From e6b2d2aef5fddaeda48e4d8e8a6eabfe4f3efb96 Mon Sep 17 00:00:00 2001
From: Nuno Cruces <ncruces@users.noreply.github.com>
Date: Mon, 24 Nov 2025 13:42:12 +0000
Subject: [PATCH] Bulk copies.

---
 vfs/shm_copy.go | 93 ++++++++++++++++++++++++++++++++++---------------
 1 file changed, 64 insertions(+), 29 deletions(-)

diff --git a/vfs/shm_copy.go b/vfs/shm_copy.go
index fce6325..540f83b 100644
--- a/vfs/shm_copy.go
+++ b/vfs/shm_copy.go
@@ -35,38 +35,44 @@ func (s *vfsShm) shmAcquire(errp *error) {
 	if errp != nil && *errp != nil {
 		return
 	}
-	if len(s.ptrs) == 0 || shmEqual(s.shadow[0][:], s.shared[0][:]) {
+	if len(s.ptrs) == 0 {
 		return
 	}
-	// Copies modified words from shared to private memory.
-	for id, p := range s.ptrs {
-		shared := shmPage(s.shared[id][:])
-		shadow := shmPage(s.shadow[id][:])
-		privat := shmPage(util.View(s.mod, p, _WALINDEX_PGSZ))
-		for i, shared := range shared {
-			if shadow[i] != shared {
-				shadow[i] = shared
-				privat[i] = shared
-			}
-		}
+	if !shmCopyHeader(
+		util.View(s.mod, s.ptrs[0], _WALINDEX_HDR_SIZE),
+		s.shadow[0][:],
+		s.shared[0][:]) {
+		return
+	}
+
+	skip := _WALINDEX_HDR_SIZE
+	for id := range s.ptrs {
+		shmCopyTables(
+			util.View(s.mod, s.ptrs[id], _WALINDEX_PGSZ)[skip:],
+			s.shadow[id][skip:],
+			s.shared[id][skip:])
+		skip = 0
 	}
 }
 
 func (s *vfsShm) shmRelease() {
-	if len(s.ptrs) == 0 || shmEqual(s.shadow[0][:], util.View(s.mod, s.ptrs[0], _WALINDEX_HDR_SIZE)) {
+	if len(s.ptrs) == 0 {
 		return
 	}
-	// Copies modified words from private to shared memory.
-	for id, p := range s.ptrs {
-		shared := shmPage(s.shared[id][:])
-		shadow := shmPage(s.shadow[id][:])
-		privat := shmPage(util.View(s.mod, p, _WALINDEX_PGSZ))
-		for i, privat := range privat {
-			if shadow[i] != privat {
-				shadow[i] = privat
-				shared[i] = privat
-			}
-		}
+	if !shmCopyHeader(
+		s.shared[0][:],
+		s.shadow[0][:],
+		util.View(s.mod, s.ptrs[0], _WALINDEX_HDR_SIZE)) {
+		return
+	}
+
+	skip := _WALINDEX_HDR_SIZE
+	for id := range s.ptrs {
+		shmCopyTables(
+			s.shared[id][skip:],
+			s.shadow[id][skip:],
+			util.View(s.mod, s.ptrs[id], _WALINDEX_PGSZ)[skip:])
+		skip = 0
 	}
 }
 
@@ -77,11 +83,40 @@ func (s *vfsShm) shmBarrier() {
 	s.Unlock()
 }
 
-func shmPage(s []byte) *[_WALINDEX_PGSZ / 4]uint32 {
-	p := (*uint32)(unsafe.Pointer(unsafe.SliceData(s)))
-	return (*[_WALINDEX_PGSZ / 4]uint32)(unsafe.Slice(p, _WALINDEX_PGSZ/4))
+func shmCopyTables(v1, v2, v3 []byte) {
+	if string(v2) != string(v3) {
+		copy(v1, v3)
+		copy(v2, v3)
+	}
 }
 
-func shmEqual(v1, v2 []byte) bool {
-	return *(*[_WALINDEX_HDR_SIZE]byte)(v1[:]) == *(*[_WALINDEX_HDR_SIZE]byte)(v2[:])
+func shmCopyHeader(s1, s2, s3 []byte) (ret bool) {
+	// First copy of the WAL Index Information.
+	if string(s2[:48]) != string(s3[:48]) {
+		copy(s1, s3[:48])
+		copy(s2, s3[:48])
+		ret = true
+	}
+	// Second copy of the WAL Index Information.
+	if string(s2[48:][:48]) != string(s3[48:][:48]) {
+		copy(s1[48:], s3[48:][:48])
+		copy(s2[48:], s3[48:][:48])
+		ret = true
+	}
+	// Checkpoint Information and Locks.
+	i1 := shmCheckpointInfo(s1)
+	i2 := shmCheckpointInfo(s2)
+	for i, i3 := range shmCheckpointInfo(s3) {
+		if i2[i] != i3 {
+			i1[i] = i3
+			i2[i] = i3
+			ret = true
+		}
+	}
+	return
+}
+
+func shmCheckpointInfo(s []byte) *[10]uint32 {
+	p := (*uint32)(unsafe.Pointer(&s[96]))
+	return (*[10]uint32)(unsafe.Slice(p, 10))
 }