feat(rustproxy-http): add HTTP/2 auto-detection via ALPN with TTL-backed protocol cache and h1-only/h2 ALPN client configs

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

@@ -0,0 +1,136 @@
+//! 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.
+
+use std::sync::Arc;
+use std::time::{Duration, Instant};
+
+use dashmap::DashMap;
+use tracing::debug;
+
+/// TTL for cached protocol detection results.
+/// After this duration, the next request will re-probe the backend.
+const PROTOCOL_CACHE_TTL: Duration = Duration::from_secs(300); // 5 minutes
+
+/// Maximum number of entries in the protocol cache.
+/// Prevents unbounded growth when backends come and go.
+const PROTOCOL_CACHE_MAX_ENTRIES: usize = 4096;
+
+/// Background cleanup interval for the protocol cache.
+const PROTOCOL_CACHE_CLEANUP_INTERVAL: Duration = Duration::from_secs(60);
+
+/// Detected backend protocol.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum DetectedProtocol {
+    H1,
+    H2,
+}
+
+/// Key for the protocol cache: (host, port).
+#[derive(Clone, Debug, Hash, Eq, PartialEq)]
+pub struct ProtocolCacheKey {
+    pub host: String,
+    pub port: u16,
+}
+
+/// A cached protocol detection result with a timestamp.
+struct CachedEntry {
+    protocol: DetectedProtocol,
+    detected_at: Instant,
+}
+
+/// Bounded, TTL-based protocol detection cache.
+///
+/// Memory safety guarantees:
+/// - Hard cap at `PROTOCOL_CACHE_MAX_ENTRIES` — cannot grow unboundedly.
+/// - TTL expiry — stale entries naturally age out on lookup.
+/// - Background cleanup task — proactively removes expired entries every 60s.
+/// - `clear()` — called on route updates to discard stale detections.
+/// - `Drop` — aborts the background task to prevent dangling tokio tasks.
+pub struct ProtocolCache {
+    cache: Arc<DashMap<ProtocolCacheKey, CachedEntry>>,
+    cleanup_handle: Option<tokio::task::JoinHandle<()>>,
+}
+
+impl ProtocolCache {
+    /// Create a new protocol cache and start the background cleanup task.
+    pub fn new() -> Self {
+        let cache: Arc<DashMap<ProtocolCacheKey, CachedEntry>> = Arc::new(DashMap::new());
+        let cache_clone = Arc::clone(&cache);
+        let cleanup_handle = tokio::spawn(async move {
+            Self::cleanup_loop(cache_clone).await;
+        });
+
+        Self {
+            cache,
+            cleanup_handle: Some(cleanup_handle),
+        }
+    }
+
+    /// Look up the cached protocol for a backend endpoint.
+    /// Returns `None` if not cached or expired (caller should probe via ALPN).
+    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);
+            Some(entry.protocol)
+        } else {
+            // Expired — remove and return None to trigger re-probe
+            drop(entry); // release DashMap ref before remove
+            self.cache.remove(key);
+            None
+        }
+    }
+
+    /// Insert a detected protocol into the cache.
+    /// If the cache is at capacity, evict the oldest entry first.
+    pub fn insert(&self, key: ProtocolCacheKey, protocol: DetectedProtocol) {
+        if self.cache.len() >= PROTOCOL_CACHE_MAX_ENTRIES && !self.cache.contains_key(&key) {
+            // Evict the oldest entry to stay within bounds
+            let oldest = self.cache.iter()
+                .min_by_key(|entry| entry.value().detected_at)
+                .map(|entry| entry.key().clone());
+            if let Some(oldest_key) = oldest {
+                self.cache.remove(&oldest_key);
+            }
+        }
+        self.cache.insert(key, CachedEntry {
+            protocol,
+            detected_at: Instant::now(),
+        });
+    }
+
+    /// Clear all entries. Called on route updates to discard stale detections.
+    pub fn clear(&self) {
+        self.cache.clear();
+    }
+
+    /// Background cleanup loop — removes expired entries every `PROTOCOL_CACHE_CLEANUP_INTERVAL`.
+    async fn cleanup_loop(cache: Arc<DashMap<ProtocolCacheKey, CachedEntry>>) {
+        let mut interval = tokio::time::interval(PROTOCOL_CACHE_CLEANUP_INTERVAL);
+        loop {
+            interval.tick().await;
+
+            let expired: Vec<ProtocolCacheKey> = cache.iter()
+                .filter(|entry| entry.value().detected_at.elapsed() >= PROTOCOL_CACHE_TTL)
+                .map(|entry| entry.key().clone())
+                .collect();
+
+            if !expired.is_empty() {
+                debug!("Protocol cache cleanup: removing {} expired entries", expired.len());
+                for key in expired {
+                    cache.remove(&key);
+                }
+            }
+        }
+    }
+}
+
+impl Drop for ProtocolCache {
+    fn drop(&mut self) {
+        if let Some(handle) = self.cleanup_handle.take() {
+            handle.abort();
+        }
+    }
+}