hooks: add custom post-command hook config

Documentation/config.adoc

@@ -490,6 +490,8 @@ include::config/pack.adoc[]
 
 include::config/pager.adoc[]
 
+include::config/postcommand.adoc[]
+
 include::config/pretty.adoc[]
 
 include::config/promisor.adoc[]

Documentation/config/postcommand.adoc

@@ -0,0 +1,13 @@
+postCommand.strategy::
+	The `post-command` hook is run on every Git process by default. This
+	config option allows running the hook only conditionally, according
+	to these values:
++
+----
+`always`;;
+	run the `post-command` hook on every process (default).
+
+`post-index-change`;;
+	run the `post-command` hook only if the current process wrote to
+	the index.
+----

hook.c

@@ -1,6 +1,7 @@
 #define USE_THE_REPOSITORY_VARIABLE
 
 #include "git-compat-util.h"
+#include "trace2/tr2_sid.h"
 #include "abspath.h"
 #include "environment.h"
 #include "advice.h"
@@ -176,6 +177,58 @@ static void run_hooks_opt_clear(struct run_hooks_opt *options)
 	strvec_clear(&options->args);
 }
 
+static char *get_post_index_change_sentinel_name(struct repository *r)
+{
+	struct strbuf path = STRBUF_INIT;
+	const char *sid = tr2_sid_get();
+	char *slash = strchr(sid, '/');
+
+	/*
+	 * Name is based on top-level SID, so children can indicate that
+	 * the top-level process should run the post-command hook.
+	 */
+	if (slash)
+		*slash = 0;
+
+	repo_git_path_replace(r, &path, "hooks/index-change-%s.snt", sid);
+
+	return strbuf_detach(&path, NULL);
+}
+
+static int write_post_index_change_sentinel(struct repository *r)
+{
+	char *path = get_post_index_change_sentinel_name(r);
+	FILE *fp = xfopen(path, "w");
+
+	if (fp) {
+		fprintf(fp, "run post-command hook");
+		fclose(fp);
+	}
+
+	free(path);
+	return fp ? 0 : -1;
+}
+
+/**
+ * Try to delete the sentinel file for this repository. If that succeeds, then
+ * return 1.
+ */
+static int post_index_change_sentinel_exists(struct repository *r)
+{
+	char *path = get_post_index_change_sentinel_name(r);
+	int res = 1;
+
+	if (unlink(path)) {
+		if (is_missing_file_error(errno))
+			res = 0;
+		else
+			warning_errno("failed to remove index-change sentinel file '%s'", path);
+	}
+
+	free(path);
+	return res;
+}
+
 int run_hooks_opt(struct repository *r, const char *hook_name,
 		  struct run_hooks_opt *options)
 {
@@ -185,7 +238,7 @@ int run_hooks_opt(struct repository *r, const char *hook_name,
 		.hook_name = hook_name,
 		.options = options,
 	};
-	const char *hook_path = find_hook(r, hook_name);
+	const char *hook_path;
 	int ret = 0;
 	const struct run_process_parallel_opts opts = {
 		.tr2_category = "hook",
@@ -201,6 +254,21 @@ int run_hooks_opt(struct repository *r, const char *hook_name,
 		.data = &cb_data,
 	};
 
+	/* Interject hook behavior depending on strategy. */
+	if (r && r->gitdir) {
+		const char *strval;
+		if (!repo_config_get_string_tmp(r, "postcommand.strategy", &strval) &&
+		    !strcasecmp(strval, "post-index-change")) {
+			if (!strcmp(hook_name, "post-index-change"))
+				return write_post_index_change_sentinel(r);
+			if (!strcmp(hook_name, "post-command") &&
+			    !post_index_change_sentinel_exists(r))
+				return 0;
+		}
+	}
+
+	hook_path = find_hook(r, hook_name);
+
 	/*
 	 * Backwards compatibility hack in VFS for Git: when originally
 	 * introduced (and used!), it was called `post-indexchanged`, but this

t/t0401-post-command-hook.sh

@@ -22,6 +22,7 @@ test_expect_success 'with succeeding hook' '
 '
 
 test_expect_success 'with failing pre-command hook' '
+	test_when_finished rm -f .git/hooks/pre-command &&
 	write_script .git/hooks/pre-command <<-EOF &&
 	exit 1
 	EOF
@@ -30,4 +31,45 @@ test_expect_success 'with failing pre-command hook' '
 	test_path_is_missing "$(cat .git/post-command.out)"
 '
 
+test_expect_success 'with post-index-change config' '
+	mkdir -p .git/hooks &&
+	write_script .git/hooks/post-command <<-EOF &&
+	echo ran >post-command.out
+	EOF
+	write_script .git/hooks/post-index-change <<-EOF &&
+	echo ran >post-index-change.out
+	EOF
+
+	# First, show expected behavior.
+	echo ran >expect &&
+	rm -f post-command.out post-index-change.out &&
+
+	# rev-parse leaves index intact, but runs post-command.
+	git rev-parse HEAD &&
+	test_path_is_missing post-index-change.out &&
+	test_cmp expect post-command.out &&
+	rm -f post-command.out &&
+
+	echo stuff >>file &&
+	# add updates the index and runs post-command.
+	git add file &&
+	test_cmp expect post-index-change.out &&
+	test_cmp expect post-command.out &&
+
+	# Now, show configured behavior
+	git config postCommand.strategy post-index-change &&
+	rm -f post-command.out post-index-change.out &&
+
+	# rev-parse leaves index intact and thus skips post-command.
+	git rev-parse HEAD &&
+	test_path_is_missing post-index-change.out &&
+	test_path_is_missing post-command.out &&
+
+	echo stuff >>file &&
+	# add updates the index and runs post-command.
+	git add file &&
+	test_path_is_missing post-index-change.out &&
+	test_cmp expect post-command.out
+'
+
 test_done