Merge pull request #33 from ecordell/win-pager

Add a pager for windows

Author: alicebob

db/pager.go

@@ -1,5 +1,12 @@
 package db
 
+const (
+	sqlitePendingByte  = 0x40000000
+	sqliteReservedByte = sqlitePendingByte + 1
+	sqliteSharedFirst  = sqlitePendingByte + 2
+	sqliteSharedSize   = 510
+)
+
 type pager interface {
 	// load a page from storage.
 	page(n int, pagesize int) ([]byte, error)

db/pager_unix.go

@@ -1,3 +1,5 @@
+// +build !windows
+
 // unix implementation of the `pager` interface (the file reader) with POSIX
 // advisory locking
 
@@ -12,11 +14,7 @@ import (
 )
 
 const (
-	seek_set             = 0 // should be defined in syscall
-	sqlite_pending_byte  = 0x40000000
-	sqlite_reserved_byte = sqlite_pending_byte + 1
-	sqlite_shared_first  = sqlite_pending_byte + 2
-	sqlite_shared_size   = 510
+	seekSet = 0 // should be defined in syscall
 )
 
 type filePager struct {
@@ -62,8 +60,8 @@ func (f *filePager) RLock() error {
 	// - get PENDING lock
 	pending := &unix.Flock_t{
 		Type:   unix.F_RDLCK,
-		Whence: seek_set,
-		Start:  sqlite_pending_byte,
+		Whence: seekSet,
+		Start:  sqlitePendingByte,
 		Len:    1,
 	}
 	if err := f.lock(pending); err != nil {
@@ -79,9 +77,9 @@ func (f *filePager) RLock() error {
 	// Get the read-lock
 	read := &unix.Flock_t{
 		Type:   unix.F_RDLCK,
-		Whence: seek_set,
-		Start:  sqlite_shared_first,
-		Len:    sqlite_shared_size,
+		Whence: seekSet,
+		Start:  sqliteSharedFirst,
+		Len:    sqliteSharedSize,
 	}
 	if err := f.lock(read); err != nil {
 		return err
@@ -105,8 +103,8 @@ func (f *filePager) CheckReservedLock() (bool, error) {
 	// per SQLite's unixCheckReservedLock()
 	lock := &unix.Flock_t{
 		Type:   unix.F_WRLCK,
-		Whence: seek_set,
-		Start:  sqlite_shared_first,
+		Whence: seekSet,
+		Start:  sqliteReservedByte,
 		Len:    1,
 	}
 	err := unix.FcntlFlock(f.f.Fd(), unix.F_GETLK, lock)

db/pager_windows.go

@@ -0,0 +1,137 @@
+// windows implementation of the `pager` interface (the file reader) with LockFileEx
+// locking
+
+// references:
+// winapi docs: https://docs.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-lockfileex
+// sqlite3 winlock: https://github.com/sqlite/sqlite/blob/c398c65bee850b6b8f24a44852872a27f114535d/src/os_win.c#L3236
+
+package db
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"time"
+
+	"golang.org/x/exp/mmap"
+	"golang.org/x/sys/windows"
+)
+
+const (
+	reserved uint32 = 0
+)
+
+// LockState is used to represent the program's opinion of whether a file is locked or not
+// this does not represent the lock state of the file - we can't distinguish between
+// a file that is locked by another process and failing to acquire a lock
+type LockState int
+
+const (
+	Unlocked LockState = iota
+	Locked
+)
+
+type filePager struct {
+	f     *os.File
+	mm    *mmap.ReaderAt
+	state LockState
+}
+
+func newFilePager(file string) (*filePager, error) {
+	f, err := os.Open(file)
+	if err != nil {
+		return nil, err
+	}
+	mm, err := mmap.Open(file)
+	if err != nil {
+		f.Close()
+		return nil, err
+	}
+	return &filePager{
+		f:     f,
+		mm:    mm,
+		state: Unlocked,
+	}, nil
+}
+
+// pages start counting at 1
+func (f *filePager) page(id int, pagesize int) ([]byte, error) {
+	buf := make([]byte, pagesize)
+	_, err := f.mm.ReadAt(buf[:], int64(id-1)*int64(pagesize))
+	return buf, err
+}
+
+func (f *filePager) lock(start, len uint32) error {
+	ol := new(windows.Overlapped)
+	ol.Offset = start
+	return windows.LockFileEx(windows.Handle(f.f.Fd()), windows.LOCKFILE_FAIL_IMMEDIATELY|windows.LOCKFILE_EXCLUSIVE_LOCK, reserved, len, 0, ol)
+}
+
+func (f *filePager) unlock(start, len uint32) error {
+	ol := new(windows.Overlapped)
+	ol.Offset = start
+	return windows.UnlockFileEx(windows.Handle(f.f.Fd()), reserved, len, 0, ol)
+}
+
+func (f *filePager) RLock() error {
+	// Set a 'SHARED' lock, following winLock() logic from sqlite3.c
+	if f.state == Locked {
+		return errors.New("trying to lock a locked lock") // panic?
+	}
+
+	// Try 3 times to get the pending lock.  This is needed to work
+	// around problems caused by indexing and/or anti-virus software on
+	// Windows systems.
+	// If you are using this code as a model for alternative VFSes, do not
+	// copy this retry logic.  It is a hack intended for Windows only.
+	//
+	// source: https://github.com/sqlite/sqlite/blob/c398c65bee850b6b8f24a44852872a27f114535d/src/os_win.c#L3280
+	for i := 0; i < 3; i++ {
+		err := f.lock(sqlitePendingByte, 1)
+		if err == nil {
+			break
+		}
+		if errors.Is(err, windows.ERROR_INVALID_HANDLE) {
+			return fmt.Errorf("failed to get lock, invalid handle")
+		}
+		time.Sleep(time.Microsecond)
+	}
+
+	defer func() {
+		// - drop the pending lock. No idea what to do with the error :/
+		f.unlock(sqlitePendingByte, 1)
+	}()
+
+	// Get the read-lock
+	if err := f.lock(sqliteSharedFirst, sqliteSharedSize); err != nil {
+		return err
+	}
+	f.state = Locked
+	return nil
+}
+
+func (f *filePager) RUnlock() error {
+	if f.state == Unlocked {
+		return errors.New("trying to unlock an unlocked lock") // panic?
+	}
+	if err := f.unlock(sqliteSharedFirst, sqliteSharedSize); err != nil {
+		return err
+	}
+	f.state = Unlocked
+	return nil
+}
+
+// True if there is a 'reserved' lock on the database, by any process.
+func (f *filePager) CheckReservedLock() (bool, error) {
+	// per SQLite's winCheckReservedLock()
+	err := f.lock(sqliteReservedByte, 1)
+	if err == nil {
+		f.unlock(sqliteReservedByte, 1)
+	}
+	return err == nil, err
+}
+
+func (f *filePager) Close() error {
+	f.f.Close()
+	return f.mm.Close()
+}