feat(api): add concurrency limiter to prevent oom under burst load

apps/api/src/index.ts

@@ -1,4 +1,4 @@
-import { BrowserPool, SessionCache } from "@trawl/browser"
+import { BrowserPool, PoolExhaustedError, SessionCache } from "@trawl/browser"
 import { scrape } from "@trawl/tiers"
 import type { FlareSolverrRequest, FlareSolverrResponse, PoolStats, ScrapeRequest } from "@trawl/types"
 import { Elysia } from "elysia"
@@ -47,6 +47,38 @@ function getDeps() {
 
 const startTime = Date.now()
 
+// Concurrency limiter — gates in-flight scrape work at exactly POOL_SIZE so we
+// reject immediately (HTTP 429) instead of letting BrowserPool.acquire() block
+// for up to 5s. Prowlarr/Jackett use the 429 to back off cleanly.
+let activeJobs = 0
+const MAX_CONCURRENT = POOL_SIZE
+
+function tryAcquireSlot(): boolean {
+  if (activeJobs < MAX_CONCURRENT) {
+    activeJobs++
+    return true
+  }
+  return false
+}
+
+function releaseSlot(): void {
+  if (activeJobs > 0) activeJobs--
+}
+
+// Build a FlareSolverr v2-shaped error envelope. Used by /v1, /scrape, and the
+// second-line PoolExhaustedError catch-arm.
+function flareSolverrError(url: string, message: string): FlareSolverrResponse {
+  const now = Date.now()
+  return {
+    status: "error",
+    message,
+    startTimestamp: now,
+    endTimestamp: now,
+    version: "2.0.0",
+    solution: { url, status: 0, headers: {}, response: "", cookies: [], userAgent: "" },
+  }
+}
+
 new Elysia()
   .get("/health", () => ({
     status: pool ? "ok" : "starting",
@@ -76,6 +108,8 @@ new Elysia()
       busy: stats.busy,
       restarts: stats.restarts,
       queueDepth: 0,
+      activeJobs,
+      maxConcurrent: MAX_CONCURRENT,
     }
   })
 
@@ -87,40 +121,17 @@ new Elysia()
 
     if (cmd !== "request.get" && cmd !== "request.post") {
       set.status = 400
-      return {
-        status: "error",
-        message: `Unknown cmd: ${cmd}`,
-        startTimestamp,
-        endTimestamp: Date.now(),
-        version: "2.0.0",
-        solution: {
-          url: "",
-          status: 0,
-          headers: {},
-          response: "",
-          cookies: [],
-          userAgent: "",
-        },
-      } satisfies FlareSolverrResponse
+      return flareSolverrError(req.url, `Unknown cmd: ${cmd}`)
     }
 
     if (!pool) {
       set.status = 503
-      return {
-        status: "error",
-        message: "Browser pool initializing, retry in a few seconds",
-        startTimestamp,
-        endTimestamp: Date.now(),
-        version: "2.0.0",
-        solution: {
-          url: "",
-          status: 0,
-          headers: {},
-          response: "",
-          cookies: [],
-          userAgent: "",
-        },
-      } satisfies FlareSolverrResponse
+      return flareSolverrError(req.url, "Browser pool initializing, retry in a few seconds")
+    }
+
+    if (!tryAcquireSlot()) {
+      set.status = 429
+      return flareSolverrError(req.url, "Browser pool saturated, retry shortly")
     }
 
     try {
@@ -141,22 +152,10 @@ new Elysia()
         },
       } satisfies FlareSolverrResponse
     } catch (err) {
-      set.status = 500
-      return {
-        status: "error",
-        message: err instanceof Error ? err.message : String(err),
-        startTimestamp,
-        endTimestamp: Date.now(),
-        version: "2.0.0",
-        solution: {
-          url: req.url,
-          status: 0,
-          headers: {},
-          response: "",
-          cookies: [],
-          userAgent: "",
-        },
-      } satisfies FlareSolverrResponse
+      set.status = err instanceof PoolExhaustedError ? 429 : 500
+      return flareSolverrError(req.url, err instanceof Error ? err.message : String(err))
+    } finally {
+      releaseSlot()
     }
   })
 
@@ -164,14 +163,23 @@ new Elysia()
   .post("/scrape", async ({ body, set }) => {
     if (!pool) {
       set.status = 503
-      return { error: "Browser pool initializing, retry in a few seconds" }
+      return flareSolverrError("", "Browser pool initializing, retry in a few seconds")
     }
+
     const req = body as ScrapeRequest
+
+    if (!tryAcquireSlot()) {
+      set.status = 429
+      return flareSolverrError(req.url ?? "", "Browser pool saturated, retry shortly")
+    }
+
     try {
       return await scrape(req, getDeps())
     } catch (err) {
-      set.status = 500
-      return { error: err instanceof Error ? err.message : String(err) }
+      set.status = err instanceof PoolExhaustedError ? 429 : 500
+      return flareSolverrError(req.url ?? "", err instanceof Error ? err.message : String(err))
+    } finally {
+      releaseSlot()
     }
   })