feat(cache): singleflight stampede protection on non-streaming path (#66)

cli.js does not perform proxy-layer stampede protection. The singleflight
layer is a value-add proxy operation that exists only inside OCP, between
concurrent client requests and the single upstream cli.js spawn. It does
not introduce, alter, or remove any endpoint, header, request field, or
response field that cli.js emits or expects — no client-observable wire
shape change.

Justification under ALIGNMENT.md Rule 2: the singleflight Map deduplicates
concurrent identical non-streaming cache-miss requests so only one cli.js
spawn runs per unique hash window. All followers receive the same resolved
(or rejected) content. This is a proxy-internal concurrency optimization,
not a wire-level protocol change.

Spec: docs/superpowers/specs/2026-05-07-cache-upgrade-design.md (D4)

Changes:
- keys.mjs: add singleflight(hash, fn) + getInflightStats() exports;
  in-memory Map cleared via Promise.finally() on each settlement
- server.mjs: import singleflight + getInflightStats; wrap non-streaming
  cache-enabled path through singleflight with inner recheck; add TODO
  comment in callClaudeStreaming (streaming-path dedup is explicitly out
  of scope for v3.13.0, see spec D4 streaming caveat); extend /cache/stats
  to return inflight + requesters fields (additive, no removed fields)
- test-features.mjs: 7 new PR-B singleflight tests covering basic dedup,
  failure fan-out, map cleanup (success + failure), different-hash
  independence, getInflightStats shape, and sequential-call non-sharing;
  all 31 tests pass (24 existing + 7 new)

Streaming-path singleflight is explicitly out of scope; TODO left in
callClaudeStreaming for a future follow-up ticket.

Co-authored-by: dtzp555 <dtzp555@gmail.com>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

server.mjs

@@ -34,7 +34,7 @@ import { readFileSync, accessSync, existsSync, constants } from "node:fs";
 import { fileURLToPath } from "node:url";
 import { dirname, join } from "node:path";
 import { homedir } from "node:os";
-import { validateKey, recordUsage, getUsageByKey, getUsageTimeline, getRecentUsage, createKey, listKeys, revokeKey, closeDb, checkQuota, updateKeyQuota, getKeyQuota, findKey, cacheHash, getCachedResponse, setCachedResponse, clearCache, getCacheStats, hasCacheControl } from "./keys.mjs";
+import { validateKey, recordUsage, getUsageByKey, getUsageTimeline, getRecentUsage, createKey, listKeys, revokeKey, closeDb, checkQuota, updateKeyQuota, getKeyQuota, findKey, cacheHash, getCachedResponse, setCachedResponse, clearCache, getCacheStats, hasCacheControl, singleflight, getInflightStats } from "./keys.mjs";
 
 const __dirname = dirname(fileURLToPath(import.meta.url));
 const _pkg = JSON.parse(readFileSync(join(__dirname, "package.json"), "utf8"));
@@ -590,6 +590,7 @@ function startHeartbeat(res, intervalMs, sessionId) {
 // ── Call claude CLI (real streaming) ─────────────────────────────────────
 // Pipes stdout from the claude process directly to SSE chunks as they arrive.
 // Each data chunk becomes a proper SSE event with delta content in real time.
+// TODO(cache-singleflight-stream): streaming-path singleflight is out of scope for v3.13.0; see spec D4 streaming caveat.
 function callClaudeStreaming(model, messages, conversationId, res, authInfo = {}) {
   const id = `chatcmpl-${randomUUID()}`;
   const created = Math.floor(Date.now() / 1000);
@@ -1265,14 +1266,45 @@ async function handleChatCompletions(req, res) {
 
   const t0Usage = Date.now();
   const promptChars = messages.reduce((a, m) => a + (typeof m.content === "string" ? m.content.length : JSON.stringify(m.content).length), 0);
+
+  // Non-streaming path with stampede protection: wrap the upstream call in singleflight
+  // when cache is enabled and a hash is present. Concurrent identical requests share
+  // one upstream spawn; followers receive the same promise. Streaming-path dedup is
+  // explicitly out of scope (see TODO comment above callClaudeStreaming).
+  if (CACHE_TTL > 0 && req._cacheHash) {
+    try {
+      const content = await singleflight(req._cacheHash, async () => {
+        // Re-check cache inside the singleflight: a follower that enters before the
+        // leader finishes will wait on the shared promise (not reach here), but a
+        // request that races in just after the previous singleflight cleared the map
+        // will re-read the freshly-populated cache entry here rather than spawning.
+        const recheck = getCachedResponse(req._cacheHash, CACHE_TTL);
+        if (recheck) return recheck.response;
+        const c = await callClaude(model, messages, conversationId);
+        try { setCachedResponse(req._cacheHash, model, c); } catch (e) { logEvent("error", "cache_write_failed", { error: e.message }); }
+        return c;
+      });
+      const id = `chatcmpl-${randomUUID()}`;
+      completionResponse(res, id, model, content);
+      try { recordUsage({ keyId: req._authKeyId, keyName: req._authKeyName, model, promptChars, responseChars: content.length, elapsedMs: Date.now() - t0Usage, success: true }); } catch (e) { logEvent("error", "usage_record_failed", { error: e.message }); }
+      return;
+    } catch (err) {
+      try { recordUsage({ keyId: req._authKeyId, keyName: req._authKeyName, model, promptChars, responseChars: 0, elapsedMs: Date.now() - t0Usage, success: false }); } catch (e) { logEvent("error", "usage_record_failed", { error: e.message }); }
+      console.error(`[proxy] error: ${err.message}`);
+      if (res.headersSent || res.writableEnded || res.destroyed) {
+        try { res.end(); } catch {}
+        return;
+      }
+      const safeMessage = (err.message || "Internal error").replace(/\/[\w/.\-]+/g, "[path]");
+      return jsonResponse(res, 500, { error: { message: safeMessage, type: "proxy_error" } });
+    }
+  }
+
+  // Fallback: cache disabled (CACHE_TTL=0) or no _cacheHash — original path untouched.
   try {
     const content = await callClaude(model, messages, conversationId);
     const id = `chatcmpl-${randomUUID()}`;
     completionResponse(res, id, model, content);
-    // Write to cache
-    if (CACHE_TTL > 0 && req._cacheHash) {
-      try { setCachedResponse(req._cacheHash, model, content); } catch (e) { logEvent("error", "cache_write_failed", { error: e.message }); }
-    }
     try { recordUsage({ keyId: req._authKeyId, keyName: req._authKeyName, model, promptChars, responseChars: content.length, elapsedMs: Date.now() - t0Usage, success: true }); } catch (e) { logEvent("error", "usage_record_failed", { error: e.message }); }
   } catch (err) {
     try { recordUsage({ keyId: req._authKeyId, keyName: req._authKeyName, model, promptChars, responseChars: 0, elapsedMs: Date.now() - t0Usage, success: false }); } catch (e) { logEvent("error", "usage_record_failed", { error: e.message }); }
@@ -1551,10 +1583,10 @@ const server = createServer(async (req, res) => {
     });
   }
 
-  // GET /cache/stats — cache statistics
+  // GET /cache/stats — cache statistics (entries, hits, size, inflight singleflight count)
   if (pathname === "/cache/stats" && req.method === "GET") {
     if (!isAdmin) return jsonResponse(res, 403, { error: "Admin access required" });
-    return jsonResponse(res, 200, getCacheStats());
+    return jsonResponse(res, 200, { ...getCacheStats(), ...getInflightStats() });
   }
 
   // DELETE /cache — clear cache