Merge branch 'scalar-gentler-config-locking'

Allow concurrent `scalar register` and `scalar unregister` calls to be
more collaborative when trying to lock the global Git config at the very
same time.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

Documentation/config/core.adoc

@@ -795,3 +795,12 @@ core.WSLCompat::
 	The default value is false. When set to true, Git will set the mode
 	bits of the file in the way of wsl, so that the executable flag of
 	files can be set or read correctly.
+
+core.configWriteLockTimeoutMS::
+	When processes try to write to the config concurrently, it is likely
+	that one process "wins" and the other process(es) fail to lock the
+	config file. By configuring a timeout larger than zero, Git can be
+	told to try to lock the config again a couple times within the
+	specified timeout. If the timeout is configure to zero (which is the
+	default), Git will fail immediately when the config is already
+	locked.

config.c

@@ -2953,6 +2953,7 @@ int repo_config_set_multivar_in_file_gently(struct repository *r,
 					    const char *comment,
 					    unsigned flags)
 {
+	static unsigned long timeout_ms = ULONG_MAX;
 	int fd = -1, in_fd = -1;
 	int ret;
 	struct lock_file lock = LOCK_INIT;
@@ -2981,11 +2982,16 @@ int repo_config_set_multivar_in_file_gently(struct repository *r,
 	if (!config_filename)
 		config_filename = filename_buf = repo_git_path(r, "config");
 
+	if ((long)timeout_ms < 0 &&
+	    repo_config_get_ulong(r, "core.configWriteLockTimeoutMS", &timeout_ms))
+		timeout_ms = 0;
+
 	/*
 	 * The lock serves a purpose in addition to locking: the new
 	 * contents of .git/config will be written into it.
 	 */
-	fd = hold_lock_file_for_update(&lock, config_filename, 0);
+	fd = hold_lock_file_for_update_timeout(&lock, config_filename, 0,
+					       timeout_ms);
 	if (fd < 0) {
 		error_errno(_("could not lock config file %s"), config_filename);
 		ret = CONFIG_NO_LOCK;

scalar.c

@@ -172,6 +172,7 @@ static int set_recommended_config(int reconfigure)
 		{ "core.safeCRLF", "false" },
 		{ "fetch.showForcedUpdates", "false" },
 		{ "pack.usePathWalk", "true" },
+		{ "core.configWriteLockTimeoutMS", "150" },
 		{ NULL, NULL },
 	};
 	int i;
@@ -219,6 +220,11 @@ static int set_recommended_config(int reconfigure)
  */
 static int toggle_maintenance(int enable)
 {
+	unsigned long ul;
+
+	if (repo_config_get_ulong(the_repository, "core.configWriteLockTimeoutMS", &ul))
+		git_config_push_parameter("core.configWriteLockTimeoutMS=150");
+
 	return run_git("maintenance",
 		       enable ? "start" : "unregister",
 		       enable ? NULL : "--force",
@@ -228,10 +234,14 @@ static int toggle_maintenance(int enable)
 static int add_or_remove_enlistment(int add)
 {
 	int res;
+	unsigned long ul;
 
 	if (!the_repository->worktree)
 		die(_("Scalar enlistments require a worktree"));
 
+	if (repo_config_get_ulong(the_repository, "core.configWriteLockTimeoutMS", &ul))
+		git_config_push_parameter("core.configWriteLockTimeoutMS=150");
+
 	res = run_git("config", "--global", "--get", "--fixed-value",
 		      "scalar.repo", the_repository->worktree, NULL);