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authorLibravatar Michael Haggerty <mhagger@alum.mit.edu>2017-06-12 10:06:13 +0200
committerLibravatar Junio C Hamano <gitster@pobox.com>2017-06-12 10:11:36 -0700
commitfed6ebebf132eb32478a5cbbebba3760c943970a (patch)
tree65dbc52e5e4da2e5b8fa9858e77ee416b62837e0 /refs
parentcache_ref_iterator_begin(): avoid priming unneeded directories (diff)
downloadtgif-fed6ebebf132eb32478a5cbbebba3760c943970a.tar.xz
lock_packed_refs(): fix cache validity check
Commit 28ed9830b1 (get_packed_ref_cache(): assume "packed-refs" won't change while locked, 2017-05-22) assumes that the "packed-refs" file cannot change while we hold the lock. That assumption is justified *if* the lock has been held the whole time since the "packed-refs" file was last read. But in `lock_packed_refs()`, we ourselves lock the "packed-refs" file and then call `get_packed_ref_cache()` to ensure that the cache agrees with the file. The intent is to guard against the possibility that another process changed the "packed-refs" file the moment before we locked it. This check was defeated because `get_packed_ref_cache()` saw that the file was locked, and therefore didn't do the `stat_validity_check()` that we want. The mistake was compounded with a misleading comment in `lock_packed_refs()` claiming that it was doing the right thing. That comment came from an earlier draft of the mh/packed-ref-store-prep patch series when the commits were in a different order. So instead: * Extract a function `validate_packed_ref_cache()` that does the validity check independent of whether the lock is held. * Change `get_packed_ref_cache()` to call the new function, but only if the lock *isn't* held. * Change `lock_packed_refs()` to call the new function in any case before calling `get_packed_ref_cache()`. * Fix the comment in `lock_packed_refs()`. Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu> Signed-off-by: Junio C Hamano <gitster@pobox.com>
Diffstat (limited to 'refs')
-rw-r--r--refs/files-backend.c32
1 files changed, 23 insertions, 9 deletions
diff --git a/refs/files-backend.c b/refs/files-backend.c
index d8b3f73147..b040bb3b0a 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -370,6 +370,18 @@ static void files_ref_path(struct files_ref_store *refs,
}
/*
+ * Check that the packed refs cache (if any) still reflects the
+ * contents of the file. If not, clear the cache.
+ */
+static void validate_packed_ref_cache(struct files_ref_store *refs)
+{
+ if (refs->packed &&
+ !stat_validity_check(&refs->packed->validity,
+ files_packed_refs_path(refs)))
+ clear_packed_ref_cache(refs);
+}
+
+/*
* Get the packed_ref_cache for the specified files_ref_store,
* creating and populating it if it hasn't been read before or if the
* file has been changed (according to its `validity` field) since it
@@ -381,10 +393,8 @@ static struct packed_ref_cache *get_packed_ref_cache(struct files_ref_store *ref
{
const char *packed_refs_file = files_packed_refs_path(refs);
- if (refs->packed &&
- !is_lock_file_locked(&refs->packed_refs_lock) &&
- !stat_validity_check(&refs->packed->validity, packed_refs_file))
- clear_packed_ref_cache(refs);
+ if (!is_lock_file_locked(&refs->packed_refs_lock))
+ validate_packed_ref_cache(refs);
if (!refs->packed)
refs->packed = read_packed_refs(packed_refs_file);
@@ -1311,13 +1321,17 @@ static int lock_packed_refs(struct files_ref_store *refs, int flags)
&refs->packed_refs_lock, files_packed_refs_path(refs),
flags, timeout_value) < 0)
return -1;
+
/*
- * Get the current packed-refs while holding the lock. It is
- * important that we call `get_packed_ref_cache()` before
- * setting `packed_ref_cache->lock`, because otherwise the
- * former will see that the file is locked and assume that the
- * cache can't be stale.
+ * Now that we hold the `packed-refs` lock, make sure that our
+ * cache matches the current version of the file. Normally
+ * `get_packed_ref_cache()` does that for us, but that
+ * function assumes that when the file is locked, any existing
+ * cache is still valid. We've just locked the file, but it
+ * might have changed the moment *before* we locked it.
*/
+ validate_packed_ref_cache(refs);
+
packed_ref_cache = get_packed_ref_cache(refs);
/* Increment the reference count to prevent it from being freed: */
acquire_packed_ref_cache(packed_ref_cache);