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[PATCH v5 25/30] walker.c: use ref transaction for ref updates

From
Ronnie Sahlberg <sahlberg@google.com>
Date
Apr 29, 2014, 22:19 UTC
Message-ID
<1398809955-32008-26-git-send-email-sahlberg@google.com>
In-Reply-To
<1398809955-32008-1-git-send-email-sahlberg@google.com>

Switch to using ref transactions in walker_fetch(). As part of the refactoring to use ref transactions we also fix a potential memory leak where in the original code if write_ref_sha1() would fail we would end up returning from the function without free()ing the msg string.

This changes the locking slightly for walker_fetch. Previously the code would lock all refs before writing them but now we do not lock the refs until the commit stage. There is thus a very short window where changes could be done locally during the fetch which would be overwritten when the fetch completes and commits its transaction. But this window should be reasonably short. Even if this race does trigger, since both the old code and the new code just overwrites the refs to the new values without checking or comparing them with the previous value, this is not too dissimilar to a similar scenario where you first do a ref change locally and then later do a fetch that overwrites the local change. With this in mind I do not see the change in locking semantics to be critical.

Note that this function is only called when fetching from a remote HTTP repository onto the local (most of the time single-user) repository which likely means that the type of collissions that the previous locking would protect against and cause the fetch to fail for to be even more rare.

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
---
 walker.c | 51 ++++++++++++++++++++++++++-------------------------
 1 file changed, 26 insertions(+), 25 deletions(-)
diff --git a/walker.c b/walker.c
index 1dd86b8..6044ccf 100644
--- a/walker.c
+++ b/walker.c
@@ -251,24 +251,18 @@ void walker_targets_free(int targets, char **target, const char **write_ref)
 int walker_fetch(struct walker *walker, int targets, char **target,
 		 const char **write_ref, const char *write_ref_log_details)
 {
-	struct ref_lock **lock = xcalloc(targets, sizeof(struct ref_lock *));
+	char ref_name[PATH_MAX];
+	struct strbuf err = STRBUF_INIT;
+	struct ref_transaction *transaction;
 	unsigned char *sha1 = xmalloc(targets * 20);
-	char *msg;
-	int ret;
+	char *msg = NULL;
 	int i;
 
 	save_commit_buffer = 0;
 
-	for (i = 0; i < targets; i++) {
-		if (!write_ref || !write_ref[i])
-			continue;
-
-		lock[i] = lock_ref_sha1(write_ref[i], NULL);
-		if (!lock[i]) {
-			error("Can't lock ref %s", write_ref[i]);
-			goto unlock_and_fail;
-		}
-	}
+	transaction = ref_transaction_begin();
+	if (!transaction)
+		return -1;
 
 	if (!walker->get_recover)
 		for_each_ref(mark_complete, NULL);
@@ -276,14 +270,14 @@ int walker_fetch(struct walker *walker, int targets, char **target,
 	for (i = 0; i < targets; i++) {
 		if (interpret_target(walker, target[i], &sha1[20 * i])) {
 			error("Could not interpret response from server '%s' as something to pull", target[i]);
-			goto unlock_and_fail;
+			goto rollback_and_fail;
 		}
 		if (process(walker, lookup_unknown_object(&sha1[20 * i])))
-			goto unlock_and_fail;
+			goto rollback_and_fail;
 	}
 
 	if (loop(walker))
-		goto unlock_and_fail;
+		goto rollback_and_fail;
 
 	if (write_ref_log_details) {
 		msg = xmalloc(strlen(write_ref_log_details) + 12);
@@ -294,19 +288,26 @@ int walker_fetch(struct walker *walker, int targets, char **target,
 	for (i = 0; i < targets; i++) {
 		if (!write_ref || !write_ref[i])
 			continue;
-		ret = write_ref_sha1(lock[i], &sha1[20 * i], msg ? msg : "fetch (unknown)");
-		lock[i] = NULL;
-		if (ret)
-			goto unlock_and_fail;
+		sprintf(ref_name, "refs/%s", write_ref[i]);
+		if (ref_transaction_update(transaction, ref_name,
+					   &sha1[20 * i], NULL,
+					   0, 0))
+			goto rollback_and_fail;
+	}
+
+	if (ref_transaction_commit(transaction, msg ? msg : "fetch (unknown)",
+				   &err)) {
+		error("%s", err.buf);
+		goto rollback_and_fail;
 	}
-	free(msg);
 
+	free(msg);
 	return 0;
 
-unlock_and_fail:
-	for (i = 0; i < targets; i++)
-		if (lock[i])
-			unlock_ref(lock[i]);
+rollback_and_fail:
+	free(msg);
+	strbuf_release(&err);
+	ref_transaction_free(transaction);
 
 	return -1;
 }

-- 
1.9.1.532.gf8485a6
Previous: Ronnie SahlbergNext: Ronnie Sahlberg
Message 26 of 31 in “Use ref transactions for all ref updates”
  1. 00/30 Use ref transactions for all ref updatesRonnie Sahlberg, Apr 29, 2014
  2. 01/30 refs.c: constify the sha arguments for ref_transaction_create|delete|updateRonnie Sahlberg, Apr 29, 2014
  3. 02/30 refs.c: allow passing NULL to ref_transaction_freeRonnie Sahlberg, Apr 29, 2014
  4. 03/30 refs.c: add a strbuf argument to ref_transaction_commit for error loggingRonnie Sahlberg, Apr 29, 2014
  5. 04/30 refs.c: make ref_update_reject_duplicates take a strbuf argument for errorsRonnie Sahlberg, Apr 29, 2014
  6. 05/30 update-ref.c: log transaction error from the update_refRonnie Sahlberg, Apr 29, 2014
  7. 06/30 refs.c: make update_ref_write update a strbuf on failureRonnie Sahlberg, Apr 29, 2014
  8. 07/30 refs.c: remove the onerr argument to ref_transaction_commitRonnie Sahlberg, Apr 29, 2014
  9. 08/30 refs.c: change ref_transaction_update() to do error checking and return statusRonnie Sahlberg, Apr 29, 2014
  10. 09/30 refs.c: change ref_transaction_create to do error checking and return statusRonnie Sahlberg, Apr 29, 2014
  11. 10/30 refs.c: ref_transaction_delete to check for error and return statusRonnie Sahlberg, Apr 29, 2014
  12. 11/30 tag.c: use ref transactions when doing updatesRonnie Sahlberg, Apr 29, 2014
  13. 12/30 replace.c: use the ref transaction functions for updatesRonnie Sahlberg, Apr 29, 2014
  14. 13/30 commit.c: use ref transactions for updatesRonnie Sahlberg, Apr 29, 2014
  15. 14/30 sequencer.c: use ref transactions for all ref updatesRonnie Sahlberg, Apr 29, 2014
  16. 15/30 fast-import.c: change update_branch to use ref transactionsRonnie Sahlberg, Apr 29, 2014
  17. 16/30 branch.c: use ref transaction for all ref updatesRonnie Sahlberg, Apr 29, 2014
  18. 17/30 refs.c: change update_ref to use a transactionRonnie Sahlberg, Apr 29, 2014
  19. 18/30 refs.c: free the transaction before returning when number of updates is 0Ronnie Sahlberg, Apr 29, 2014
  20. 19/30 refs.c: ref_transaction_commit should not free the transactionRonnie Sahlberg, Apr 29, 2014
  21. 20/30 fetch.c: clear errno before calling functions that might set itRonnie Sahlberg, Apr 29, 2014
  22. 21/30 fetch.c: change s_update_ref to use a ref transactionRonnie Sahlberg, Apr 29, 2014
  23. 22/30 fetch.c: use a single ref transaction for all ref updatesRonnie Sahlberg, Apr 29, 2014
  24. 23/30 receive-pack.c: use a reference transaction for updating the refsRonnie Sahlberg, Apr 29, 2014
  25. 24/30 fast-import.c: use a ref transaction when dumping tagsRonnie Sahlberg, Apr 29, 2014
  26. 25/30 walker.c: use ref transaction for ref updatesRonnie Sahlberg, Apr 29, 2014
  27. 26/30 refs.c: make write_ref_sha1 staticRonnie Sahlberg, Apr 29, 2014
  28. 27/30 refs.c: make lock_ref_sha1 staticRonnie Sahlberg, Apr 29, 2014
  29. 28/30 refs.c: add transaction.status and track OPEN/CLOSED/ERRORRonnie Sahlberg, Apr 29, 2014
  30. 29/30 refs.c: remove the update_ref_lock functionRonnie Sahlberg, Apr 29, 2014
  31. 30/30 refs.c: remove the update_ref_write functionRonnie Sahlberg, Apr 29, 2014

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