os: use poll.fdMutex for Plan 9 files

This permits us to safely support concurrent access to files on Plan 9.
Concurrent access was already safe on other systems.

This does introduce a change: if one goroutine calls a blocking read
on a pipe, and another goroutine closes the pipe, then before this CL
the close would occur. Now the close will be delayed until the blocking
read completes.

Also add tests that concurrent I/O and Close on a pipe are OK.

For golang#50436
For golang#56043

Change-Id: I969c869ea3b8c5c2f2ef319e441a56a3c64e7bf5
Reviewed-on: https://go-review.googlesource.com/c/go/+/438347
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: David du Colombier <0intro@gmail.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

Author: ianlancetaylor

Diff for: src/internal/poll/export_test.go

@@ -10,26 +10,26 @@ package poll
 
 var Consume = consume
 
-type FDMutex struct {
+type XFDMutex struct {
 	fdMutex
 }
 
-func (mu *FDMutex) Incref() bool {
+func (mu *XFDMutex) Incref() bool {
 	return mu.incref()
 }
 
-func (mu *FDMutex) IncrefAndClose() bool {
+func (mu *XFDMutex) IncrefAndClose() bool {
 	return mu.increfAndClose()
 }
 
-func (mu *FDMutex) Decref() bool {
+func (mu *XFDMutex) Decref() bool {
 	return mu.decref()
 }
 
-func (mu *FDMutex) RWLock(read bool) bool {
+func (mu *XFDMutex) RWLock(read bool) bool {
 	return mu.rwlock(read)
 }
 
-func (mu *FDMutex) RWUnlock(read bool) bool {
+func (mu *XFDMutex) RWUnlock(read bool) bool {
 	return mu.rwunlock(read)
 }

Diff for: src/internal/poll/fd_mutex_test.go

@@ -14,7 +14,7 @@ import (
 )
 
 func TestMutexLock(t *testing.T) {
-	var mu FDMutex
+	var mu XFDMutex
 
 	if !mu.Incref() {
 		t.Fatal("broken")
@@ -39,7 +39,7 @@ func TestMutexLock(t *testing.T) {
 }
 
 func TestMutexClose(t *testing.T) {
-	var mu FDMutex
+	var mu XFDMutex
 	if !mu.IncrefAndClose() {
 		t.Fatal("broken")
 	}
@@ -60,7 +60,7 @@ func TestMutexClose(t *testing.T) {
 
 func TestMutexCloseUnblock(t *testing.T) {
 	c := make(chan bool, 4)
-	var mu FDMutex
+	var mu XFDMutex
 	mu.RWLock(true)
 	for i := 0; i < 4; i++ {
 		go func() {
@@ -104,7 +104,7 @@ func TestMutexPanic(t *testing.T) {
 		f()
 	}
 
-	var mu FDMutex
+	var mu XFDMutex
 	ensurePanics(func() { mu.Decref() })
 	ensurePanics(func() { mu.RWUnlock(true) })
 	ensurePanics(func() { mu.RWUnlock(false) })
@@ -137,7 +137,7 @@ func TestMutexOverflowPanic(t *testing.T) {
 		}
 	}()
 
-	var mu1 FDMutex
+	var mu1 XFDMutex
 	for i := 0; i < 1<<21; i++ {
 		mu1.Incref()
 	}
@@ -152,7 +152,7 @@ func TestMutexStress(t *testing.T) {
 	}
 	defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(P))
 	done := make(chan bool, P)
-	var mu FDMutex
+	var mu XFDMutex
 	var readState [2]uint64
 	var writeState [2]uint64
 	for p := 0; p < P; p++ {

Diff for: src/internal/poll/file_plan9.go

@@ -0,0 +1,42 @@
+// Copyright 2022 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.
+
+package poll
+
+// Expose fdMutex for use by the os package on Plan 9.
+// On Plan 9 we don't want to use async I/O for file operations,
+// but we still want the locking semantics that fdMutex provides.
+
+// FDMutex is an exported fdMutex, only for Plan 9.
+type FDMutex struct {
+	fdmu fdMutex
+}
+
+func (fdmu *FDMutex) Incref() bool {
+	return fdmu.fdmu.incref()
+}
+
+func (fdmu *FDMutex) Decref() bool {
+	return fdmu.fdmu.decref()
+}
+
+func (fdmu *FDMutex) IncrefAndClose() bool {
+	return fdmu.fdmu.increfAndClose()
+}
+
+func (fdmu *FDMutex) ReadLock() bool {
+	return fdmu.fdmu.rwlock(true)
+}
+
+func (fdmu *FDMutex) ReadUnlock() bool {
+	return fdmu.fdmu.rwunlock(true)
+}
+
+func (fdmu *FDMutex) WriteLock() bool {
+	return fdmu.fdmu.rwlock(false)
+}
+
+func (fdmu *FDMutex) WriteUnlock() bool {
+	return fdmu.fdmu.rwunlock(false)
+}

Diff for: src/os/file_mutex_plan9.go

@@ -0,0 +1,70 @@
+// Copyright 2022 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.
+
+package os
+
+// File locking support for Plan 9. This uses fdMutex from the
+// internal/poll package.
+
+// incref adds a reference to the file. It returns an error if the file
+// is already closed. This method is on File so that we can incorporate
+// a nil test.
+func (f *File) incref(op string) (err error) {
+	if f == nil {
+		return ErrInvalid
+	}
+	if !f.fdmu.Incref() {
+		err = ErrClosed
+		if op != "" {
+			err = &PathError{Op: op, Path: f.name, Err: err}
+		}
+	}
+	return err
+}
+
+// decref removes a reference to the file. If this is the last
+// remaining reference, and the file has been marked to be closed,
+// then actually close it.
+func (file *file) decref() error {
+	if file.fdmu.Decref() {
+		return file.destroy()
+	}
+	return nil
+}
+
+// readLock adds a reference to the file and locks it for reading.
+// It returns an error if the file is already closed.
+func (file *file) readLock() error {
+	if !file.fdmu.ReadLock() {
+		return ErrClosed
+	}
+	return nil
+}
+
+// readUnlock removes a reference from the file and unlocks it for reading.
+// It also closes the file if it marked as closed and there is no remaining
+// reference.
+func (file *file) readUnlock() {
+	if file.fdmu.ReadUnlock() {
+		file.destroy()
+	}
+}
+
+// writeLock adds a reference to the file and locks it for writing.
+// It returns an error if the file is already closed.
+func (file *file) writeLock() error {
+	if !file.fdmu.WriteLock() {
+		return ErrClosed
+	}
+	return nil
+}
+
+// writeUnlock removes a reference from the file and unlocks it for writing.
+// It also closes the file if it is marked as closed and there is no remaining
+// reference.
+func (file *file) writeUnlock() {
+	if file.fdmu.WriteUnlock() {
+		file.destroy()
+	}
+}

Diff for: src/os/file_plan9.go

@@ -22,6 +22,7 @@ func fixLongPath(path string) string {
 // can overwrite this data, which could cause the finalizer
 // to close the wrong file descriptor.
 type file struct {
+	fdmu       poll.FDMutex
 	fd         int
 	name       string
 	dirinfo    *dirInfo // nil unless directory being read
@@ -142,24 +143,35 @@ func openFileNolog(name string, flag int, perm FileMode) (*File, error) {
 // be canceled and return immediately with an ErrClosed error.
 // Close will return an error if it has already been called.
 func (f *File) Close() error {
-	if err := f.checkValid("close"); err != nil {
-		return err
+	if f == nil {
+		return ErrInvalid
 	}
 	return f.file.close()
 }
 
 func (file *file) close() error {
-	if file == nil || file.fd == badFd {
-		return ErrInvalid
+	if !file.fdmu.IncrefAndClose() {
+		return &PathError{Op: "close", Path: file.name, Err: ErrClosed}
 	}
+
+	// At this point we should cancel any pending I/O.
+	// How do we do that on Plan 9?
+
+	err := file.decref()
+
+	// no need for a finalizer anymore
+	runtime.SetFinalizer(file, nil)
+	return err
+}
+
+// destroy actually closes the descriptor. This is called when
+// there are no remaining references, by the decref, readUnlock,
+// and writeUnlock methods.
+func (file *file) destroy() error {
 	var err error
 	if e := syscall.Close(file.fd); e != nil {
 		err = &PathError{Op: "close", Path: file.name, Err: e}
 	}
-	file.fd = badFd // so it can't be closed again
-
-	// no need for a finalizer anymore
-	runtime.SetFinalizer(file, nil)
 	return err
 }
 
@@ -193,6 +205,12 @@ func (f *File) Truncate(size int64) error {
 	if err != nil {
 		return &PathError{Op: "truncate", Path: f.name, Err: err}
 	}
+
+	if err := f.incref("truncate"); err != nil {
+		return err
+	}
+	defer f.decref()
+
 	if err = syscall.Fwstat(f.fd, buf[:n]); err != nil {
 		return &PathError{Op: "truncate", Path: f.name, Err: err}
 	}
@@ -219,6 +237,12 @@ func (f *File) chmod(mode FileMode) error {
 	if err != nil {
 		return &PathError{Op: "chmod", Path: f.name, Err: err}
 	}
+
+	if err := f.incref("chmod"); err != nil {
+		return err
+	}
+	defer f.decref()
+
 	if err = syscall.Fwstat(f.fd, buf[:n]); err != nil {
 		return &PathError{Op: "chmod", Path: f.name, Err: err}
 	}
@@ -240,6 +264,12 @@ func (f *File) Sync() error {
 	if err != nil {
 		return &PathError{Op: "sync", Path: f.name, Err: err}
 	}
+
+	if err := f.incref("sync"); err != nil {
+		return err
+	}
+	defer f.decref()
+
 	if err = syscall.Fwstat(f.fd, buf[:n]); err != nil {
 		return &PathError{Op: "sync", Path: f.name, Err: err}
 	}
@@ -249,6 +279,10 @@ func (f *File) Sync() error {
 // read reads up to len(b) bytes from the File.
 // It returns the number of bytes read and an error, if any.
 func (f *File) read(b []byte) (n int, err error) {
+	if err := f.readLock(); err != nil {
+		return 0, err
+	}
+	defer f.readUnlock()
 	n, e := fixCount(syscall.Read(f.fd, b))
 	if n == 0 && len(b) > 0 && e == nil {
 		return 0, io.EOF
@@ -260,6 +294,10 @@ func (f *File) read(b []byte) (n int, err error) {
 // It returns the number of bytes read and the error, if any.
 // EOF is signaled by a zero count with err set to nil.
 func (f *File) pread(b []byte, off int64) (n int, err error) {
+	if err := f.readLock(); err != nil {
+		return 0, err
+	}
+	defer f.readUnlock()
 	n, e := fixCount(syscall.Pread(f.fd, b, off))
 	if n == 0 && len(b) > 0 && e == nil {
 		return 0, io.EOF
@@ -272,6 +310,10 @@ func (f *File) pread(b []byte, off int64) (n int, err error) {
 // Since Plan 9 preserves message boundaries, never allow
 // a zero-byte write.
 func (f *File) write(b []byte) (n int, err error) {
+	if err := f.writeLock(); err != nil {
+		return 0, err
+	}
+	defer f.writeUnlock()
 	if len(b) == 0 {
 		return 0, nil
 	}
@@ -283,6 +325,10 @@ func (f *File) write(b []byte) (n int, err error) {
 // Since Plan 9 preserves message boundaries, never allow
 // a zero-byte write.
 func (f *File) pwrite(b []byte, off int64) (n int, err error) {
+	if err := f.writeLock(); err != nil {
+		return 0, err
+	}
+	defer f.writeUnlock()
 	if len(b) == 0 {
 		return 0, nil
 	}
@@ -294,6 +340,10 @@ func (f *File) pwrite(b []byte, off int64) (n int, err error) {
 // relative to the current offset, and 2 means relative to the end.
 // It returns the new offset and an error, if any.
 func (f *File) seek(offset int64, whence int) (ret int64, err error) {
+	if err := f.incref(""); err != nil {
+		return 0, err
+	}
+	defer f.decref()
 	if f.dirinfo != nil {
 		// Free cached dirinfo, so we allocate a new one if we
 		// access this file as a directory again. See #35767 and #37161.
@@ -493,9 +543,10 @@ func tempDir() string {
 // which must be a directory.
 // If there is an error, it will be of type *PathError.
 func (f *File) Chdir() error {
-	if err := f.checkValid("chdir"); err != nil {
+	if err := f.incref("chdir"); err != nil {
 		return err
 	}
+	defer f.decref()
 	if e := syscall.Fchdir(f.fd); e != nil {
 		return &PathError{Op: "chdir", Path: f.name, Err: e}
 	}
@@ -526,16 +577,17 @@ func (f *File) setWriteDeadline(time.Time) error {
 	return poll.ErrNoDeadline
 }
 
-// checkValid checks whether f is valid for use.
-// If not, it returns an appropriate error, perhaps incorporating the operation name op.
+// checkValid checks whether f is valid for use, but does not prepare
+// to actually use it. If f is not ready checkValid returns an appropriate
+// error, perhaps incorporating the operation name op.
 func (f *File) checkValid(op string) error {
 	if f == nil {
 		return ErrInvalid
 	}
-	if f.fd == badFd {
-		return &PathError{Op: op, Path: f.name, Err: ErrClosed}
+	if err := f.incref(op); err != nil {
+		return err
 	}
-	return nil
+	return f.decref()
 }
 
 type rawConn struct{}