internal/poll: use fdMutex to provide read/write locking on Windows

On Windows it is not possible to do concurrent I/O on file handles due
to the way FD.Pread and FD.Pwrite are implemented. This serialization is
achieved by having a dedicated mutex locked in the affected FD methods.

This makes the code difficult to reason about, as there is another
layer of locking introduced by the fdMutex. For example, it is not
obvious that concurrent I/O operations are serialized.

This CL removed the dedicated mutex and uses the fdMutex to provide
read/write locking.

Change-Id: I00389662728ce29428a587c3189bab90a0399215
Reviewed-on: https://go-review.googlesource.com/c/go/+/698096
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>

src/internal/poll/fd_mutex.go

@@ -251,6 +251,23 @@ func (fd *FD) writeUnlock() {
 	}
 }
 
+// readWriteLock adds a reference to fd and locks fd for reading and writing.
+// It returns an error when fd cannot be used for reading and writing.
+func (fd *FD) readWriteLock() error {
+	if !fd.fdmu.rwlock(true) || !fd.fdmu.rwlock(false) {
+		return errClosing(fd.isFile)
+	}
+	return nil
+}
+
+// readWriteUnlock removes a reference from fd and unlocks fd for reading and writing.
+// It also closes fd when the state of fd is set to closed and there
+// is no remaining reference.
+func (fd *FD) readWriteUnlock() {
+	fd.fdmu.rwunlock(true)
+	fd.fdmu.rwunlock(false)
+}
+
 // closing returns true if fd is closing.
 func (fd *FD) closing() bool {
 	return atomic.LoadUint64(&fd.fdmu.state)&mutexClosed != 0

src/internal/poll/fd_windows.go

@@ -276,9 +276,6 @@ type FD struct {
 	// I/O poller.
 	pd pollDesc
 
-	// Used to implement pread/pwrite.
-	l sync.Mutex
-
 	// The file offset for the next read or write.
 	// Overlapped IO operations don't use the real file pointer,
 	// so we need to keep track of the offset ourselves.
@@ -316,7 +313,7 @@ type FD struct {
 }
 
 // setOffset sets the offset fields of the overlapped object
-// to the given offset. The fd.l lock must be held.
+// to the given offset. The fd read/write lock must be held.
 //
 // Overlapped IO operations don't update the offset fields
 // of the overlapped object nor the file pointer automatically,
@@ -476,13 +473,16 @@ const maxRW = 1 << 30 // 1GB is large enough and keeps subsequent reads aligned
 
 // Read implements io.Reader.
 func (fd *FD) Read(buf []byte) (int, error) {
-	if err := fd.readLock(); err != nil {
-		return 0, err
-	}
-	defer fd.readUnlock()
 	if fd.kind == kindFile {
-		fd.l.Lock()
-		defer fd.l.Unlock()
+		if err := fd.readWriteLock(); err != nil {
+			return 0, err
+		}
+		defer fd.readWriteUnlock()
+	} else {
+		if err := fd.readLock(); err != nil {
+			return 0, err
+		}
+		defer fd.readUnlock()
 	}
 
 	if len(buf) > maxRW {
@@ -609,19 +609,16 @@ func (fd *FD) Pread(b []byte, off int64) (int, error) {
 		// Pread does not work with pipes
 		return 0, syscall.ESPIPE
 	}
-	// Call incref, not readLock, because since pread specifies the
-	// offset it is independent from other reads.
-	if err := fd.incref(); err != nil {
+
+	if err := fd.readWriteLock(); err != nil {
 		return 0, err
 	}
-	defer fd.decref()
+	defer fd.readWriteUnlock()
 
 	if len(b) > maxRW {
 		b = b[:maxRW]
 	}
 
-	fd.l.Lock()
-	defer fd.l.Unlock()
 	if fd.isBlocking {
 		curoffset, err := syscall.Seek(fd.Sysfd, 0, io.SeekCurrent)
 		if err != nil {
@@ -749,13 +746,16 @@ func (fd *FD) ReadFromInet6(buf []byte, sa6 *syscall.SockaddrInet6) (int, error)
 
 // Write implements io.Writer.
 func (fd *FD) Write(buf []byte) (int, error) {
-	if err := fd.writeLock(); err != nil {
-		return 0, err
-	}
-	defer fd.writeUnlock()
 	if fd.kind == kindFile {
-		fd.l.Lock()
-		defer fd.l.Unlock()
+		if err := fd.readWriteLock(); err != nil {
+			return 0, err
+		}
+		defer fd.readWriteUnlock()
+	} else {
+		if err := fd.writeLock(); err != nil {
+			return 0, err
+		}
+		defer fd.writeUnlock()
 	}
 
 	var ntotal int
@@ -848,15 +848,12 @@ func (fd *FD) Pwrite(buf []byte, off int64) (int, error) {
 		// Pwrite does not work with pipes
 		return 0, syscall.ESPIPE
 	}
-	// Call incref, not writeLock, because since pwrite specifies the
-	// offset it is independent from other writes.
-	if err := fd.incref(); err != nil {
+
+	if err := fd.readWriteLock(); err != nil {
 		return 0, err
 	}
-	defer fd.decref()
+	defer fd.readWriteUnlock()
 
-	fd.l.Lock()
-	defer fd.l.Unlock()
 	if fd.isBlocking {
 		curoffset, err := syscall.Seek(fd.Sysfd, 0, io.SeekCurrent)
 		if err != nil {
@@ -1119,13 +1116,10 @@ func (fd *FD) Seek(offset int64, whence int) (int64, error) {
 	if fd.kind == kindPipe {
 		return 0, syscall.ESPIPE
 	}
-	if err := fd.incref(); err != nil {
+	if err := fd.readWriteLock(); err != nil {
 		return 0, err
 	}
-	defer fd.decref()
-
-	fd.l.Lock()
-	defer fd.l.Unlock()
+	defer fd.readWriteUnlock()
 
 	if !fd.isBlocking && whence == io.SeekCurrent {
 		// Windows doesn't keep the file pointer for overlapped file handles.