runtime: for kqueue treat EVFILT_READ with EV_EOF as permitting a write

On systems that use kqueue, we always register descriptors for both
EVFILT_READ and EVFILT_WRITE. On at least FreeBSD and OpenBSD, when
the write end of a pipe is registered for EVFILT_READ and EVFILT_WRITE
events, and the read end of the pipe is closed, kqueue reports an
EVFILT_READ event with EV_EOF set, but does not report an EVFILT_WRITE
event. Since the write to the pipe is waiting for an EVFILT_WRITE
event, closing the read end of a pipe can cause the write end to hang
rather than attempt another write which will fail with EPIPE.

Fix this by treating EVFILT_READ with EV_EOF set as making both reads
and writes ready to proceed.

The real test for this is in CL 71770, which tests using various
timeouts with pipes.

Updates #22114

Change-Id: Ib23fbaaddbccd8eee77bdf18f27a7f0aa50e2742
Reviewed-on: https://go-review.googlesource.com/71973
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

1 files changed, 66 insertions, 0 deletions

diff --git a/src/net/write_unix_test.go b/src/net/write_unix_test.go
new file mode 100644
index 0000000000..6d8cb6a6f8
--- /dev/null
+++ b/src/net/write_unix_test.go
@@ -0,0 +1,66 @@
+// Copyright 2017 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.
+
+// +build darwin dragonfly freebsd linux netbsd openbsd solaris
+
+package net
+
+import (
+	"bytes"
+	"syscall"
+	"testing"
+	"time"
+)
+
+// Test that a client can't trigger an endless loop of write system
+// calls on the server by shutting down the write side on the client.
+// Possibility raised in the discussion of https://golang.org/cl/71973.
+func TestEndlessWrite(t *testing.T) {
+	t.Parallel()
+	c := make(chan bool)
+	server := func(cs *TCPConn) error {
+		cs.CloseWrite()
+		<-c
+		return nil
+	}
+	client := func(ss *TCPConn) error {
+		// Tell the server to return when we return.
+		defer close(c)
+
+		// Loop writing to the server. The server is not reading
+		// anything, so this will eventually block, and then time out.
+		b := bytes.Repeat([]byte{'a'}, 8192)
+		cagain := 0
+		for {
+			n, err := ss.conn.fd.pfd.WriteOnce(b)
+			if n > 0 {
+				cagain = 0
+			}
+			switch err {
+			case nil:
+			case syscall.EAGAIN:
+				if cagain == 0 {
+					// We've written enough data to
+					// start blocking. Set a deadline
+					// so that we will stop.
+					ss.SetWriteDeadline(time.Now().Add(5 * time.Millisecond))
+				}
+				cagain++
+				if cagain > 20 {
+					t.Error("looping on EAGAIN")
+					return nil
+				}
+				if err = ss.conn.fd.pfd.WaitWrite(); err != nil {
+					t.Logf("client WaitWrite: %v", err)
+					return nil
+				}
+			default:
+				// We expect to eventually get an error.
+				t.Logf("client WriteOnce: %v", err)
+				return nil
+			}
+		}
+	}
+	withTCPConnPair(t, client, server)
+}