fix(protocol-cache): Include requested_host in protocol detection cache key to avoid cache oscillation when multiple frontend domains share the same backend

rust/crates/rustproxy-http/src/protocol_cache.rs

@@ -1,7 +1,8 @@
 //! Bounded, TTL-based protocol detection cache for HTTP/2 auto-detection.
 //!
-//! Caches the ALPN-negotiated protocol (H1 or H2) per backend endpoint (host:port).
-//! Prevents repeated ALPN probes for backends whose protocol is already known.
+//! Caches the ALPN-negotiated protocol (H1 or H2) per backend endpoint and requested
+//! domain (host:port + requested_host). This prevents cache oscillation when multiple
+//! frontend domains share the same backend but differ in HTTP/2 support.
 
 use std::sync::Arc;
 use std::time::{Duration, Instant};
@@ -27,11 +28,14 @@ pub enum DetectedProtocol {
     H2,
 }
 
-/// Key for the protocol cache: (host, port).
+/// Key for the protocol cache: (host, port, requested_host).
 #[derive(Clone, Debug, Hash, Eq, PartialEq)]
 pub struct ProtocolCacheKey {
     pub host: String,
     pub port: u16,
+    /// The incoming request's domain (Host header / :authority).
+    /// Distinguishes protocol detection when multiple domains share the same backend.
+    pub requested_host: Option<String>,
 }
 
 /// A cached protocol detection result with a timestamp.
@@ -73,7 +77,7 @@ impl ProtocolCache {
     pub fn get(&self, key: &ProtocolCacheKey) -> Option<DetectedProtocol> {
         let entry = self.cache.get(key)?;
         if entry.detected_at.elapsed() < PROTOCOL_CACHE_TTL {
-            debug!("Protocol cache hit: {:?} for {}:{}", entry.protocol, key.host, key.port);
+            debug!("Protocol cache hit: {:?} for {}:{} (requested: {:?})", entry.protocol, key.host, key.port, key.requested_host);
             Some(entry.protocol)
         } else {
             // Expired — remove and return None to trigger re-probe