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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,84 @@ | ||
| //go:build (windows && (386 || arm || amd64 || arm64 || riscv64 || ppc64le) && !sqlite3_nosys) || sqlite3_dotlk | ||
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| package vfs | ||
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| import ( | ||
| "unsafe" | ||
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| "github.com/ncruces/go-sqlite3/internal/util" | ||
| ) | ||
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| const ( | ||
| _WALINDEX_HDR_SIZE = 136 | ||
| _WALINDEX_PGSZ = 32768 | ||
| ) | ||
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| // 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 | ||
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| 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 | ||
| } | ||
| } | ||
| } | ||
| } | ||
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| 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 | ||
| } | ||
| } | ||
| } | ||
| } | ||
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| func (s *vfsShm) shmBarrier() { | ||
| s.Lock() | ||
| s.shmAcquire() | ||
| s.shmRelease() | ||
| s.Unlock() | ||
| } | ||
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| 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)) | ||
| } | ||
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| func shmUnmodified(v1, v2 []byte) bool { | ||
| return *(*[_WALINDEX_HDR_SIZE]byte)(v1[:]) == *(*[_WALINDEX_HDR_SIZE]byte)(v2[:]) | ||
| } |
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