internal/poll: if poller init fails, assume blocking mode

Fixes #23943

Change-Id: I16e604872f1615963925ec3c4710106bcce1330c
Reviewed-on: https://go-review.googlesource.com/99015
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/internal/poll/fd_unix.go

@@ -59,7 +59,13 @@ func (fd *FD) Init(net string, pollable bool) error {
 		fd.isBlocking = true
 		return nil
 	}
-	return fd.pd.init(fd)
+	err := fd.pd.init(fd)
+	if err != nil {
+		// If we could not initialize the runtime poller,
+		// assume we are using blocking mode.
+		fd.isBlocking = true
+	}
+	return err
 }
 
 // Destroy closes the file descriptor. This is called when there are

src/os/timeout_test.go

@@ -587,3 +587,36 @@ func TestRacyWrite(t *testing.T) {
 		}()
 	}
 }
+
+// Closing a TTY while reading from it should not hang.  Issue 23943.
+func TestTTYClose(t *testing.T) {
+	f, err := os.Open("/dev/tty")
+	if err != nil {
+		t.Skipf("skipping because opening /dev/tty failed: %v", err)
+	}
+
+	go func() {
+		var buf [1]byte
+		f.Read(buf[:])
+	}()
+
+	// Give the goroutine a chance to enter the read.
+	// It doesn't matter much if it occasionally fails to do so,
+	// we won't be testing what we want to test but the test will pass.
+	time.Sleep(time.Millisecond)
+
+	c := make(chan bool)
+	go func() {
+		defer close(c)
+		f.Close()
+	}()
+
+	select {
+	case <-c:
+	case <-time.After(time.Second):
+		t.Error("timed out waiting for close")
+	}
+
+	// On some systems the goroutines may now be hanging.
+	// There's not much we can do about that.
+}