[IMPROVED] Consumer create time when stream has large number of blks. (#4269)

When creating a consumer on a stream with a very large number of msg
blks, calculating numPending could be slow.

This aims to optimize a bit, more work to be done on streams with a very
large (> 200k) number of msg blks.

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

server/filestore.go

@@ -1868,12 +1868,24 @@ func (fs *fileStore) NumPending(sseq uint64, filter string, lastPerSubject bool)
 		seqStart, _ = fs.selectMsgBlockWithIndex(sseq)
 	}
 
-	tsa := [32]string{}
-	fsa := [32]string{}
+	var tsa, fsa [32]string
 	fts := tokenizeSubjectIntoSlice(fsa[:0], filter)
 	isAll := filter == _EMPTY_ || filter == fwcs
 	wc := subjectHasWildcard(filter)
 
+	// See if filter was provided but its the only subject.
+	if !isAll && !wc && len(fs.psim) == 1 && fs.psim[filter] != nil {
+		isAll = true
+	}
+
+	// If we are isAll and have no deleted we can do a simpler calculation.
+	if isAll && (fs.state.LastSeq-fs.state.FirstSeq+1) == fs.state.Msgs {
+		if sseq == 0 {
+			return fs.state.Msgs, validThrough
+		}
+		return fs.state.LastSeq - sseq + 1, validThrough
+	}
+
 	isMatch := func(subj string) bool {
 		if isAll {
 			return true
@@ -1900,6 +1912,7 @@ func (fs *fileStore) NumPending(sseq uint64, filter string, lastPerSubject bool)
 			var t uint64
 			if isAll && sseq <= mb.first.seq {
 				if lastPerSubject {
+					mb.ensurePerSubjectInfoLoaded()
 					for subj := range mb.fss {
 						if !seen[subj] {
 							total++
@@ -2023,16 +2036,20 @@ func (fs *fileStore) NumPending(sseq uint64, filter string, lastPerSubject bool)
 		mb.mu.Lock()
 		// Check if we should include all of this block in adjusting. If so work with metadata.
 		if sseq > mb.last.seq {
-			// We need to adjust for all matches in this block.
-			// We will scan fss state vs messages themselves.
-			// Make sure we have fss loaded.
-			mb.ensurePerSubjectInfoLoaded()
-			for subj, ss := range mb.fss {
-				if isMatch(subj) {
-					if lastPerSubject {
-						adjust++
-					} else {
-						adjust += ss.Msgs
+			if isAll && !lastPerSubject {
+				adjust += mb.msgs
+			} else {
+				// We need to adjust for all matches in this block.
+				// We will scan fss state vs messages themselves.
+				// Make sure we have fss loaded.
+				mb.ensurePerSubjectInfoLoaded()
+				for subj, ss := range mb.fss {
+					if isMatch(subj) {
+						if lastPerSubject {
+							adjust++
+						} else {
+							adjust += ss.Msgs
+						}
 					}
 				}
 			}

server/filestore_test.go

@@ -5457,3 +5457,45 @@ func TestFileStoreConsumerStoreEncodeAfterRestart(t *testing.T) {
 		}
 	})
 }
+
+func TestFileStoreNumPendingLargeNumBlks(t *testing.T) {
+	// No need for all permutations here.
+	storeDir := t.TempDir()
+	fcfg := FileStoreConfig{
+		StoreDir:  storeDir,
+		BlockSize: 128, // Small on purpose to create alot of blks.
+	}
+	fs, err := newFileStore(fcfg, StreamConfig{Name: "zzz", Subjects: []string{"zzz"}, Storage: FileStorage})
+	require_NoError(t, err)
+
+	subj, msg := "zzz", bytes.Repeat([]byte("X"), 100)
+	numMsgs := 10_000
+
+	for i := 0; i < numMsgs; i++ {
+		fs.StoreMsg(subj, nil, msg)
+	}
+
+	start := time.Now()
+	total, _ := fs.NumPending(4000, "zzz", false)
+	require_True(t, time.Since(start) < 5*time.Millisecond)
+	require_True(t, total == 6001)
+
+	start = time.Now()
+	total, _ = fs.NumPending(6000, "zzz", false)
+	require_True(t, time.Since(start) < 5*time.Millisecond)
+	require_True(t, total == 4001)
+
+	// Now delete a message in first half and second half.
+	fs.RemoveMsg(1000)
+	fs.RemoveMsg(9000)
+
+	start = time.Now()
+	total, _ = fs.NumPending(4000, "zzz", false)
+	require_True(t, time.Since(start) < 50*time.Millisecond)
+	require_True(t, total == 6000)
+
+	start = time.Now()
+	total, _ = fs.NumPending(6000, "zzz", false)
+	require_True(t, time.Since(start) < 50*time.Millisecond)
+	require_True(t, total == 4000)
+}