refs: retry acquiring reference locks for 100ms

The philosophy of reference locking has been, "if another process is
changing a reference, then whatever I'm trying to do to it will
probably fail anyway because my old-SHA-1 value is probably no longer
current". But this argument falls down if the other process has locked
the reference to do something that doesn't actually change the value
of the reference, such as `pack-refs` or `reflog expire`. There
actually *is* a decent chance that a planned reference update will
still be able to go through after the other process has released the
lock.

So when trying to lock an individual reference (e.g., when creating
"refs/heads/master.lock"), if it is already locked, then retry the
lock acquisition for approximately 100 ms before giving up. This
should eliminate some unnecessary lock conflicts without wasting a lot
of time.

Add a configuration setting, `core.filesRefLockTimeout`, to allow this
setting to be tweaked.

Note: the function `get_files_ref_lock_timeout_ms()` cannot be private
to the files backend because it is also used by `write_pseudoref()`
and `delete_pseudoref()`, which are defined in `refs.c` so that they
can be used by other reference backends.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 21 insertions, 3 deletions

diff --git a/refs.c b/refs.c
index ba22f4ace..748e34d7c 100644
--- a/refs.c
+++ b/refs.c
@@ -561,6 +561,21 @@ enum ref_type ref_type(const char *refname)
        return REF_TYPE_NORMAL;
 }
 
+long get_files_ref_lock_timeout_ms(void)
+{
+	static int configured = 0;
+
+	/* The default timeout is 100 ms: */
+	static int timeout_ms = 100;
+
+	if (!configured) {
+		git_config_get_int("core.filesreflocktimeout", &timeout_ms);
+		configured = 1;
+	}
+
+	return timeout_ms;
+}
+
 static int write_pseudoref(const char *pseudoref, const unsigned char *sha1,
 			   const unsigned char *old_sha1, struct strbuf *err)
 {
@@ -573,7 +588,9 @@ static int write_pseudoref(const char *pseudoref, const unsigned char *sha1,
 	strbuf_addf(&buf, "%s\n", sha1_to_hex(sha1));
 
 	filename = git_path("%s", pseudoref);
-	fd = hold_lock_file_for_update(&lock, filename, LOCK_DIE_ON_ERROR);
+	fd = hold_lock_file_for_update_timeout(&lock, filename,
+					       LOCK_DIE_ON_ERROR,
+					       get_files_ref_lock_timeout_ms());
 	if (fd < 0) {
 		strbuf_addf(err, "could not open '%s' for writing: %s",
 			    filename, strerror(errno));
@@ -616,8 +633,9 @@ static int delete_pseudoref(const char *pseudoref, const unsigned char *old_sha1
 		int fd;
 		unsigned char actual_old_sha1[20];
 
-		fd = hold_lock_file_for_update(&lock, filename,
-					       LOCK_DIE_ON_ERROR);
+		fd = hold_lock_file_for_update_timeout(
+				&lock, filename, LOCK_DIE_ON_ERROR,
+				get_files_ref_lock_timeout_ms());
 		if (fd < 0)
 			die_errno(_("Could not open '%s' for writing"), filename);
 		if (read_ref(pseudoref, actual_old_sha1))