text/tabwriter: improve panic backtraces

Fixes #7117.

LGTM=gri
R=golang-codereviews, gobot, gri
CC=golang-codereviews
https://golang.org/cl/53310044

src/pkg/text/tabwriter/tabwriter.go

@@ -434,9 +434,13 @@ func (b *Writer) terminateCell(htab bool) int {
 	return len(*line)
 }
 
-func handlePanic(err *error) {
+func handlePanic(err *error, op string) {
 	if e := recover(); e != nil {
-		*err = e.(osError).err // re-panics if it's not a local osError
+		if nerr, ok := e.(osError); ok {
+			*err = nerr.err
+			return
+		}
+		panic("tabwriter: panic during " + op)
 	}
 }
 
@@ -447,7 +451,7 @@ func handlePanic(err *error) {
 //
 func (b *Writer) Flush() (err error) {
 	defer b.reset() // even in the presence of errors
-	defer handlePanic(&err)
+	defer handlePanic(&err, "Flush")
 
 	// add current cell if not empty
 	if b.cell.size > 0 {
@@ -471,7 +475,7 @@ var hbar = []byte("---\n")
 // while writing to the underlying output stream.
 //
 func (b *Writer) Write(buf []byte) (n int, err error) {
-	defer handlePanic(&err)
+	defer handlePanic(&err, "Write")
 
 	// split text into cells
 	n = 0

src/pkg/text/tabwriter/tabwriter_test.go

@@ -613,3 +613,40 @@ func Test(t *testing.T) {
 		check(t, e.testname, e.minwidth, e.tabwidth, e.padding, e.padchar, e.flags, e.src, e.expected)
 	}
 }
+
+type panicWriter struct{}
+
+func (panicWriter) Write([]byte) (int, error) {
+	panic("cannot write")
+}
+
+func wantPanicString(t *testing.T, want string) {
+	if e := recover(); e != nil {
+		got, ok := e.(string)
+		switch {
+		case !ok:
+			t.Errorf("got %v (%T), want panic string", e, e)
+		case got != want:
+			t.Errorf("wrong panic message: got %q, want %q", got, want)
+		}
+	}
+}
+
+func TestPanicDuringFlush(t *testing.T) {
+	defer wantPanicString(t, "tabwriter: panic during Flush")
+	var p panicWriter
+	w := new(Writer)
+	w.Init(p, 0, 0, 5, ' ', 0)
+	io.WriteString(w, "a")
+	w.Flush()
+	t.Errorf("failed to panic during Flush")
+}
+
+func TestPanicDuringWrite(t *testing.T) {
+	defer wantPanicString(t, "tabwriter: panic during Write")
+	var p panicWriter
+	w := new(Writer)
+	w.Init(p, 0, 0, 5, ' ', 0)
+	io.WriteString(w, "a\n\n") // the second \n triggers a call to w.Write and thus a panic
+	t.Errorf("failed to panic during Write")
+}