Michael Haggerty 6e30b2f652 lock_ref_for_update(): don't resolve symrefs ... If a transaction includes a non-NODEREF update to a symbolic reference, we don't have to look it up in lock_ref_for_update(). The reference will be dereferenced anyway when the split-off update is processed. This change requires that we store a backpointer from the split-off update to its parent update, for two reasons: * We still want to report the original reference name in error messages. So if an error occurs when checking the split-off update's old_sha1, walk the parent_update pointers back to find the original reference name, and report that one. * We still need to write the old_sha1 of the symref to its reflog. So after we read the split-off update's reference value, walk the parent_update pointers back and fill in their old_sha1 fields. Aside from eliminating unnecessary reads, this change fixes a subtle (though not very serious) race condition: in the old code, the old_sha1 of the symref was resolved before the reference that it pointed at was locked. So it was possible that the old_sha1 value logged to the symref's reflog could be wrong if another process changed the downstream reference before it was locked. Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>		2016-06-13 11:23:50 +02:00
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files-backend.c	lock_ref_for_update(): don't resolve symrefs	2016-06-13 11:23:50 +02:00
refs-internal.h	lock_ref_for_update(): don't resolve symrefs	2016-06-13 11:23:50 +02:00