plugin/cache: Add refresh mode setting to serve_stale (#5131)

This PR adds an optional REFRESH_MODE parameter on the serve_stale configuration directive of the
cache plugin, which verifies that the upstream is still unavailable before returning stale entries.

Signed-off-by: Antoine Tollenaere <atollena@gmail.com>

1 files changed, 10 insertions, 2 deletions

diff --git a/plugin/cache/setup.go b/plugin/cache/setup.go
index afbf361c5..e5258dc06 100644
--- a/plugin/cache/setup.go
+++ b/plugin/cache/setup.go
@@ -166,11 +166,11 @@ func cacheParse(c *caddy.Controller) (*Cache, error) {
 
 			case "serve_stale":
 				args := c.RemainingArgs()
-				if len(args) > 1 {
+				if len(args) > 2 {
 					return nil, c.ArgErr()
 				}
 				ca.staleUpTo = 1 * time.Hour
-				if len(args) == 1 {
+				if len(args) > 0 {
 					d, err := time.ParseDuration(args[0])
 					if err != nil {
 						return nil, err
@@ -180,6 +180,14 @@ func cacheParse(c *caddy.Controller) (*Cache, error) {
 					}
 					ca.staleUpTo = d
 				}
+				ca.verifyStale = false
+				if len(args) > 1 {
+					mode := strings.ToLower(args[1])
+					if mode != "immediate" && mode != "verify" {
+						return nil, fmt.Errorf("invalid value for serve_stale refresh mode: %s", mode)
+					}
+					ca.verifyStale = mode == "verify"
+				}
 			default:
 				return nil, c.ArgErr()
 			}