btrfs: check for commit error at btrfs_attach_transaction_barrier()

btrfs_attach_transaction_barrier() is used to get a handle pointing to the
current running transaction if the transaction has not started its commit
yet (its state is < TRANS_STATE_COMMIT_START). If the transaction commit
has started, then we wait for the transaction to commit and finish before
returning - however we completely ignore if the transaction was aborted
due to some error during its commit, we simply return ERR_PT(-ENOENT),
which makes the caller assume everything is fine and no errors happened.

This could make an fsync return success (0) to user space when in fact we
had a transaction abort and the target inode changes were therefore not
persisted.

Fix this by checking for the return value from btrfs_wait_for_commit(),
and if it returned an error, return it back to the caller.

Fixes: d4edf39 ("Btrfs: fix uncompleted transaction")
CC: stable@vger.kernel.org # 4.19+
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/transaction.c

@@ -826,8 +826,13 @@ btrfs_attach_transaction_barrier(struct btrfs_root *root)
 
 	trans = start_transaction(root, 0, TRANS_ATTACH,
 				  BTRFS_RESERVE_NO_FLUSH, true);
-	if (trans == ERR_PTR(-ENOENT))
-		btrfs_wait_for_commit(root->fs_info, 0);
+	if (trans == ERR_PTR(-ENOENT)) {
+		int ret;
+
+		ret = btrfs_wait_for_commit(root->fs_info, 0);
+		if (ret)
+			return ERR_PTR(ret);
+	}
 
 	return trans;
 }