From 0eb7e95f054f9801340e08f44c81acd472a3d448 Mon Sep 17 00:00:00 2001 From: Andrew Kryczka Date: Wed, 24 Apr 2019 15:00:42 -0700 Subject: [PATCH] storage: fix possible raft log panic after fsync error Detected with #36989 applied by running `./bin/roachtest run --local '^system-crash/sync-errors=true$'`. With some slight modification to that test's constants it could repro errors like this within a minute: ``` panic: tocommit(375) is out of range [lastIndex(374)]. Was the raft log corrupted, truncated, or lost? ``` Debugging showed `DBSyncWAL` can be called even after a sync failure. I guess if it returns success any time after it fails it will ack writes that aren't recoverable in WAL. They aren't recoverable because RocksDB stops recovery upon hitting the offset corresponding to the lost write (typically there should be a corruption there). Meanwhile, there are still successfully synced writes at later offsets in the file. The fix is simple. If `DBSyncWAL` returns an error once, keep track of that error and return it for all future writes. Release note (bug fix): Fixed possible panic while recovering from a WAL on which a sync operation failed. --- pkg/storage/engine/rocksdb.go | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-) diff --git a/pkg/storage/engine/rocksdb.go b/pkg/storage/engine/rocksdb.go index ae79e5056ea2..3462c4fdeca1 100644 --- a/pkg/storage/engine/rocksdb.go +++ b/pkg/storage/engine/rocksdb.go @@ -649,6 +649,7 @@ func (r *RocksDB) syncLoop() { s.Lock() var lastSync time.Time + var err error for { for len(s.pending) == 0 && !s.closed { @@ -674,8 +675,14 @@ func (r *RocksDB) syncLoop() { s.Unlock() - var err error - if r.cfg.Dir != "" { + // Linux only guarantees we'll be notified of a writeback error once + // during a sync call. After sync fails once, we cannot rely on any + // future data written to WAL being crash-recoverable. That's because + // any future writes will be appended after a potential corruption in + // the WAL, and RocksDB's recovery terminates upon encountering any + // corruption. So, we must not call `DBSyncWAL` again after it has + // failed once. + if r.cfg.Dir != "" && err == nil { err = statusToError(C.DBSyncWAL(r.rdb)) lastSync = timeutil.Now() }