Add AIX support with fcntl

AIX doesn't provide a true flock() syscall. It does exist but it's just
a wrapper around fcntl. It doesn't provide safe locks under file
descriptors of a same process.

The current implementation is based on the file
cmd/go/internal/lockedfile/internal/filelock/filelock_fcntl.go.

Using fcntl implementation doesn't allow to have several RLocks at the
same time as closing a file descriptor might release the lock if even
others RLocks remain attached.

flock.go

@@ -19,6 +19,7 @@ package flock
 import (
 	"context"
 	"os"
+	"runtime"
 	"sync"
 	"time"
 )
@@ -116,7 +117,15 @@ func tryCtx(ctx context.Context, fn func() (bool, error), retryDelay time.Durati
 func (f *Flock) setFh() error {
 	// open a new os.File instance
 	// create it if it doesn't exist, and open the file read-only.
-	fh, err := os.OpenFile(f.path, os.O_CREATE|os.O_RDONLY, os.FileMode(0600))
+	flags := os.O_CREATE
+	if runtime.GOOS == "aix" {
+		// AIX cannot preform write-lock (ie exclusive) on a
+		// read-only file.
+		flags |= os.O_RDWR
+	} else {
+		flags |= os.O_RDONLY
+	}
+	fh, err := os.OpenFile(f.path, flags, os.FileMode(0600))
 	if err != nil {
 		return err
 	}

flock_aix.go

@@ -0,0 +1,271 @@
+// Copyright 2019 Tim Heckman. All rights reserved. Use of this source code is
+// governed by the BSD 3-Clause license that can be found in the LICENSE file.
+
+// Copyright 2018 The 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.
+
+// This code implements the filelock API using POSIX 'fcntl' locks, which attach
+// to an (inode, process) pair rather than a file descriptor. To avoid unlocking
+// files prematurely when the same file is opened through different descriptors,
+// we allow only one read-lock at a time.
+//
+// This code is adapted from the Go package:
+// cmd/go/internal/lockedfile/internal/filelock
+
+//+build aix
+
+package flock
+
+import (
+	"errors"
+	"io"
+	"os"
+	"sync"
+	"syscall"
+
+	"golang.org/x/sys/unix"
+)
+
+type lockType int16
+
+const (
+	readLock  lockType = unix.F_RDLCK
+	writeLock lockType = unix.F_WRLCK
+)
+
+type inode = uint64
+
+type inodeLock struct {
+	owner *Flock
+	queue []<-chan *Flock
+}
+
+var (
+	mu     sync.Mutex
+	inodes = map[*Flock]inode{}
+	locks  = map[inode]inodeLock{}
+)
+
+// Lock is a blocking call to try and take an exclusive file lock. It will wait
+// until it is able to obtain the exclusive file lock. It's recommended that
+// TryLock() be used over this function. This function may block the ability to
+// query the current Locked() or RLocked() status due to a RW-mutex lock.
+//
+// If we are already exclusive-locked, this function short-circuits and returns
+// immediately assuming it can take the mutex lock.
+//
+// If the *Flock has a shared lock (RLock), this may transparently replace the
+// shared lock with an exclusive lock on some UNIX-like operating systems. Be
+// careful when using exclusive locks in conjunction with shared locks
+// (RLock()), because calling Unlock() may accidentally release the exclusive
+// lock that was once a shared lock.
+func (f *Flock) Lock() error {
+	return f.lock(&f.l, writeLock)
+}
+
+// RLock is a blocking call to try and take a shared file lock. It will wait
+// until it is able to obtain the shared file lock. It's recommended that
+// TryRLock() be used over this function. This function may block the ability to
+// query the current Locked() or RLocked() status due to a RW-mutex lock.
+//
+// If we are already shared-locked, this function short-circuits and returns
+// immediately assuming it can take the mutex lock.
+func (f *Flock) RLock() error {
+	return f.lock(&f.r, readLock)
+}
+
+func (f *Flock) lock(locked *bool, flag lockType) error {
+	f.m.Lock()
+	defer f.m.Unlock()
+
+	if *locked {
+		return nil
+	}
+
+	if f.fh == nil {
+		if err := f.setFh(); err != nil {
+			return err
+		}
+		defer f.ensureFhState()
+	}
+
+	if _, err := f.doLock(flag, true); err != nil {
+		return err
+	}
+
+	*locked = true
+	return nil
+}
+
+func (f *Flock) doLock(lt lockType, blocking bool) (bool, error) {
+	// POSIX locks apply per inode and process, and the lock for an inode is
+	// released when *any* descriptor for that inode is closed. So we need to
+	// synchronize access to each inode internally, and must serialize lock and
+	// unlock calls that refer to the same inode through different descriptors.
+	fi, err := f.fh.Stat()
+	if err != nil {
+		return false, err
+	}
+	ino := inode(fi.Sys().(*syscall.Stat_t).Ino)
+
+	mu.Lock()
+	if i, dup := inodes[f]; dup && i != ino {
+		mu.Unlock()
+		return false, &os.PathError{
+			Path: f.Path(),
+			Err:  errors.New("inode for file changed since last Lock or RLock"),
+		}
+	}
+
+	inodes[f] = ino
+
+	var wait chan *Flock
+	l := locks[ino]
+	if l.owner == f {
+		// This file already owns the lock, but the call may change its lock type.
+	} else if l.owner == nil {
+		// No owner: it's ours now.
+		l.owner = f
+	} else if !blocking {
+		// Already owned: cannot take the lock.
+		mu.Unlock()
+		return false, nil
+	} else {
+		// Already owned: add a channel to wait on.
+		wait = make(chan *Flock)
+		l.queue = append(l.queue, wait)
+	}
+	locks[ino] = l
+	mu.Unlock()
+
+	if wait != nil {
+		wait <- f
+	}
+
+	err = setlkw(f.fh.Fd(), lt)
+
+	if err != nil {
+		f.doUnlock()
+		return false, err
+	}
+
+	return true, nil
+}
+
+func (f *Flock) Unlock() error {
+	f.m.Lock()
+	defer f.m.Unlock()
+
+	// if we aren't locked or if the lockfile instance is nil
+	// just return a nil error because we are unlocked
+	if (!f.l && !f.r) || f.fh == nil {
+		return nil
+	}
+
+	if err := f.doUnlock(); err != nil {
+		return err
+	}
+
+	f.fh.Close()
+
+	f.l = false
+	f.r = false
+	f.fh = nil
+
+	return nil
+}
+
+func (f *Flock) doUnlock() (err error) {
+	var owner *Flock
+	mu.Lock()
+	ino, ok := inodes[f]
+	if ok {
+		owner = locks[ino].owner
+	}
+	mu.Unlock()
+
+	if owner == f {
+		err = setlkw(f.fh.Fd(), unix.F_UNLCK)
+	}
+
+	mu.Lock()
+	l := locks[ino]
+	if len(l.queue) == 0 {
+		// No waiters: remove the map entry.
+		delete(locks, ino)
+	} else {
+		// The first waiter is sending us their file now.
+		// Receive it and update the queue.
+		l.owner = <-l.queue[0]
+		l.queue = l.queue[1:]
+		locks[ino] = l
+	}
+	delete(inodes, f)
+	mu.Unlock()
+
+	return err
+}
+
+// TryLock is the preferred function for taking an exclusive file lock. This
+// function takes an RW-mutex lock before it tries to lock the file, so there is
+// the possibility that this function may block for a short time if another
+// goroutine is trying to take any action.
+//
+// The actual file lock is non-blocking. If we are unable to get the exclusive
+// file lock, the function will return false instead of waiting for the lock. If
+// we get the lock, we also set the *Flock instance as being exclusive-locked.
+func (f *Flock) TryLock() (bool, error) {
+	return f.try(&f.l, writeLock)
+}
+
+// TryRLock is the preferred function for taking a shared file lock. This
+// function takes an RW-mutex lock before it tries to lock the file, so there is
+// the possibility that this function may block for a short time if another
+// goroutine is trying to take any action.
+//
+// The actual file lock is non-blocking. If we are unable to get the shared file
+// lock, the function will return false instead of waiting for the lock. If we
+// get the lock, we also set the *Flock instance as being share-locked.
+func (f *Flock) TryRLock() (bool, error) {
+	return f.try(&f.r, readLock)
+}
+
+func (f *Flock) try(locked *bool, flag lockType) (bool, error) {
+	f.m.Lock()
+	defer f.m.Unlock()
+
+	if *locked {
+		return true, nil
+	}
+
+	if f.fh == nil {
+		if err := f.setFh(); err != nil {
+			return false, err
+		}
+		defer f.ensureFhState()
+	}
+
+	haslock, err := f.doLock(flag, false)
+	if err != nil {
+		return false, err
+	}
+
+	*locked = haslock
+	return haslock, nil
+}
+
+// setlkw calls FcntlFlock with F_SETLKW for the entire file indicated by fd.
+func setlkw(fd uintptr, lt lockType) error {
+	for {
+		err := unix.FcntlFlock(fd, unix.F_SETLKW, &unix.Flock_t{
+			Type:   int16(lt),
+			Whence: io.SeekStart,
+			Start:  0,
+			Len:    0, // All bytes.
+		})
+		if err != unix.EINTR {
+			return err
+		}
+	}
+}

flock_test.go

@@ -9,6 +9,7 @@ import (
 	"context"
 	"io/ioutil"
 	"os"
+	"runtime"
 	"testing"
 	"time"
 
@@ -123,7 +124,16 @@ func (t *TestSuite) TestFlock_TryRLock(c *C) {
 	flock2 := flock.New(t.path)
 	locked, err = flock2.TryRLock()
 	c.Assert(err, IsNil)
-	c.Check(locked, Equals, true)
+	if runtime.GOOS == "aix" {
+		// When using POSIX locks, we can't safely read-lock the same
+		// inode through two different descriptors at the same time:
+		// when the first descriptor is closed, the second descriptor
+		// would still be open but silently unlocked. So a second
+		// TryRLock must return false.
+		c.Check(locked, Equals, false)
+	} else {
+		c.Check(locked, Equals, true)
+	}
 
 	// make sure we just return false with no error in cases
 	// where we would have been blocked

flock_unix.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by the BSD 3-Clause
 // license that can be found in the LICENSE file.
 
-// +build !windows
+// +build !aix,!windows
 
 package flock