vm: fix spurious crash detection caused by trimmed lines

We've got a case when "ODEBUG:" was incorrectly detected as crash.
That was caused by a flaw in matchPos logic. Fix that.
See the added test for details.

2 files changed, 34 insertions, 2 deletions

diff --git a/vm/vm.go b/vm/vm.go
index 28eddeef0..96298912b 100644
--- a/vm/vm.go
+++ b/vm/vm.go
@@ -209,7 +209,19 @@ func (inst *Instance) MonitorExecution(outc <-chan []byte, errc <-chan error,
 				copy(mon.output, mon.output[len(mon.output)-beforeContext:])
 				mon.output = mon.output[:beforeContext]
 			}
+			// Find the starting position for crash matching on the next iteration.
+			// We step back from the end of output by maxErrorLength to handle the case
+			// when a crash line is currently split/incomplete. And then we try to find
+			// the preceeding '\n' to have a full line. This is required to handle
+			// the case when a particular pattern is ignored as crash, but a suffix
+			// of the pattern is detected as crash (e.g. "ODEBUG:" is trimmed to "BUG:").
 			mon.matchPos = len(mon.output) - maxErrorLength
+			for i := 0; i < maxErrorLength; i++ {
+				if mon.matchPos <= 0 || mon.output[mon.matchPos-1] == '\n' {
+					break
+				}
+				mon.matchPos--
+			}
 			if mon.matchPos < 0 {
 				mon.matchPos = 0
 			}
@@ -330,7 +342,7 @@ func (mon *monitor) waitForOutput() {
 }
 
 const (
-	maxErrorLength = 512
+	maxErrorLength = 256
 
 	lostConnectionCrash  = "lost connection to test machine"
 	noOutputCrash        = "no output from test machine"
diff --git a/vm/vm_test.go b/vm/vm_test.go
index e55cf2184..73c3198a0 100644
--- a/vm/vm_test.go
+++ b/vm/vm_test.go
@@ -70,7 +70,7 @@ func (inst *testInstance) Close() {
 }
 
 func init() {
-	beforeContext = 200
+	beforeContext = maxErrorLength + 100
 	tickerPeriod = 1 * time.Second
 	NoOutputTimeout = 5 * time.Second
 	waitForOutputTimeout = 3 * time.Second
@@ -275,6 +275,26 @@ var tests = []*Test{
 			errc <- vmimpl.ErrTimeout
 		},
 	},
+	{
+		Name: "split-line",
+		Exit: ExitNormal,
+		Body: func(outc chan []byte, errc chan error) {
+			// "ODEBUG:" lines should be ignored, however the matchPos logic
+			// used to trim the lines so that we could see just "BUG:" later
+			// and detect it as crash.
+			buf := new(bytes.Buffer)
+			for i := 0; i < 50; i++ {
+				buf.WriteString("[ 2886.597572] ODEBUG: Out of memory. ODEBUG disabled\n")
+				buf.Write(bytes.Repeat([]byte{'-'}, i))
+				buf.WriteByte('\n')
+			}
+			output := buf.Bytes()
+			for i := range output {
+				outc <- output[i : i+1]
+			}
+			errc <- nil
+		},
+	},
 }
 
 func TestMonitorExecution(t *testing.T) {