules";
+	case CONFIG_SCOPE_SUPERPROJECT:
+		return "superproject";
 	default:
 		return "unknown";
 	}
diff --git a/config.h b/config.h
index 535f5517b8..b42e1d13eb 100644
--- a/config.h
+++ b/config.h
@@ -43,6 +43,7 @@ enum config_scope {
 	CONFIG_SCOPE_WORKTREE,
 	CONFIG_SCOPE_COMMAND,
 	CONFIG_SCOPE_GITMODULES,
+	CONFIG_SCOPE_SUPERPROJECT,
 };
 const char *config_scope_name(enum config_scope scope);
 
@@ -84,6 +85,7 @@ typedef int (*config_parser_event_fn_t)(enum config_event_t type,
 struct config_options {
 	unsigned int respect_includes : 1;
 	unsigned int ignore_repo : 1;
+	unsigned int ignore_superproject : 1;
 	unsigned int ignore_worktree : 1;
 	unsigned int ignore_cmdline : 1;
 	unsigned int system_gently : 1;
@@ -319,6 +321,7 @@ int git_config_rename_section_in_file(const char *, const char *, const char *);
 int git_config_copy_section(const char *, const char *);
 int git_config_copy_section_in_file(const char *, const char *, const char *);
 const char *git_etc_gitconfig(void);
+void git_config_superproject(struct strbuf *, const char *);
 int git_env_bool(const char *, int);
 unsigned long git_env_ulong(const char *, unsigned long);
 int git_config_system(void);
diff --git a/submodule.c b/submodule.c
index 9767ba9893..92b00f8697 100644
--- a/submodule.c
+++ b/submodule.c
@@ -2178,6 +2178,35 @@ void absorb_git_dir_into_superproject(const char *path,
 	}
 }
 
+int get_superproject_gitdir(struct strbuf *buf)
+{
+	struct strbuf sb = STRBUF_INIT;
+	struct child_process cp = CHILD_PROCESS_INIT;
+	int rc = 0;
+
+	/* NEEDSWORK: this call also calls out to a subprocess! */
+	rc = get_superproject_working_tree(&sb);
+	strbuf_release(&sb);
+
+	if (!rc)
+		return rc;
+
+	prepare_submodule_repo_env_no_git_dir(&cp.env_array);
+
+	strvec_pushl(&cp.args, "-C", "..", "rev-parse", "--absolute-git-dir", NULL);
+	cp.git_cmd = 1;
+
+	rc = capture_command(&cp, buf, 0);
+	strbuf_trim_trailing_newline(buf);
+
+	/* leave buf empty if we didn't have a superproject gitdir */
+	if (rc)
+		strbuf_reset(buf);
+
+	/* rc reflects the exit code of the rev-parse; invert into a bool */
+	return !rc;
+}
+
 int get_superproject_working_tree(struct strbuf *buf)
 {
 	struct child_process cp = CHILD_PROCESS_INIT;
diff --git a/submodule.h b/submodule.h
index 4ac6e31cf1..1308d5ae2d 100644
--- a/submodule.h
+++ b/submodule.h
@@ -149,6 +149,12 @@ void prepare_submodule_repo_env(struct strvec *out);
 void absorb_git_dir_into_superproject(const char *path,
 				      unsigned flags);
 
+/*
+ * Return the gitdir of the superproject, which this project is a submodule of.
+ * If this repository is not a submodule of another repository, return 0.
+ */
+int get_superproject_gitdir(struct strbuf *buf);
+
 /*
  * Return the absolute path of the working tree of the superproject, which this
  * project is a submodule of. If this repository is not a submodule of
diff --git a/t/t1311-superproject-config.sh b/t/t1311-superproject-config.sh
new file mode 100755
index 0000000000..650c4d24c7
--- /dev/null
+++ b/t/t1311-superproject-config.sh
@@ -0,0 +1,124 @@
+#!/bin/sh
+
+test_description='Test git config --superproject in different settings'
+
+. ./test-lib.sh
+
+# follow t7400's example and use the trash dir repo as a submodule to add
+submodurl=$(pwd -P)
+
+# since only the configs are modified, set up the repo structure only once
+test_expect_success 'setup repo structure' '
+	test_commit "base" &&
+	git submodule add "${submodurl}" sub/ &&
+	git commit -m "add a submodule"
+'
+
+test_expect_success 'superproject config applies to super and submodule' '
+	cat >.git/config.superproject <<-EOF &&
+	[foo]
+		bar = baz
+	EOF
+
+	git config --get foo.bar &&
+	git -C sub config --get foo.bar &&
+
+	rm .git/config.superproject
+'
+
+test_expect_success 'can add from super or sub' '
+	git config --superproject apple.species honeycrisp &&
+	git -C sub config --superproject banana.species cavendish &&
+
+	cat >expect <<-EOF &&
+	apple.species=honeycrisp
+	banana.species=cavendish
+	EOF
+
+	git config --list >actual &&
+	grep -Ff expect actual &&
+
+	git -C sub config --list >actual &&
+	grep -Ff expect actual &&
+
+	rm .git/config.superproject
+'
+
+test_expect_success 'can --unset from super or sub' '
+	git config --superproject apple.species honeycrisp &&
+	git -C sub config --superproject banana.species cavendish &&
+
+	git config --unset --superproject banana.species &&
+	git -C sub config --unset --superproject apple.species
+'
+
+test_expect_success 'can --edit superproject config' '
+	test_config core.editor "echo [foo]bar=baz >" &&
+	git config --edit --superproject &&
+
+	git config --get foo.bar &&
+
+	rm .git/config.superproject
+'
+
+test_expect_success 'can --show-origin the superproject config' '
+	git config --superproject --add foo.bar baz &&
+
+	git config --list --show-origin >actual &&
+	grep -F "config.superproject" actual &&
+
+	rm .git/config.superproject
+'
+
+test_expect_success 'can --show-scope the superproject config' '
+	git config --superproject --add foo.bar baz &&
+
+	git config --list --show-scope >actual &&
+	grep "superproject" actual &&
+
+	rm .git/config.superproject
+'
+
+test_expect_success 'pre-existing config applies to new submodule' '
+	git config --superproject --add foo.bar baz &&
+
+	git submodule add "${submodurl}" sub2/ &&
+	git commit -m "add a second submodule" &&
+
+	git -C sub2 config --get foo.bar &&
+
+	# clean up
+	git reset HEAD^ &&
+	rm -fr sub2 &&
+	rm .git/config.superproject
+'
+
+# NEEDSWORK: submodule.c:get_superproject_working_tree doesn't support worktree
+test_expect_failure 'worktrees can still access config.superproject' '
+	git config --superproject --add foo.bar baz &&
+
+	git worktree add wt &&
+	(
+		cd wt &&
+		git config --get foo.bar
+	) &&
+
+	# clean up
+	git worktree remove wt &&
+	rm .git/config.superproject
+'
+
+# This test deletes the submodule! Keep it at the end of the test suite.
+test_expect_success 'config.submodule works even with no submodules' '
+	# get rid of the submodule
+	git reset HEAD^ &&
+	rm -fr sub &&
+
+	git config --superproject --add foo.bar baz &&
+
+	git config --get foo.bar &&
+
+	rm .git/config.superproject
+'
+
+test_done
-- 
2.31.1.295.g9ea45b61b8-goog