lock: add linux/arm support. From upstream 172e5cc1b6

Change-Id: Ia110eb027f68deb239757f1fb06cfe4574d8bf2d

third_party/github.com/camlistore/lock/lock_linux_arm.go

@@ -0,0 +1,81 @@
+// +build linux,arm
+// +build !appengine
+
+/*
+Copyright 2013 The Go Authors
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+     http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package lock
+
+import (
+	"fmt"
+	"io"
+	"os"
+	"path/filepath"
+	"syscall"
+	"unsafe"
+)
+
+func init() {
+	lockFn = lockFcntl
+}
+
+func lockFcntl(name string) (io.Closer, error) {
+	abs, err := filepath.Abs(name)
+	if err != nil {
+		return nil, err
+	}
+	lockmu.Lock()
+	if locked[abs] {
+		lockmu.Unlock()
+		return nil, fmt.Errorf("file %q already locked", abs)
+	}
+	locked[abs] = true
+	lockmu.Unlock()
+
+	fi, err := os.Stat(name)
+	if err == nil && fi.Size() > 0 {
+		return nil, fmt.Errorf("can't Lock file %q: has non-zero size", name)
+	}
+
+	f, err := os.Create(name)
+	if err != nil {
+		return nil, err
+	}
+
+	// This type matches C's "struct flock" defined in /usr/include/bits/fcntl.h.
+	// TODO: move this into the standard syscall package.
+	k := struct {
+		Type   uint16
+		Whence uint16
+		Start  uint32
+		Len    uint32
+		Pid    uint32
+	}{
+		Type:   syscall.F_WRLCK,
+		Whence: uint16(os.SEEK_SET),
+		Start:  0,
+		Len:    0, // 0 means to lock the entire file.
+		Pid:    uint32(os.Getpid()),
+	}
+
+	const F_SETLK = 6 // actual value. syscall package is wrong: golang.org/issue/7059
+	_, _, errno := syscall.Syscall(syscall.SYS_FCNTL, f.Fd(), uintptr(F_SETLK), uintptr(unsafe.Pointer(&k)))
+	if errno != 0 {
+		f.Close()
+		return nil, errno
+	}
+	return &unlocker{f, abs}, nil
+}