internal/poll: don't return from Close until descriptor is closed

This permits the program to reliably know that when the Close method
returns, the descriptor has definitely been closed. This matters at
least for listeners.

Fixes #21856
Updates #7970

Change-Id: I1fd0cfd2333649e6e67c6ae956e19fdff3a35a83
Reviewed-on: https://go-review.googlesource.com/66150
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Joe Tsai <joetsai@google.com>

src/internal/poll/fd_unix.go

@@ -26,6 +26,9 @@ type FD struct {
 	// Writev cache.
 	iovecs *[]syscall.Iovec
 
+	// Semaphore signaled when file is closed.
+	csema uint32
+
 	// Whether this is a streaming descriptor, as opposed to a
 	// packet-based descriptor like a UDP socket. Immutable.
 	IsStream bool
@@ -62,6 +65,7 @@ func (fd *FD) destroy() error {
 	fd.pd.close()
 	err := CloseFunc(fd.Sysfd)
 	fd.Sysfd = -1
+	runtime_Semrelease(&fd.csema)
 	return err
 }
 
@@ -79,7 +83,11 @@ func (fd *FD) Close() error {
 	fd.pd.evict()
 	// The call to decref will call destroy if there are no other
 	// references.
-	return fd.decref()
+	err := fd.decref()
+	// Wait until the descriptor is closed. If this was the only
+	// reference, it is already closed.
+	runtime_Semacquire(&fd.csema)
+	return err
 }
 
 // Shutdown wraps the shutdown network call.

src/internal/poll/fd_windows.go

@@ -278,6 +278,9 @@ type FD struct {
 	readbyte       []byte   // buffer to hold decoding of readuint16 from utf16 to utf8
 	readbyteOffset int      // readbyte[readOffset:] is yet to be consumed with file.Read
 
+	// Semaphore signaled when file is closed.
+	csema uint32
+
 	skipSyncNotif bool
 
 	// Whether this is a streaming descriptor, as opposed to a
@@ -399,6 +402,7 @@ func (fd *FD) destroy() error {
 		err = CloseFunc(fd.Sysfd)
 	}
 	fd.Sysfd = syscall.InvalidHandle
+	runtime_Semrelease(&fd.csema)
 	return err
 }
 
@@ -410,7 +414,11 @@ func (fd *FD) Close() error {
 	}
 	// unblock pending reader and writer
 	fd.pd.evict()
-	return fd.decref()
+	err := fd.decref()
+	// Wait until the descriptor is closed. If this was the only
+	// reference, it is already closed.
+	runtime_Semacquire(&fd.csema)
+	return err
 }
 
 // Shutdown wraps the shutdown network call.

src/net/listen_test.go

@@ -13,6 +13,7 @@ import (
 	"runtime"
 	"syscall"
 	"testing"
+	"time"
 )
 
 func (ln *TCPListener) port() string {
@@ -696,3 +697,34 @@ func multicastRIBContains(ip IP) (bool, error) {
 	}
 	return false, nil
 }
+
+// Issue 21856.
+func TestClosingListener(t *testing.T) {
+	listener, err := Listen("tcp", ":0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	addr := listener.Addr()
+
+	go func() {
+		for {
+			c, err := listener.Accept()
+			if err != nil {
+				return
+			}
+			c.Close()
+		}
+	}()
+
+	// Let the goroutine start. We don't sleep long: if the
+	// goroutine doesn't start, the test will pass without really
+	// testing anything, which is OK.
+	time.Sleep(time.Millisecond)
+
+	listener.Close()
+
+	_, err = Listen("tcp", addr.String())
+	if err != nil {
+		t.Error(err)
+	}
+}