text/tabwriter: improve panic backtraces

Fixes #7117.

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

diff --git a/src/pkg/text/tabwriter/tabwriter.go b/src/pkg/text/tabwriter/tabwriter.go
index 722ac8d877a7fecb5ce30ddc7088137daafc3809..c0c32d5deca51d2057b4c4658be67486f2159284 100644 (file)
--- a/src/pkg/text/tabwriter/tabwriter.go
+++ b/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

diff --git a/src/pkg/text/tabwriter/tabwriter_test.go b/src/pkg/text/tabwriter/tabwriter_test.go
index b0526a03f738b616787e3a4ff4fab555b79c9b1a..9d3111e2c2730502017b5c86696dbbd10558cb74 100644 (file)
--- a/src/pkg/text/tabwriter/tabwriter_test.go
+++ b/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")
+}