refs: resolve symbolic refs first

Before committing ref updates, split symbolic ref updates into two
parts: an update to the underlying ref, and a log-only update to the
symbolic ref. This ensures that both references are locked correctly
during the transaction, including while their reflogs are updated.

Similarly, if the reference pointed to by HEAD is modified directly, add
a separate log-only update to HEAD, rather than leaving the job of
updating HEAD's reflog to commit_ref_update(). This change ensures that
HEAD is locked correctly while its reflog is being modified, as well as
being cheaper (HEAD only needs to be resolved once).

This makes use of a new function, lock_raw_ref(), which is analogous to
read_raw_ref(), but acquires a lock on the reference before reading it.

This change still has two problems:

* There are redundant read_ref_full() reference lookups.

* It is still possible to get incorrect reflogs for symbolic references
  if there is a concurrent update by another process, since the old_oid
  of a symref is determined before the lock on the pointed-to ref is
  held.

Both problems will soon be fixed.

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

WIP

1 files changed, 9 insertions, 2 deletions

diff --git a/refs/refs-internal.h b/refs/refs-internal.h
index babdf2769f..cccd76b28c 100644
--- a/refs/refs-internal.h
+++ b/refs/refs-internal.h
@@ -50,6 +50,12 @@
 #define REF_LOG_ONLY 0x80
 
 /*
+ * Internal flag, meaning that the containing ref_update was via an
+ * update to HEAD.
+ */
+#define REF_UPDATE_VIA_HEAD 0x100
+
+/*
  * Return true iff refname is minimally safe. "Safe" here means that
  * deleting a loose reference by this name will not do any damage, for
  * example by causing a file that is not a reference to be deleted.
@@ -148,11 +154,12 @@ struct ref_update {
 	unsigned char old_sha1[20];
 	/*
 	 * One or more of REF_HAVE_NEW, REF_HAVE_OLD, REF_NODEREF,
-	 * REF_DELETING, and REF_ISPRUNING:
+	 * REF_DELETING, REF_ISPRUNING, REF_LOG_ONLY, and
+	 * REF_UPDATE_VIA_HEAD:
 	 */
 	unsigned int flags;
 	struct ref_lock *lock;
-	int type;
+	unsigned int type;
 	char *msg;
 	const char refname[FLEX_ARRAY];
 };