[IMPROVED] Do not load blocks when calling LoadNextMsg for the first …

…time after restart or long inactivity (#4969)

If LoadNextMsg was called with start sequence == FirstSeq, we would walk
all blocks, which if fss was loaded we could skip fairly quickly.
However on a restart or no activity past a sync interval this could
cause memory blocks to be loaded in just to be skipped.

#4906

Signed-off-by: Derek Collison <derek@nats.io>

server/filestore.go

@@ -5960,6 +5960,16 @@ func (fs *fileStore) LoadNextMsg(filter string, wc bool, start uint64, sm *Store
 		start = fs.state.FirstSeq
 	}
 
+	// If start is less than or equal to beginning of our stream, meaning our first call,
+	// let's check the psim to see if we can skip ahead.
+	if start <= fs.state.FirstSeq {
+		var ss SimpleState
+		fs.numFilteredPending(filter, &ss)
+		if ss.First > start {
+			start = ss.First
+		}
+	}
+
 	if bi, _ := fs.selectMsgBlockWithIndex(start); bi >= 0 {
 		for i := bi; i < len(fs.blks); i++ {
 			mb := fs.blks[i]

server/filestore_test.go

@@ -6496,6 +6496,43 @@ func TestFileStoreSkipMsgs(t *testing.T) {
 	require_Equal(t, state.NumDeleted, 10)
 }
 
+func TestFileStoreOptimizeFirstLoadNextMsgWithSequenceZero(t *testing.T) {
+	sd := t.TempDir()
+	fs, err := newFileStore(
+		FileStoreConfig{StoreDir: sd, BlockSize: 4096},
+		StreamConfig{Name: "zzz", Subjects: []string{"foo.*"}, Storage: FileStorage})
+	require_NoError(t, err)
+	defer fs.Stop()
+
+	msg := bytes.Repeat([]byte("ZZZ"), 33) // ~100bytes
+
+	for i := 0; i < 5000; i++ {
+		fs.StoreMsg("foo.A", nil, msg)
+	}
+	// This will create alot of blocks, ~167.
+	// Just used to check that we do not load these in when searching.
+	// Now add in 10 for foo.bar at the end.
+	for i := 0; i < 10; i++ {
+		fs.StoreMsg("foo.B", nil, msg)
+	}
+	// The bug would not be visible on running server per se since we would have had fss loaded
+	// and that sticks around a bit longer, we would use that to skip over the early blocks. So stop
+	// and restart the filestore.
+	fs.Stop()
+	fs, err = newFileStore(
+		FileStoreConfig{StoreDir: sd, BlockSize: 4096},
+		StreamConfig{Name: "zzz", Subjects: []string{"foo.*"}, Storage: FileStorage})
+	require_NoError(t, err)
+	defer fs.Stop()
+
+	// Now fetch the next message for foo.B but set starting sequence to 0.
+	_, nseq, err := fs.LoadNextMsg("foo.B", false, 0, nil)
+	require_NoError(t, err)
+	require_Equal(t, nseq, 5001)
+	// Now check how many blks are loaded, should be only 1.
+	require_Equal(t, fs.cacheLoads(), 1)
+}
+
 ///////////////////////////////////////////////////////////////////////////
 // Benchmarks
 ///////////////////////////////////////////////////////////////////////////