From fed6ebebf132eb32478a5cbbebba3760c943970a Mon Sep 17 00:00:00 2001 From: Michael Haggerty Date: Mon, 12 Jun 2017 10:06:13 +0200 Subject: 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 Signed-off-by: Junio C Hamano --- refs/files-backend.c | 32 +++++++++++++++++++++++--------- 1 file changed, 23 insertions(+), 9 deletions(-) (limited to 'refs') 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 @@ -369,6 +369,18 @@ 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 @@ -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); -- cgit v1.2.3