net: return error from pollster rather than panicing

Fixes #3590.

R=bradfitz, mikioh.mikioh, iant, bsiegert
CC=golang-dev
https://golang.org/cl/6684054

src/pkg/net/fd_unix.go

@@ -97,15 +97,11 @@ func (s *pollServer) AddFD(fd *netFD, mode int) error {
 	}
 
 	wake, err := s.poll.AddFD(intfd, mode, false)
+	s.Unlock()
 	if err != nil {
-		panic("pollServer AddFD " + err.Error())
-	}
-	if wake {
-		doWakeup = true
+		return &OpError{"addfd", fd.net, fd.laddr, err}
 	}
-	s.Unlock()
-
-	if doWakeup {
+	if wake || doWakeup {
 		s.Wakeup()
 	}
 	return nil

src/pkg/net/fd_unix_test.go

+// Copyright 2012 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 freebsd linux netbsd openbsd
+
+package net
+
+import (
+	"testing"
+)
+
+// Issue 3590. netFd.AddFD should return an error 
+// from the underlying pollster rather than panicing.
+func TestAddFDReturnsError(t *testing.T) {
+	l, err := Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer l.Close()
+
+	go func() {
+		for {
+			c, err := l.Accept()
+			if err != nil {
+				return
+			}
+			defer c.Close()
+		}
+	}()
+
+	c, err := Dial("tcp", l.Addr().String())
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer c.Close()
+
+	// replace c's pollServer with a closed version.
+	ps, err := newPollServer()
+	if err != nil {
+		t.Fatal(err)
+	}
+	ps.poll.Close()
+	c.(*TCPConn).conn.fd.pollServer = ps
+
+	var b [1]byte
+	_, err = c.Read(b[:])
+	if err, ok := err.(*OpError); ok {
+		if err.Op == "addfd" {
+			return
+		}
+		if err, ok := err.Err.(*OpError); ok {
+			// the err is sometimes wrapped by another OpError
+			if err.Op == "addfd" {
+				return
+			}
+		}
+	}
+	t.Error(err)
+}