From 5609d12e5e6efc63cbc69c21299d8dcca4815707 Mon Sep 17 00:00:00 2001
From: Matthew Holt <mholt@users.noreply.github.com>
Date: Mon, 20 Apr 2026 16:55:16 -0600
Subject: [PATCH] reverseproxy: Add ability to clear dynamic upstreams cache
 during retries

This is an optional interface for dynamic upstream modules to implement if they cache results.

TODO: More documentation; this is an experiment.
---
 .../caddyhttp/reverseproxy/reverseproxy.go    | 16 ++++++++++++++
 modules/caddyhttp/reverseproxy/upstreams.go   | 21 +++++++++++++++----
 2 files changed, 33 insertions(+), 4 deletions(-)

diff --git a/modules/caddyhttp/reverseproxy/reverseproxy.go b/modules/caddyhttp/reverseproxy/reverseproxy.go
index 3b9b56a05..c5375793f 100644
--- a/modules/caddyhttp/reverseproxy/reverseproxy.go
+++ b/modules/caddyhttp/reverseproxy/reverseproxy.go
@@ -574,6 +574,15 @@ func (h *Handler) proxyLoopIteration(r *http.Request, origReq *http.Request, w h
 	// get the updated list of upstreams
 	upstreams := h.Upstreams
 	if h.DynamicUpstreams != nil {
+		if retries > 0 {
+			if cachingDynamicUpstreams, ok := h.DynamicUpstreams.(cachingUpstreamSource); ok {
+				if err := cachingDynamicUpstreams.ResetCache(r); err != nil {
+					if c := h.logger.Check(zapcore.ErrorLevel, "failed clearing dynamic upstream source's cache"); c != nil {
+						c.Write(zap.Error(err))
+					}
+				}
+			}
+		}
 		dUpstreams, err := h.DynamicUpstreams.GetUpstreams(r)
 		if err != nil {
 			if c := h.logger.Check(zapcore.ErrorLevel, "failed getting dynamic upstreams; falling back to static upstreams"); c != nil {
@@ -1590,6 +1599,13 @@ type UpstreamSource interface {
 	GetUpstreams(*http.Request) ([]*Upstream, error)
 }
 
+// cachingUpstreamSource is an upstream source that caches its upstreams.
+// EXPERIMENTAL: Subject to change.
+type cachingUpstreamSource interface {
+	UpstreamSource
+	ResetCache(*http.Request) error
+}
+
 // Hop-by-hop headers. These are removed when sent to the backend.
 // As of RFC 7230, hop-by-hop headers are required to appear in the
 // Connection header field. These are the headers defined by the
diff --git a/modules/caddyhttp/reverseproxy/upstreams.go b/modules/caddyhttp/reverseproxy/upstreams.go
index e9120725a..7ada6a501 100644
--- a/modules/caddyhttp/reverseproxy/upstreams.go
+++ b/modules/caddyhttp/reverseproxy/upstreams.go
@@ -119,6 +119,18 @@ func (su *SRVUpstreams) Provision(ctx caddy.Context) error {
 	return nil
 }
 
+func (su *SRVUpstreams) ResetCache(r *http.Request) error {
+	srvsMu.Lock()
+	if r == nil {
+		srvs = make(map[string]srvLookup)
+	} else {
+		suAddr, _, _, _ := su.expandedAddr(r)
+		delete(srvs, suAddr)
+	}
+	srvsMu.Unlock()
+	return nil
+}
+
 func (su SRVUpstreams) GetUpstreams(r *http.Request) ([]*Upstream, error) {
 	suAddr, service, proto, name := su.expandedAddr(r)
 
@@ -554,8 +566,9 @@ var (
 
 // Interface guards
 var (
-	_ caddy.Provisioner = (*SRVUpstreams)(nil)
-	_ UpstreamSource    = (*SRVUpstreams)(nil)
-	_ caddy.Provisioner = (*AUpstreams)(nil)
-	_ UpstreamSource    = (*AUpstreams)(nil)
+	_ caddy.Provisioner     = (*SRVUpstreams)(nil)
+	_ UpstreamSource        = (*SRVUpstreams)(nil)
+	_ cachingUpstreamSource = (*SRVUpstreams)(nil)
+	_ caddy.Provisioner     = (*AUpstreams)(nil)
+	_ UpstreamSource        = (*AUpstreams)(nil)
 )