1//go:build (windows && (386 || arm || amd64 || arm64 || riscv64 || ppc64le)) || sqlite3_dotlk
 2
 3package vfs
 4
 5import (
 6	"unsafe"
 7
 8	"github.com/ncruces/go-sqlite3/internal/util"
 9)
10
11const (
12	_WALINDEX_HDR_SIZE = 136
13	_WALINDEX_PGSZ     = 32768
14)
15
16// This seems a safe way of keeping the WAL-index in sync.
17//
18// The WAL-index file starts with a header,
19// and the index doesn't meaningfully change if the header doesn't change.
20//
21// The header starts with two 48 byte, checksummed, copies of the same information,
22// which are accessed independently between memory barriers.
23// The checkpoint information that follows uses 4 byte aligned words.
24//
25// Finally, we have the WAL-index hash tables,
26// which are only modified holding the exclusive WAL_WRITE_LOCK.
27//
28// Since all the data is either redundant+checksummed,
29// 4 byte aligned, or modified under an exclusive lock,
30// the copies below should correctly keep memory in sync.
31//
32// https://sqlite.org/walformat.html#the_wal_index_file_format
33
34func (s *vfsShm) shmAcquire(ptr *_ErrorCode) {
35	if ptr != nil && *ptr != _OK {
36		return
37	}
38	if len(s.ptrs) == 0 || shmEqual(s.shadow[0][:], s.shared[0][:]) {
39		return
40	}
41	// Copies modified words from shared to private memory.
42	for id, p := range s.ptrs {
43		shared := shmPage(s.shared[id][:])
44		shadow := shmPage(s.shadow[id][:])
45		privat := shmPage(util.View(s.mod, p, _WALINDEX_PGSZ))
46		for i, shared := range shared {
47			if shadow[i] != shared {
48				shadow[i] = shared
49				privat[i] = shared
50			}
51		}
52	}
53}
54
55func (s *vfsShm) shmRelease() {
56	if len(s.ptrs) == 0 || shmEqual(s.shadow[0][:], util.View(s.mod, s.ptrs[0], _WALINDEX_HDR_SIZE)) {
57		return
58	}
59	// Copies modified words from private to shared memory.
60	for id, p := range s.ptrs {
61		shared := shmPage(s.shared[id][:])
62		shadow := shmPage(s.shadow[id][:])
63		privat := shmPage(util.View(s.mod, p, _WALINDEX_PGSZ))
64		for i, privat := range privat {
65			if shadow[i] != privat {
66				shadow[i] = privat
67				shared[i] = privat
68			}
69		}
70	}
71}
72
73func (s *vfsShm) shmBarrier() {
74	s.Lock()
75	s.shmAcquire(nil)
76	s.shmRelease()
77	s.Unlock()
78}
79
80func shmPage(s []byte) *[_WALINDEX_PGSZ / 4]uint32 {
81	p := (*uint32)(unsafe.Pointer(unsafe.SliceData(s)))
82	return (*[_WALINDEX_PGSZ / 4]uint32)(unsafe.Slice(p, _WALINDEX_PGSZ/4))
83}
84
85func shmEqual(v1, v2 []byte) bool {
86	return *(*[_WALINDEX_HDR_SIZE]byte)(v1[:]) == *(*[_WALINDEX_HDR_SIZE]byte)(v2[:])
87}