ref_transaction_commit(): fix atomicity and avoid fd exhaustion

The old code was roughly

    for update in updates:
        acquire locks and check old_sha
    for update in updates:
        if changing value:
            write_ref_to_lockfile()
            commit_ref_update()
    for update in updates:
        if deleting value:
            unlink()
    rewrite packed-refs file
    for update in updates:
        if reference still locked:
            unlock_ref()

This has two problems.

Non-atomic updates
==================

The atomicity of the reference transaction depends on all pre-checks
being done in the first loop, before any changes have started being
committed in the second loop. The problem is that
write_ref_to_lockfile() (previously part of write_ref_sha1()), which
is called from the second loop, contains two more checks:

* It verifies that new_sha1 is a valid object

* If the reference being updated is a branch, it verifies that
  new_sha1 points at a commit object (as opposed to a tag, tree, or
  blob).

If either of these checks fails, the "transaction" is aborted during
the second loop. But this might happen after some reference updates
have already been permanently committed. In other words, the
all-or-nothing promise of "git update-ref --stdin" could be violated.

So these checks have to be moved to the first loop.

File descriptor exhaustion
==========================

The old code locked all of the references in the first loop, leaving
all of the lockfiles open until later loops. Since we might be
updating a lot of references, this could result in file descriptor
exhaustion.

The solution
============

After this patch, the code looks like

    for update in updates:
        acquire locks and check old_sha
        if changing value:
            write_ref_to_lockfile()
        else:
            close_ref()
    for update in updates:
        if changing value:
            commit_ref_update()
    for update in updates:
        if deleting value:
            unlink()
    rewrite packed-refs file
    for update in updates:
        if reference still locked:
            unlock_ref()

This fixes both problems:

1. The pre-checks in write_ref_to_lockfile() are now done in the first
   loop, before any changes have been committed. If any of the checks
   fails, the whole transaction can now be rolled back correctly.

2. All lockfiles are closed in the first loop immediately after they
   are created (either by write_ref_to_lockfile() or by close_ref()).
   This means that there is never more than one open lockfile at a
   time, preventing file descriptor exhaustion.

To simplify the bookkeeping across loops, add a new REF_NEEDS_COMMIT
bit to update->flags, which keeps track of whether the corresponding
lockfile needs to be committed, as opposed to just unlocked. (Since
"struct ref_update" is internal to the refs module, this change is not
visible to external callers.)

This change fixes two tests in t1400.

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

1 files changed, 53 insertions, 21 deletions

diff --git a/refs.c b/refs.c
index 8dad0df..96f5046 100644
--- a/refs.c
+++ b/refs.c
@@ -58,6 +58,12 @@ static unsigned char refname_disposition[256] = {
 #define REF_HAVE_OLD	0x10
 
 /*
+ * Used as a flag in ref_update::flags when the lockfile needs to be
+ * committed.
+ */
+#define REF_NEEDS_COMMIT 0x20
+
+/*
  * Try to read one refname component from the front of refname.
  * Return the length of the component found, or -1 if the component is
  * not legal.  It is legal if it is something reasonable to have under
@@ -3760,7 +3766,12 @@ int ref_transaction_commit(struct ref_transaction *transaction,
 		goto cleanup;
 	}
 
-	/* Acquire all locks while verifying old values */
+	/*
+	 * Acquire all locks, verify old values if provided, check
+	 * that new values are valid, and write new values to the
+	 * lockfiles, ready to be activated. Only keep one lockfile
+	 * open at a time to avoid running out of file descriptors.
+	 */
 	for (i = 0; i < n; i++) {
 		struct ref_update *update = updates[i];
 
@@ -3782,38 +3793,60 @@ int ref_transaction_commit(struct ref_transaction *transaction,
 				    update->refname);
 			goto cleanup;
 		}
-	}
-
-	/* Perform updates first so live commits remain referenced */
-	for (i = 0; i < n; i++) {
-		struct ref_update *update = updates[i];
-
-		if ((update->flags & REF_HAVE_NEW)
-		    && !is_null_sha1(update->new_sha1)) {
+		if ((update->flags & REF_HAVE_NEW) &&
+		    !(update->flags & REF_DELETING)) {
 			int overwriting_symref = ((update->type & REF_ISSYMREF) &&
 						  (update->flags & REF_NODEREF));
 
-			if (!overwriting_symref
-			    && !hashcmp(update->lock->old_sha1, update->new_sha1)) {
+			if (!overwriting_symref &&
+			    !hashcmp(update->lock->old_sha1, update->new_sha1)) {
 				/*
 				 * The reference already has the desired
 				 * value, so we don't need to write it.
 				 */
-				unlock_ref(update->lock);
-				update->lock = NULL;
 			} else if (write_ref_to_lockfile(update->lock,
-							 update->new_sha1) ||
-				   commit_ref_update(update->lock,
-						     update->new_sha1,
-						     update->msg)) {
-				/* freed by one of the above calls: */
+							 update->new_sha1)) {
+				/*
+				 * The lock was freed upon failure of
+				 * write_ref_to_lockfile():
+				 */
+				update->lock = NULL;
+				strbuf_addf(err, "Cannot update the ref '%s'.",
+					    update->refname);
+				ret = TRANSACTION_GENERIC_ERROR;
+				goto cleanup;
+			} else {
+				update->flags |= REF_NEEDS_COMMIT;
+			}
+		}
+		if (!(update->flags & REF_NEEDS_COMMIT)) {
+			/*
+			 * We didn't have to write anything to the lockfile.
+			 * Close it to free up the file descriptor:
+			 */
+			if (close_ref(update->lock)) {
+				strbuf_addf(err, "Couldn't close %s.lock",
+					    update->refname);
+				goto cleanup;
+			}
+		}
+	}
+
+	/* Perform updates first so live commits remain referenced */
+	for (i = 0; i < n; i++) {
+		struct ref_update *update = updates[i];
+
+		if (update->flags & REF_NEEDS_COMMIT) {
+			if (commit_ref_update(update->lock,
+					      update->new_sha1, update->msg)) {
+				/* freed by commit_ref_update(): */
 				update->lock = NULL;
 				strbuf_addf(err, "Cannot update the ref '%s'.",
 					    update->refname);
 				ret = TRANSACTION_GENERIC_ERROR;
 				goto cleanup;
 			} else {
-				/* freed by write_ref_sha1(): */
+				/* freed by commit_ref_update(): */
 				update->lock = NULL;
 			}
 		}
@@ -3823,8 +3856,7 @@ int ref_transaction_commit(struct ref_transaction *transaction,
 	for (i = 0; i < n; i++) {
 		struct ref_update *update = updates[i];
 
-		if ((update->flags & REF_HAVE_NEW)
-		    && is_null_sha1(update->new_sha1)) {
+		if (update->flags & REF_DELETING) {
 			if (delete_ref_loose(update->lock, update->type, err)) {
 				ret = TRANSACTION_GENERIC_ERROR;
 				goto cleanup;