mirror of
https://github.com/superseriousbusiness/gotosocial.git
synced 2024-11-24 12:46:38 +00:00
85 lines
2.3 KiB
Go
85 lines
2.3 KiB
Go
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//go:build (windows && (386 || arm || amd64 || arm64 || riscv64 || ppc64le) && !sqlite3_nosys) || sqlite3_dotlk
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package vfs
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import (
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"unsafe"
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"github.com/ncruces/go-sqlite3/internal/util"
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)
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const (
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_WALINDEX_HDR_SIZE = 136
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_WALINDEX_PGSZ = 32768
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)
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// This looks like a safe way of keeping the WAL-index in sync.
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//
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// The WAL-index file starts with a header,
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// and the index doesn't meaningfully change if the header doesn't change.
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//
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// The header starts with two 48 byte, checksummed, copies of the same information,
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// which are accessed independently between memory barriers.
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// The checkpoint information that follows uses 4 byte aligned words.
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//
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// Finally, we have the WAL-index hash tables,
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// which are only modified holding the exclusive WAL_WRITE_LOCK.
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//
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// Since all the data is either redundant+checksummed,
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// 4 byte aligned, or modified under an exclusive lock,
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// the copies below should correctly keep copies in sync.
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//
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// https://sqlite.org/walformat.html#the_wal_index_file_format
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func (s *vfsShm) shmAcquire() {
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if len(s.ptrs) == 0 || shmUnmodified(s.shadow[0][:], s.shared[0][:]) {
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return
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}
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// Copies modified words from shared to private memory.
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for id, p := range s.ptrs {
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shared := shmPage(s.shared[id][:])
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shadow := shmPage(s.shadow[id][:])
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privat := shmPage(util.View(s.mod, p, _WALINDEX_PGSZ))
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for i, shared := range shared {
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if shadow[i] != shared {
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shadow[i] = shared
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privat[i] = shared
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}
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}
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}
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}
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func (s *vfsShm) shmRelease() {
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if len(s.ptrs) == 0 || shmUnmodified(s.shadow[0][:], util.View(s.mod, s.ptrs[0], _WALINDEX_HDR_SIZE)) {
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return
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}
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// Copies modified words from private to shared memory.
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for id, p := range s.ptrs {
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shared := shmPage(s.shared[id][:])
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shadow := shmPage(s.shadow[id][:])
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privat := shmPage(util.View(s.mod, p, _WALINDEX_PGSZ))
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for i, privat := range privat {
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if shadow[i] != privat {
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shadow[i] = privat
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shared[i] = privat
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}
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}
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}
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}
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func (s *vfsShm) shmBarrier() {
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s.Lock()
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s.shmAcquire()
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s.shmRelease()
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s.Unlock()
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}
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func shmPage(s []byte) *[_WALINDEX_PGSZ / 4]uint32 {
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p := (*uint32)(unsafe.Pointer(unsafe.SliceData(s)))
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return (*[_WALINDEX_PGSZ / 4]uint32)(unsafe.Slice(p, _WALINDEX_PGSZ/4))
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}
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func shmUnmodified(v1, v2 []byte) bool {
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return *(*[_WALINDEX_HDR_SIZE]byte)(v1[:]) == *(*[_WALINDEX_HDR_SIZE]byte)(v2[:])
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}
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