commit f8c69a179645d6b19ee60c05c3ab4e6725a49ccc
from: Noel Cower <ncower@gmail.com>
date: Thu Jul 21 00:12:06 2016 UTC

Fix race-y channel close/error assignment

Data race occurred when assigning to err and then subsequently reading
it from anywhere else. Moving the channel close down could potentially
help, but far easier to just gate both with a sync.Once since this
isn't a code path that seems like it needs to be super-performant.

Changes cause CloseWithError to be call-able once, ensuring that
there's no case where err is written to while being read (as would
happen when closing and reading c.err from another goroutine the moment
c.closed is closed).

Signed-off-by: Noel Cower <ncower@gmail.com>

commit - 87c6cf4550e8bf9dac847f1c6437e453aadef7c8
commit + f8c69a179645d6b19ee60c05c3ab4e6725a49ccc
blob - 80b1e9479bb7f916e95e8c9e0734ed4a889ac404
blob + 2c5ec955994e20ddbba22d86b7c3b9c475d23344
--- server.go
+++ server.go
@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"log"
 	"net"
+	"sync"
 	"time"
 
 	"golang.org/x/net/context"
@@ -53,8 +54,10 @@ type conn struct {
 	session Session
 	ch      Channel
 	handler Handler
-	closed  chan struct{}
-	err     error // terminal error for the conn
+
+	once   sync.Once
+	closed chan struct{}
+	err    error // terminal error for the conn
 }
 
 // activeRequest includes information about the active request.
@@ -233,17 +236,14 @@ func (c *conn) Close() error {
 }
 
 func (c *conn) CloseWithError(err error) error {
-	select {
-	case <-c.closed:
-		return c.err
-	default:
-		close(c.closed)
+	c.once.Do(func() {
 		if err == nil {
-			c.err = err
-		} else {
-			c.err = ErrClosed
+			err = ErrClosed
 		}
 
-		return c.err
-	}
+		c.err = err
+		close(c.closed)
+	})
+
+	return c.err
 }