Handle Retry-After header for 429 and 503 in gateway prober

When a gateway returns 429 Too Many Requests or 503 Service
Unavailable, the Retry-After response header specifies how long
to wait before retrying. Previously these responses fell back to
static TTL defaults, ignoring the server-specified duration.

Add a retry_after_or helper that parses the Retry-After header
value as seconds and uses it as the cache TTL, falling back to
the configured default when the header is absent.

Closes #1474

Author: codaMW

payjoin-mailroom/src/ohttp_relay/gateway_prober.rs

@@ -246,6 +246,15 @@ impl Prober {
     }
 
     /// Probes a target gateway by attempting to send a GET request.
+    fn retry_after_or<B>(res: &hyper::Response<B>, default: Duration) -> Duration {
+        res.headers()
+            .get(hyper::header::RETRY_AFTER)
+            .and_then(|v| v.to_str().ok())
+            .and_then(|s| s.parse::<u64>().ok())
+            .map(Duration::from_secs)
+            .unwrap_or(default)
+    }
+
     async fn probe(&self, base_url: &GatewayUri) -> Policy {
         // Create a GET request without a body
         let req = hyper::Request::builder()
@@ -281,11 +290,13 @@ impl Prober {
                     }
                 } else if status == hyper::StatusCode::GATEWAY_TIMEOUT {
                     ttls.http_504_gateway_timeout
+                } else if status == hyper::StatusCode::TOO_MANY_REQUESTS {
+                    Self::retry_after_or(res, ttls.http_4xx)
                 } else if status.is_client_error() {
-                    // TODO handle Retry-After for 429 too many requests
                     ttls.http_4xx
+                } else if status == hyper::StatusCode::SERVICE_UNAVAILABLE {
+                    Self::retry_after_or(res, ttls.http_5xx)
                 } else if status.is_server_error() {
-                    // TODO handle Retry-After for 503 service unavailable
                     ttls.http_5xx
                 } else {
                     ttls.default