errors: detect unknown frames correctly (#192)

With Go 1.12, we will now be doing mid-stack inlining. This exposes
inlined frames to runtime.Callers and friends.

The spec says that a runtime.Frame may have a nil Func field for
inlined functions. Instead, use the Function field to detect whether
we really have an empty Frame.

format_test.go

@@ -360,6 +360,30 @@ func TestFormatGeneric(t *testing.T) {
 	}
 }
 
+func wrappedNew(message string) error { // This function will be mid-stack inlined in go 1.12+
+	return New(message)
+}
+
+func TestFormatWrappedNew(t *testing.T) {
+	tests := []struct {
+		error
+		format string
+		want   string
+	}{{
+		wrappedNew("error"),
+		"%+v",
+		"error\n" +
+			"github.com/pkg/errors.wrappedNew\n" +
+			"\t.+/github.com/pkg/errors/format_test.go:364\n" +
+			"github.com/pkg/errors.TestFormatWrappedNew\n" +
+			"\t.+/github.com/pkg/errors/format_test.go:373",
+	}}
+
+	for i, tt := range tests {
+		testFormatRegexp(t, i, tt.error, tt.format, tt.want)
+	}
+}
+
 func testFormatRegexp(t *testing.T, n int, arg interface{}, format, want string) {
 	got := fmt.Sprintf(format, arg)
 	gotLines := strings.SplitN(got, "\n", -1)

stack.go

@@ -35,11 +35,10 @@ func (f Frame) format(w io.Writer, s fmt.State, verb rune) {
 	case 's':
 		switch {
 		case s.Flag('+'):
-			fn := f.Func
-			if fn == nil {
+			if f.Function == "" {
 				io.WriteString(w, "unknown")
 			} else {
-				io.WriteString(w, fn.Name())
+				io.WriteString(w, f.Function)
 				io.WriteString(w, "\n\t")
 				io.WriteString(w, f.File)
 			}