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 #50436
For #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>

3 files changed, 54 insertions, 12 deletions

diff --git a/src/internal/poll/export_test.go b/src/internal/poll/export_test.go
index 02664d9ea3..66d7c3274b 100644
--- a/src/internal/poll/export_test.go
+++ b/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 --git a/src/internal/poll/fd_mutex_test.go b/src/internal/poll/fd_mutex_test.go
index 3029b9a681..62f953192d 100644
--- a/src/internal/poll/fd_mutex_test.go
+++ b/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 --git a/src/internal/poll/file_plan9.go b/src/internal/poll/file_plan9.go
new file mode 100644
index 0000000000..57dc0c668f
--- /dev/null
+++ b/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)
+}