Only close epoller FD at most once.

sipsma · sipsma · commit 38c5469e7522 · 2019-12-18T18:55:43.000Z
Previously, multiple calls to epoller.Close() could result in unix.Close being
called on the same FD value multiple times. This made it easy for consumers to
create race conditions where calls to Close after the first one were closing
unrelated FDs in the process that happened to use the same value of the epoller
FD.

This change uses sync.Once to ensure that unix.Close is only called once, with
subsequent calls to Close returning "file already closed" instead.

Signed-off-by: Erik Sipsma &lt;erik@sipsma.dev&gt;
diff --git a/console_linux.go b/console_linux.go
@@ -58,6 +58,7 @@ type Epoller struct {
 	efd       int
 	mu        sync.Mutex
 	fdMapping map[int]*EpollConsole
+	closeOnce sync.Once
 }
 
 // NewEpoller returns an instance of epoller with a valid epoll fd.
@@ -151,7 +152,11 @@ func (e *Epoller) getConsole(sysfd int) *EpollConsole {
 
 // Close closes the epoll fd
 func (e *Epoller) Close() error {
-	return unix.Close(e.efd)
+	closeErr := os.ErrClosed // default to "file already closed"
+	e.closeOnce.Do(func() {
+		closeErr = unix.Close(e.efd)
+	})
+	return closeErr
 }
 
 // EpollConsole acts like a console but registers its file descriptor with an
diff --git a/console_linux_test.go b/console_linux_test.go
@@ -85,4 +85,12 @@ func TestEpollConsole(t *testing.T) {
 	if out := b.String(); out != expectedOutput {
 		t.Errorf("unexpected output %q", out)
 	}
+
+	// make sure multiple Close calls return os.ErrClosed after the first
+	if err := epoller.Close(); err != nil {
+		t.Fatal(err)
+	}
+	if err := epoller.Close(); err != os.ErrClosed {
+		t.Fatalf("unexpected error returned from second call to epoller.Close(): %v", err)
+	}
 }

Original file line number	Diff line number	Diff line change
`@@ -85,4 +85,12 @@ func TestEpollConsole(t *testing.T) {`
`85`	`85`	`if out := b.String(); out != expectedOutput {`
`86`	`86`	`t.Errorf("unexpected output %q", out)`
`87`	`87`	`}`
	`88`	`+`
	`89`	`+ // make sure multiple Close calls return os.ErrClosed after the first`
	`90`	`+ if err := epoller.Close(); err != nil {`
	`91`	`+ t.Fatal(err)`
	`92`	`+ }`
	`93`	`+ if err := epoller.Close(); err != os.ErrClosed {`
	`94`	`+ t.Fatalf("unexpected error returned from second call to epoller.Close(): %v", err)`
	`95`	`+ }`
`88`	`96`	`}`