@matheusbsbs / matheus-bagliano / commits / fed6ebebf1

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>

Michael Haggerty committed Jun 12, 2017 at 10:06 UTC fed6ebebf132eb32478a5cbbebba3760c943970a

1 file changed +23 -9

refs/files-backend.c

+23 -9

@@ -369,6 +369,18 @@ static void files_ref_path(struct files_ref_store *refs,

369 }

370 }

371

372 +/*

373 + * Check that the packed refs cache (if any) still reflects the

374 + * contents of the file. If not, clear the cache.

375 + */

376 +static void validate_packed_ref_cache(struct files_ref_store *refs)

377 +{

378 + if (refs->packed &&

379 + !stat_validity_check(&refs->packed->validity,

380 + files_packed_refs_path(refs)))

381 + clear_packed_ref_cache(refs);

382 +}

383 +

384 /*

385 * Get the packed_ref_cache for the specified files_ref_store,

386 * 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

393 {

394 const char *packed_refs_file = files_packed_refs_path(refs);

395

384 - if (refs->packed &&

385 - !is_lock_file_locked(&refs->packed_refs_lock) &&

386 - !stat_validity_check(&refs->packed->validity, packed_refs_file))

387 - clear_packed_ref_cache(refs);

396 + if (!is_lock_file_locked(&refs->packed_refs_lock))

397 + validate_packed_ref_cache(refs);

398

399 if (!refs->packed)

400 refs->packed = read_packed_refs(packed_refs_file);

@@ -1311,13 +1321,17 @@ static int lock_packed_refs(struct files_ref_store *refs, int flags)

1321 &refs->packed_refs_lock, files_packed_refs_path(refs),

1322 flags, timeout_value) < 0)

1323 return -1;

1324 +

1325 /*

1315 - * Get the current packed-refs while holding the lock. It is

1316 - * important that we call `get_packed_ref_cache()` before

1317 - * setting `packed_ref_cache->lock`, because otherwise the

1318 - * former will see that the file is locked and assume that the

1319 - * cache can't be stale.

1326 + * Now that we hold the `packed-refs` lock, make sure that our

1327 + * cache matches the current version of the file. Normally

1328 + * `get_packed_ref_cache()` does that for us, but that

1329 + * function assumes that when the file is locked, any existing

1330 + * cache is still valid. We've just locked the file, but it

1331 + * might have changed the moment *before* we locked it.

1332 */

1333 + validate_packed_ref_cache(refs);

1334 +

1335 packed_ref_cache = get_packed_ref_cache(refs);

1336 /* Increment the reference count to prevent it from being freed: */

1337 acquire_packed_ref_cache(packed_ref_cache);