allow basic logging for http Get err with RegisterErrLogFn

groupcache.go

@@ -60,6 +60,7 @@ var (
 
 	initPeerServerOnce sync.Once
 	initPeerServer     func()
+	errLogFn           func(error)
 )
 
 // GetGroup returns the named group previously created with NewGroup, or
@@ -121,6 +122,20 @@ func RegisterNewGroupHook(fn func(*Group)) {
 	newGroupHook = fn
 }
 
+// RegisterErrLogFn registers a log func that is used for http error logging
+func RegisterErrLogFn(fn func(error)) {
+	if errLogFn != nil {
+		panic("RegisterLogFunc called more than once")
+	}
+	errLogFn = fn
+}
+
+func logErr(err error) {
+	if errLogFn != nil {
+		errLogFn(err)
+	}
+}
+
 // RegisterServerStart registers a hook that is run when the first
 // group is created.
 func RegisterServerStart(fn func()) {
@@ -271,10 +286,7 @@ func (g *Group) load(ctx Context, key string, dest Sink) (value ByteView, destPo
 				return value, nil
 			}
 			g.Stats.PeerErrors.Add(1)
-			// TODO(bradfitz): log the peer's error? keep
-			// log of the past few for /groupcachez?  It's
-			// probably boring (normal task movement), so not
-			// worth logging I imagine.
+			logErr(err)
 		}
 		value, err = g.getLocally(ctx, key, dest)
 		if err != nil {