From 654ed7dd45490fa90680d85600656fec19dd1891 Mon Sep 17 00:00:00 2001
From: Derek Collison <derek@nats.io>
Date: Fri, 21 Jun 2024 10:32:36 -0700
Subject: [PATCH] When we miss on LoadNextMsg but not starting at first
 sequence, optimize jump ahead to not need blk access from file.

Signed-off-by: Derek Collison <derek@nats.io>
---
 server/filestore.go | 33 +++++++++++++++++++++++++++++----
 1 file changed, 29 insertions(+), 4 deletions(-)

diff --git a/server/filestore.go b/server/filestore.go
index e5ddae76a2e..556646c0edb 100644
--- a/server/filestore.go
+++ b/server/filestore.go
@@ -2590,6 +2590,32 @@ func (fs *fileStore) FilteredState(sseq uint64, subj string) SimpleState {
 	return ss
 }
 
+// This is used to see if we can selectively jump start blocks based on filter subject and a floor block index.
+// Will return -1 if no matches at all.
+func (fs *fileStore) checkSkipFirstBlock(filter string, wc bool) int {
+	start := uint32(math.MaxUint32)
+	if wc {
+		fs.psim.Match(stringToBytes(filter), func(_ []byte, psi *psi) {
+			if psi.fblk < start {
+				start = psi.fblk
+			}
+		})
+	} else if psi, ok := fs.psim.Find(stringToBytes(filter)); ok {
+		start = psi.fblk
+	}
+	// Nothing found.
+	if start == uint32(math.MaxUint32) {
+		return -1
+	}
+	// Here we need to translate this to index into fs.blks.
+	mb := fs.bim[start]
+	if mb == nil {
+		return -1
+	}
+	bi, _ := fs.selectMsgBlockWithIndex(atomic.LoadUint64(&mb.last.seq))
+	return bi
+}
+
 // Optimized way for getting all num pending matching a filter subject.
 // Lock should be held.
 func (fs *fileStore) numFilteredPending(filter string, ss *SimpleState) {
@@ -6438,16 +6464,15 @@ func (fs *fileStore) LoadNextMsg(filter string, wc bool, start uint64, sm *Store
 				// Similar to above if start <= first seq.
 				// TODO(dlc) - For v2 track these by filter subject since they will represent filtered consumers.
 				if i == bi {
-					var ss SimpleState
-					fs.numFilteredPendingNoLast(filter, &ss)
+					nbi := fs.checkSkipFirstBlock(filter, wc)
 					// Nothing available.
-					if ss.Msgs == 0 {
+					if nbi < 0 {
 						return nil, fs.state.LastSeq, ErrStoreEOF
 					}
 					// See if we can jump ahead here.
 					// Right now we can only spin on first, so if we have interior sparseness need to favor checking per block fss if loaded.
 					// For v2 will track all blocks that have matches for psim.
-					if nbi, _ := fs.selectMsgBlockWithIndex(ss.First); nbi > i {
+					if nbi > i {
 						i = nbi - 1 // For the iterator condition i++
 					}
 				}