bump ncruces/go-sqlite3 to v0.20.2 (#3524)

vendor/github.com/ncruces/go-sqlite3/vfs/shm_copy.go

@@ -0,0 +1,84 @@
+//go:build (windows && (386 || arm || amd64 || arm64 || riscv64 || ppc64le) && !sqlite3_nosys) || sqlite3_dotlk
+
+package vfs
+
+import (
+	"unsafe"
+
+	"github.com/ncruces/go-sqlite3/internal/util"
+)
+
+const (
+	_WALINDEX_HDR_SIZE = 136
+	_WALINDEX_PGSZ     = 32768
+)
+
+// This looks like a safe way of keeping the WAL-index in sync.
+//
+// The WAL-index file starts with a header,
+// and the index doesn't meaningfully change if the header doesn't change.
+//
+// The header starts with two 48 byte, checksummed, copies of the same information,
+// which are accessed independently between memory barriers.
+// The checkpoint information that follows uses 4 byte aligned words.
+//
+// Finally, we have the WAL-index hash tables,
+// which are only modified holding the exclusive WAL_WRITE_LOCK.
+//
+// Since all the data is either redundant+checksummed,
+// 4 byte aligned, or modified under an exclusive lock,
+// the copies below should correctly keep copies in sync.
+//
+// https://sqlite.org/walformat.html#the_wal_index_file_format
+
+func (s *vfsShm) shmAcquire() {
+	if len(s.ptrs) == 0 || shmUnmodified(s.shadow[0][:], s.shared[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
+			}
+		}
+	}
+}
+
+func (s *vfsShm) shmRelease() {
+	if len(s.ptrs) == 0 || shmUnmodified(s.shadow[0][:], util.View(s.mod, s.ptrs[0], _WALINDEX_HDR_SIZE)) {
+		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
+			}
+		}
+	}
+}
+
+func (s *vfsShm) shmBarrier() {
+	s.Lock()
+	s.shmAcquire()
+	s.shmRelease()
+	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 shmUnmodified(v1, v2 []byte) bool {
+	return *(*[_WALINDEX_HDR_SIZE]byte)(v1[:]) == *(*[_WALINDEX_HDR_SIZE]byte)(v2[:])
+}