net: only remove Unix domain socket file on the first call to Close

Fixes #17131.

Change-Id: I60b381687746fadce12ef18a190cbe3f435172f2
Reviewed-on: https://go-review.googlesource.com/32098
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Quentin Smith <quentin@golang.org>

src/net/unixsock.go

@@ -7,6 +7,7 @@ package net
 import (
 	"context"
 	"os"
+	"sync"
 	"syscall"
 	"time"
 )
@@ -206,9 +207,10 @@ func DialUnix(net string, laddr, raddr *UnixAddr) (*UnixConn, error) {
 // typically use variables of type Listener instead of assuming Unix
 // domain sockets.
 type UnixListener struct {
-	fd     *netFD
-	path   string
-	unlink bool
+	fd         *netFD
+	path       string
+	unlink     bool
+	unlinkOnce sync.Once
 }
 
 func (ln *UnixListener) ok() bool { return ln != nil && ln.fd != nil }

src/net/unixsock_posix.go

@@ -173,9 +173,12 @@ func (ln *UnixListener) close() error {
 	// is at least compatible with the auto-remove
 	// sequence in ListenUnix. It's only non-Go
 	// programs that can mess us up.
-	if ln.path[0] != '@' && ln.unlink {
-		syscall.Unlink(ln.path)
-	}
+	// Even if there are racy calls to Close, we want to unlink only for the first one.
+	ln.unlinkOnce.Do(func() {
+		if ln.path[0] != '@' && ln.unlink {
+			syscall.Unlink(ln.path)
+		}
+	})
 	return ln.fd.Close()
 }
 

src/net/unixsock_test.go

@@ -9,6 +9,7 @@ package net
 import (
 	"bytes"
 	"internal/testenv"
+	"io/ioutil"
 	"os"
 	"reflect"
 	"runtime"
@@ -443,4 +444,15 @@ func TestUnixUnlink(t *testing.T) {
 	if _, err := os.Stat(name); err == nil {
 		t.Fatal("closing unix listener did not remove unix socket")
 	}
+	if err := ioutil.WriteFile(name, []byte("hello world"), 0666); err != nil {
+		t.Fatalf("cannot recreate socket file: %v", err)
+	}
+	if _, err := os.Stat(name); err != nil {
+		t.Fatal("recreating unix listener as file failed: %v", err)
+	}
+	l.Close()
+	if _, err := os.Stat(name); err != nil {
+		t.Fatalf("second close of unix socket did second remove: %v", err)
+	}
+	os.Remove(name)
 }