git_config_set_multivar_in_file_gently(): add a lock timeout

In particular when multiple processes want to write to the config
simultaneously, it would come in handy to not fail immediately when
another process locked the config, but to gently try again.

This will help with Scalar's functional test suite which wants to
register multiple repositories for maintenance semi-simultaneously.

As not all code paths calling this function read the config (e.g. `git
config`), we have to read the config setting via
`git_config_get_ulong()`.

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

Documentation/config/core.txt

@@ -755,3 +755,12 @@ 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

@@ -3374,6 +3374,7 @@ int git_config_set_multivar_in_file_gently(const char *config_filename,
 					   const char *value_pattern,
 					   unsigned flags)
 {
+	static unsigned long timeout_ms = ULONG_MAX;
 	int fd = -1, in_fd = -1;
 	int ret;
 	struct lock_file lock = LOCK_INIT;
@@ -3394,11 +3395,16 @@ int git_config_set_multivar_in_file_gently(const char *config_filename,
 	if (!config_filename)
 		config_filename = filename_buf = git_pathdup("config");
 
+	if ((long)timeout_ms < 0 &&
+	    git_config_get_ulong("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;