x/crypto/ssh: interpret disconnect message as error in the transport layer.

This ensures that higher level parts (e.g. the client authentication
loop) never have to deal with disconnect messages.

Fixes https://github.com/coreos/fleet/issues/565.

Change-Id: Ie164b6c4b0982c7ed9af6d3bf91697a78a911a20
Reviewed-on: https://go-review.googlesource.com/20801
Reviewed-by: Anton Khramov <anton@endocode.com>
Reviewed-by: Adam Langley <agl@golang.org>

ssh/handshake_test.go

@@ -10,6 +10,7 @@ import (
 	"errors"
 	"fmt"
 	"net"
+	"reflect"
 	"runtime"
 	"strings"
 	"sync"
@@ -413,3 +414,45 @@ func testHandshakeErrorHandlingN(t *testing.T, readLimit, writeLimit int) {
 
 	wg.Wait()
 }
+
+func TestDisconnect(t *testing.T) {
+	if runtime.GOOS == "plan9" {
+		t.Skip("see golang.org/issue/7237")
+	}
+	checker := &testChecker{}
+	trC, trS, err := handshakePair(&ClientConfig{HostKeyCallback: checker.Check}, "addr")
+	if err != nil {
+		t.Fatalf("handshakePair: %v", err)
+	}
+
+	defer trC.Close()
+	defer trS.Close()
+
+	trC.writePacket([]byte{msgRequestSuccess, 0, 0})
+	errMsg := &disconnectMsg{
+		Reason: 42,
+		Message: "such is life",
+	}
+	trC.writePacket(Marshal(errMsg))
+	trC.writePacket([]byte{msgRequestSuccess, 0, 0})
+
+	packet, err := trS.readPacket()
+	if err != nil {
+		t.Fatalf("readPacket 1: %v", err)
+	}
+	if packet[0] != msgRequestSuccess {
+		t.Errorf("got packet %v, want packet type %d", packet,  msgRequestSuccess)
+	}
+
+	_, err = trS.readPacket()
+	if err == nil {
+		t.Errorf("readPacket 2 succeeded")
+	} else if !reflect.DeepEqual(err, errMsg) {
+		t.Errorf("got error %#v, want %#v", err, errMsg)
+	}
+
+	_, err = trS.readPacket()
+	if err == nil {
+		t.Errorf("readPacket 3 succeeded")
+	}
+}