log/syslog: fix flakey test on slow hosts

Fixes #4467.

The syslog tests can fail if the timeout fires before the data arrives at the mock server. Moving the timeout onto the goroutine that is calling ReadFrom() and always processing the data returned before handling the error should improve the reliability of the test.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/6920047

src/pkg/log/syslog/syslog_test.go

@@ -20,13 +20,14 @@ var serverAddr string
 
 func runSyslog(c net.PacketConn, done chan<- string) {
 	var buf [4096]byte
-	var rcvd string = ""
+	var rcvd string
 	for {
-		n, _, err := c.ReadFrom(buf[0:])
-		if err != nil || n == 0 {
+		c.SetReadDeadline(time.Now().Add(100 * time.Millisecond))
+		n, _, err := c.ReadFrom(buf[:])
+		rcvd += string(buf[:n])
+		if err != nil {
 			break
 		}
-		rcvd += string(buf[0:n])
 	}
 	done <- rcvd
 }
@@ -37,7 +38,6 @@ func startServer(done chan<- string) {
 		log.Fatalf("net.ListenPacket failed udp :0 %v", e)
 	}
 	serverAddr = c.LocalAddr().String()
-	c.SetReadDeadline(time.Now().Add(100 * time.Millisecond))
 	go runSyslog(c, done)
 }