leveldb: file locking for OSX/amd64.

This was tested by running two instances of the program in manualtest/filelock.

Also add a very basic test to ease implementation for other systems.

R=nigeltao
CC=golang-dev
http://codereview.appspot.com/6446087

Committer: Nigel Tao <nigeltao@golang.org>

--HG--
rename : leveldb/db/file_lock_linux_amd64.go => leveldb/db/file_lock_darwin_amd64.go

Author: nictuku

leveldb/db/file.go

@@ -45,9 +45,9 @@ type FileSystem interface {
 	// A nil Closer is returned if an error occurred. Otherwise, close that
 	// Closer to release the lock.
 	//
-	// On Linux, a lock has the same semantics as fcntl(2)'s advisory locks.
-	// In particular, closing any other file descriptor for the same file will
-	// release the lock prematurely.
+	// On Linux and OSX, a lock has the same semantics as fcntl(2)'s advisory
+	// locks.  In particular, closing any other file descriptor for the same
+	// file will release the lock prematurely.
 	//
 	// Attempting to lock a file that is already locked by the current process
 	// has undefined behavior.

leveldb/db/file_lock_darwin_amd64.go

@@ -0,0 +1,51 @@
+// Copyright 2012 The LevelDB-Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package db
+
+import (
+	"io"
+	"os"
+	"syscall"
+	"unsafe"
+)
+
+// lockCloser hides all of an os.File's methods, except for Close.
+type lockCloser struct {
+	f *os.File
+}
+
+func (l lockCloser) Close() error {
+	return l.f.Close()
+}
+
+func (defFS) Lock(name string) (io.Closer, error) {
+	f, err := os.Create(name)
+	if err != nil {
+		return nil, err
+	}
+
+	// This type matches C's "struct flock" defined in /usr/include/sys/fcntl.h.
+	// TODO: move this into the standard syscall package.
+	k := struct {
+		Start  uint64 // sizeof(off_t): 8
+		Len    uint64 // sizeof(off_t): 8
+		Pid    uint32 // sizeof(pid_t): 4
+		Type   uint16 // sizeof(short): 2
+		Whence uint16 // sizeof(short): 2
+	}{
+		Type:   syscall.F_WRLCK,
+		Whence: uint16(os.SEEK_SET),
+		Start:  0,
+		Len:    0, // 0 means to lock the entire file.
+		Pid:    uint32(os.Getpid()),
+	}
+
+	_, _, errno := syscall.Syscall(syscall.SYS_FCNTL, f.Fd(), uintptr(syscall.F_SETLK), uintptr(unsafe.Pointer(&k)))
+	if errno != 0 {
+		f.Close()
+		return nil, errno
+	}
+	return lockCloser{f}, nil
+}

leveldb/db/file_lock_generic.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build !linux !amd64
+// +build !linux,!darwin !amd64
 
 package db