fix: detect a panic header split across reads

CHANGELOG.md

@@ -1,3 +1,9 @@
+## Unreleased
+
+### Bug fixes
+
+* Detect a panic header that is split across stderr reads
+
 ## 1.3.4 (2021-08-10)
 
 ### Bug fixes

panicwrap.go

@@ -264,7 +264,17 @@ func trackPanic(r io.Reader, w io.Writer, dur time.Duration, result chan<- strin
 		[]byte("panic:"),
 		[]byte("fatal error:"),
 	}
+	// A header can start in one read and finish in the next. Hold back
+	// one byte less than the longest header so that tail can be joined
+	// with the following read before it is treated as ordinary output.
+	keep := 0
+	for _, header := range panicHeaders {
+		if len(header)-1 > keep {
+			keep = len(header) - 1
+		}
+	}
 	panicType := -1
+	var carry []byte
 
 	tempBuf := make([]byte, 2048)
 	for {
@@ -288,17 +298,31 @@ func trackPanic(r io.Reader, w io.Writer, dur time.Duration, result chan<- strin
 			var err error
 			buf = tempBuf
 			n, err = r.Read(buf)
-			if n <= 0 && err == io.EOF {
+			if n > 0 && panicTimer == nil && len(carry) > 0 {
+				merged := make([]byte, 0, len(carry)+n)
+				merged = append(merged, carry...)
+				merged = append(merged, buf[:n]...)
+				buf = merged
+				n = len(merged)
+				carry = nil
+			}
+			if n <= 0 && err != nil {
 				if panicBuf.Len() > 0 {
 					// We were tracking a panic, assume it was a panic
 					// and return that as the result.
 					result <- panicBuf.String()
+				} else if len(carry) > 0 {
+					w.Write(carry)
 				}
 
 				return
 			}
 		}
 
+		if n <= 0 {
+			continue
+		}
+
 		if panicTimer != nil {
 			// We're tracking what we think is a panic right now.
 			// If the timer ended, then it is not a panic.
@@ -333,14 +357,22 @@ func trackPanic(r io.Reader, w io.Writer, dur time.Duration, result chan<- strin
 			}
 		}
 
-		// Flush to stderr what isn't a panic
-		w.Write(buf[0:flushIdx])
-
 		if panicType == -1 {
-			// Not a panic so just continue along
+			// Not a panic. Hold back the tail so a header split across
+			// this read and the next one is still visible.
+			if n > keep {
+				w.Write(buf[0 : n-keep])
+				carry = append([]byte(nil), buf[n-keep:n]...)
+			} else {
+				carry = append([]byte(nil), buf[:n]...)
+			}
 			continue
 		}
 
+		// Flush to stderr what isn't a panic
+		w.Write(buf[0:flushIdx])
+		carry = nil
+
 		// We have a panic header. Write we assume is a panic os far.
 		panicBuf.Write(buf[flushIdx:n])
 		panicTimer = time.After(dur)

panicwrap_test.go

@@ -3,6 +3,7 @@ package panicwrap
 import (
 	"bytes"
 	"fmt"
+	"io"
 	"os"
 	"os/exec"
 	"runtime"
@@ -221,9 +222,9 @@ func TestHelperProcess(*testing.T) {
 	case "panic-monitor":
 
 		config := &WrapConfig{
-			Handler: panicHandler,
+			Handler:   panicHandler,
 			HidePanic: true,
-			Monitor: true,
+			Monitor:   true,
 		}
 
 		exitStatus, err := Wrap(config)
@@ -396,9 +397,6 @@ func TestPanicWrap_concurrentMapWrite(t *testing.T) {
 }
 
 func TestPanicWrap_panicBoundary(t *testing.T) {
-	// TODO(mitchellh): panics are currently lost on boundaries
-	t.SkipNow()
-
 	stdout := new(bytes.Buffer)
 
 	p := helperProcess("panic-boundary")
@@ -408,11 +406,48 @@ func TestPanicWrap_panicBoundary(t *testing.T) {
 		t.Fatalf("err: %s", err)
 	}
 
-	if !strings.Contains(stdout.String(), "wrapped: 1015") {
+	// The child writes "pan" and then "ic: oh crap".
+	if !strings.Contains(stdout.String(), "wrapped: 14") {
 		t.Fatalf("didn't wrap: %#v", stdout.String())
 	}
 }
 
+func TestTrackPanic_splitHeader(t *testing.T) {
+	r, w := io.Pipe()
+	out := new(bytes.Buffer)
+	ch := make(chan string)
+	go trackPanic(r, out, time.Second, ch)
+	go func() {
+		w.Write([]byte("before pan"))
+		w.Write([]byte("ic: split\n"))
+		w.Close()
+	}()
+	got := <-ch
+	if got != "panic: split\n" {
+		t.Fatalf("panic text: %q", got)
+	}
+	if out.String() != "before " {
+		t.Fatalf("forwarded: %q", out.String())
+	}
+}
+
+func TestTrackPanic_plainTextFlushed(t *testing.T) {
+	r, w := io.Pipe()
+	out := new(bytes.Buffer)
+	ch := make(chan string)
+	go trackPanic(r, out, time.Second, ch)
+	go func() {
+		w.Write([]byte("hello world, not a panic"))
+		w.Close()
+	}()
+	if got := <-ch; got != "" {
+		t.Fatalf("panic text: %q", got)
+	}
+	if out.String() != "hello world, not a panic" {
+		t.Fatalf("forwarded: %q", out.String())
+	}
+}
+
 func TestWrapped(t *testing.T) {
 	stdout := new(bytes.Buffer)