mirror of
https://github.com/superseriousbusiness/gotosocial.git
synced 2024-11-25 13:16:40 +00:00
85 lines
2.3 KiB
Go
85 lines
2.3 KiB
Go
//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[:])
|
|
}
|