vm: don't call Diagnose when VM hasn't crashed

Fixes #875

2 files changed, 22 insertions, 9 deletions

diff --git a/vm/vm.go b/vm/vm.go
index 26bd21745..b403956a6 100644
--- a/vm/vm.go
+++ b/vm/vm.go
@@ -230,10 +230,13 @@ type monitor struct {
 }
 
 func (mon *monitor) extractError(defaultError string) *report.Report {
+	crashed := defaultError != "" || !mon.canExit
+	if crashed {
+		mon.inst.Diagnose()
+	}
 	// Give it some time to finish writing the error message.
-	mon.inst.Diagnose()
 	mon.waitForOutput()
-	if bytes.Contains(mon.output, []byte(fuzzerPreemptedStr)) {
+	if bytes.Contains(mon.output, []byte("SYZ-FUZZER: PREEMPTED")) {
 		return nil
 	}
 	if !mon.reporter.ContainsCrash(mon.output[mon.matchPos:]) {
@@ -250,6 +253,9 @@ func (mon *monitor) extractError(defaultError string) *report.Report {
 		}
 		return rep
 	}
+	if !crashed && mon.inst.Diagnose() {
+		mon.waitForOutput()
+	}
 	rep := mon.reporter.Parse(mon.output[mon.matchPos:])
 	if rep == nil {
 		panic(fmt.Sprintf("reporter.ContainsCrash/Parse disagree:\n%s", mon.output[mon.matchPos:]))
diff --git a/vm/vm_test.go b/vm/vm_test.go
index 1c66166f7..626ad62cb 100644
--- a/vm/vm_test.go
+++ b/vm/vm_test.go
@@ -107,11 +107,16 @@ var tests = []*Test{
 		Body: func(outc chan []byte, errc chan error) {
 			errc <- nil
 		},
+	},
+	{
+		Name: "#875-diagnose-bugs-2",
+		Body: func(outc chan []byte, errc chan error) {
+			errc <- nil
+		},
 		Report: &report.Report{
-			Title: "BUG: DIAGNOSE",
-			// TODO: this is wrong.
-			Report: []byte(
-				"BUG: DIAGNOSE\n",
+			Title: lostConnectionCrash,
+			Output: []byte(
+				"DIAGNOSE\n",
 			),
 		},
 	},
@@ -148,10 +153,9 @@ var tests = []*Test{
 		},
 		Report: &report.Report{
 			Title: "BUG: bad",
-			// TODO: this is wrong.
 			Report: []byte(
-				"DIAGNOSE\n" +
-					"BUG: bad\n",
+				"BUG: bad\n" +
+					"DIAGNOSE\n",
 			),
 		},
 	},
@@ -298,4 +302,7 @@ func testMonitorExecution(t *testing.T, test *Test) {
 	if !bytes.Equal(test.Report.Report, rep.Report) {
 		t.Fatalf("want report:\n%s\n\ngot report:\n%s\n", test.Report.Report, rep.Report)
 	}
+	if test.Report.Output != nil && !bytes.Equal(test.Report.Output, rep.Output) {
+		t.Fatalf("want output:\n%s\n\ngot output:\n%s\n", test.Report.Output, rep.Output)
+	}
 }