M39: harden application and acceptance gates

backend/app/core/ratelimit.py

@@ -0,0 +1,49 @@
+"""Small in-process failed-attempt limiter for credential endpoints.
+
+Fleet Ops runs as a single API process per deployment, so an in-memory sliding window
+is sufficient to blunt online password guessing (and the scrypt CPU amplification that
+comes with it) without adding Redis. Only *failed* attempts count, so legitimate users
+and the automated test suite are never throttled.
+"""
+
+from __future__ import annotations
+
+import threading
+import time
+from collections import deque
+
+
+class FailedAttemptLimiter:
+    def __init__(self, *, max_failures: int, window_seconds: float) -> None:
+        self.max_failures = max_failures
+        self.window_seconds = window_seconds
+        self._failures: dict[str, deque[float]] = {}
+        self._lock = threading.Lock()
+
+    def _prune(self, key: str, now: float) -> deque[float]:
+        bucket = self._failures.setdefault(key, deque())
+        cutoff = now - self.window_seconds
+        while bucket and bucket[0] <= cutoff:
+            bucket.popleft()
+        if not bucket:
+            self._failures.pop(key, None)
+        return bucket
+
+    def retry_after_seconds(self, key: str) -> int:
+        """Return >0 seconds to wait when the key is currently blocked, else 0."""
+        now = time.monotonic()
+        with self._lock:
+            bucket = self._prune(key, now)
+            if len(bucket) < self.max_failures:
+                return 0
+            return max(1, int(bucket[0] + self.window_seconds - now + 0.999))
+
+    def record_failure(self, key: str) -> None:
+        now = time.monotonic()
+        with self._lock:
+            self._prune(key, now)
+            self._failures.setdefault(key, deque()).append(now)
+
+    def reset(self, key: str) -> None:
+        with self._lock:
+            self._failures.pop(key, None)