proc/gdbserial: backward continue should stop at start of process

ContinueOnce didn't detect the way RR signals that it has reached the
start of the process and would never finish.

Fixes #1376

pkg/proc/gdbserial/gdbserver.go

@@ -697,6 +697,14 @@ continueLoop:
 		// process so we must stop here.
 		case 0x91, 0x92, 0x93, 0x94, 0x95, 0x96: /* TARGET_EXC_BAD_ACCESS */
 			break continueLoop
+
+		// Signal 0 is returned by rr when it reaches the start of the process
+		// in backward continue mode.
+		case 0:
+			if p.conn.direction == proc.Backward {
+				break continueLoop
+			}
+
 		default:
 			// any other signal is always propagated to inferior
 		}

pkg/proc/gdbserial/rr_test.go

 package gdbserial_test
 
 import (
+	"flag"
 	"fmt"
 	"os"
 	"os/exec"
@@ -8,12 +9,17 @@ import (
 	"runtime"
 	"testing"
 
+	"github.com/derekparker/delve/pkg/logflags"
 	"github.com/derekparker/delve/pkg/proc"
 	"github.com/derekparker/delve/pkg/proc/gdbserial"
 	protest "github.com/derekparker/delve/pkg/proc/test"
 )
 
 func TestMain(m *testing.M) {
+	var logConf string
+	flag.StringVar(&logConf, "log", "", "configures logging")
+	flag.Parse()
+	logflags.Setup(logConf != "", logConf)
 	os.Exit(protest.RunTestsWithFixtures(m))
 }
 
@@ -251,3 +257,16 @@ func TestCheckpoints(t *testing.T) {
 		}
 	})
 }
+
+func TestIssue1376(t *testing.T) {
+	// Backward Continue should terminate when it encounters the start of the process.
+	protest.AllowRecording(t)
+	withTestRecording("continuetestprog", t, func(p *gdbserial.Process, fixture protest.Fixture) {
+		bp := setFunctionBreakpoint(p, t, "main.main")
+		assertNoError(proc.Continue(p), t, "Continue (forward)")
+		_, err := p.ClearBreakpoint(bp.Addr)
+		assertNoError(err, t, "ClearBreakpoint")
+		assertNoError(p.Direction(proc.Backward), t, "Switching to backward direction")
+		assertNoError(proc.Continue(p), t, "Continue (backward)")
+	})
+}