Add basic panic recovery.

Based on Martini's.

app/app.go

@@ -126,9 +126,16 @@ func NotSignedIn(r *http.Request) *AppError {
 	return RedirectToRoute("index", map[string]string{"continue_url": r.URL.String()})
 }
 
+func Panic(panicData interface{}) *AppError {
+	return InternalError(
+		errors.New(fmt.Sprintf("Panic: %+v\n\n%s", panicData, stack(3))),
+		"Panic")
+}
+
 type AppHandler func(http.ResponseWriter, *http.Request) *AppError
 
 func (fn AppHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	defer panicRecovery(w, r)
 	if e := fn(w, r); e != nil {
 		handleAppError(e, w, r)
 	}
@@ -137,6 +144,7 @@ func (fn AppHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 type SignedInAppHandler func(http.ResponseWriter, *http.Request, *AppSignedInState) *AppError
 
 func (fn SignedInAppHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	defer panicRecovery(w, r)
 	session, _ := sessionStore.Get(r, sessionConfig.CookieName)
 	userId, ok := session.Values[sessionConfig.UserIdKey].(int)
 	if !ok {
@@ -167,6 +175,12 @@ func (fn SignedInAppHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	}
 }
 
+func panicRecovery(w http.ResponseWriter, r *http.Request) {
+	if panicData := recover(); panicData != nil {
+		handleAppError(Panic(panicData), w, r)
+	}
+}
+
 func handleAppError(e *AppError, w http.ResponseWriter, r *http.Request) {
 	c := appengine.NewContext(r)
 	if e.Type == AppErrorTypeGitHubFetch {

app/recovery.go

@@ -0,0 +1,79 @@
+package retrogit
+
+// From Martini's recovery package:
+// https://github.com/go-martini/martini/blob/master/recovery.go
+
+import (
+	"bytes"
+	"fmt"
+	"io/ioutil"
+	"runtime"
+)
+
+var (
+	dunno     = []byte("???")
+	centerDot = []byte("·")
+	dot       = []byte(".")
+	slash     = []byte("/")
+)
+
+// stack returns a nicely formated stack frame, skipping skip frames
+func stack(skip int) []byte {
+	buf := new(bytes.Buffer) // the returned data
+	// As we loop, we open files and read them. These variables record the currently
+	// loaded file.
+	var lines [][]byte
+	var lastFile string
+	for i := skip; ; i++ { // Skip the expected number of frames
+		pc, file, line, ok := runtime.Caller(i)
+		if !ok {
+			break
+		}
+		// Print this much at least.  If we can't find the source, it won't show.
+		fmt.Fprintf(buf, "%s:%d (0x%x)\n", file, line, pc)
+		if file != lastFile {
+			data, err := ioutil.ReadFile(file)
+			if err != nil {
+				continue
+			}
+			lines = bytes.Split(data, []byte{'\n'})
+			lastFile = file
+		}
+		fmt.Fprintf(buf, "\t%s: %s\n", function(pc), source(lines, line))
+	}
+	return buf.Bytes()
+}
+
+// source returns a space-trimmed slice of the n'th line.
+func source(lines [][]byte, n int) []byte {
+	n-- // in stack trace, lines are 1-indexed but our array is 0-indexed
+	if n < 0 || n >= len(lines) {
+		return dunno
+	}
+	return bytes.TrimSpace(lines[n])
+}
+
+// function returns, if possible, the name of the function containing the PC.
+func function(pc uintptr) []byte {
+	fn := runtime.FuncForPC(pc)
+	if fn == nil {
+		return dunno
+	}
+	name := []byte(fn.Name())
+	// The name includes the path name to the package, which is unnecessary
+	// since the file name is already included.  Plus, it has center dots.
+	// That is, we see
+	//	runtime/debug.*T·ptrmethod
+	// and want
+	//	*T.ptrmethod
+	// Also the package path might contains dot (e.g. code.google.com/...),
+	// so first eliminate the path prefix
+	if lastslash := bytes.LastIndex(name, slash); lastslash >= 0 {
+		name = name[lastslash+1:]
+	}
+	if period := bytes.Index(name, dot); period >= 0 {
+		name = name[period+1:]
+	}
+	name = bytes.Replace(name, centerDot, dot, -1)
+	return name
+}