From 2c09a97f47615b07bb1a4fc426a7163433c0d3ea Mon Sep 17 00:00:00 2001
From: Agrim Prasad <AgrimPrasad@users.noreply.github.com>
Date: Thu, 20 Aug 2020 16:50:03 +0800
Subject: [PATCH 1/2] HandlerWithKey:recover panic in callback need a panic
 recoverer as separate recoverer middleware can't catch panics in new
 goroutine

---
 http.go | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/http.go b/http.go
index f0d92d6..4b2144b 100644
--- a/http.go
+++ b/http.go
@@ -7,6 +7,7 @@ import (
 	"io"
 	"io/ioutil"
 	"net/http"
+	"runtime/debug"
 	"strings"
 	"time"
 )
@@ -76,10 +77,23 @@ func HandlerWithKey(cacheSize int, ttl time.Duration, keyFunc ...func(r *http.Re
 			}
 
 			// mark the request that actually processes the response
-			first := false
+			var (
+				err   error
+				first bool
+				val   interface{}
+			)
 
 			// process request (single flight)
-			val, err := cache.GetFresh(r.Context(), key, func(ctx context.Context) (interface{}, error) {
+			val, err = cache.GetFresh(r.Context(), key, func(ctx context.Context) (interface{}, error) {
+				defer func() {
+					// need a panic recoverer as separate recoverer middleware can't catch panics in new goroutine
+					if r := recover(); r != nil {
+						// shouldn't return in case of panic as it has not responded to client
+						first = false
+						err = fmt.Errorf("recovered panicking request:%#v, stack:%s", r, string(debug.Stack()))
+					}
+				}()
+
 				first = true
 				buf := bytes.NewBuffer(nil)
 				ww := &responseWriter{ResponseWriter: w, tee: buf}

From 43cd1d9dd66eea05c28bd946ae1aa23f2bcaca78 Mon Sep 17 00:00:00 2001
From: Agrim Prasad <AgrimPrasad@users.noreply.github.com>
Date: Thu, 20 Aug 2020 16:56:09 +0800
Subject: [PATCH 2/2] HandlerWithKey: handle panic error separately for
 stacktrace

---
 http.go | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/http.go b/http.go
index 4b2144b..2c3ba91 100644
--- a/http.go
+++ b/http.go
@@ -78,9 +78,10 @@ func HandlerWithKey(cacheSize int, ttl time.Duration, keyFunc ...func(r *http.Re
 
 			// mark the request that actually processes the response
 			var (
-				err   error
-				first bool
-				val   interface{}
+				err      error
+				panicErr error
+				first    bool
+				val      interface{}
 			)
 
 			// process request (single flight)
@@ -88,9 +89,7 @@ func HandlerWithKey(cacheSize int, ttl time.Duration, keyFunc ...func(r *http.Re
 				defer func() {
 					// need a panic recoverer as separate recoverer middleware can't catch panics in new goroutine
 					if r := recover(); r != nil {
-						// shouldn't return in case of panic as it has not responded to client
-						first = false
-						err = fmt.Errorf("recovered panicking request:%#v, stack:%s", r, string(debug.Stack()))
+						panicErr = fmt.Errorf("recovered panicking request:%#v, stack:%s", r, string(debug.Stack()))
 					}
 				}()
 
@@ -108,6 +107,11 @@ func HandlerWithKey(cacheSize int, ttl time.Duration, keyFunc ...func(r *http.Re
 				return val, nil
 			})
 
+			// handle panic for first or other listeners
+			if panicErr != nil {
+				panic(fmt.Sprintf("stampede: encountered unexpected panic, %v", panicErr))
+			}
+
 			// the first request to trigger the fetch should return as it's already
 			// responded to the client
 			if first {