fix(auth): prune expired sessions on every verify to prevent memory leak (#196)

* fix(auth): prune expired sessions on every verify to prevent memory leak

The in-memory _sessions dict accumulated expired tokens indefinitely —
entries were only removed when that specific token was verified. Add a
lazy _prune_expired_sessions() call at the top of verify_session() so
all expired entries are swept during normal traffic.

Addresses #192.

* test(auth): add 8 unit tests for session lifecycle and lazy pruning

Tests verify:
- Fresh session creation and validation
- Expired entries are pruned during verify_session() calls
- Valid sessions are never removed by pruning
- Empty dict is safe for pruning
- Session TTL matches expected 24-hour window
- invalidate_session() actually removes the token
- Invalidating non-existent tokens is safe

api/auth.py

@@ -108,10 +108,18 @@ def create_session() -> str:
     return f"{token}.{sig}"
 
 
+def _prune_expired_sessions():
+    """Remove all expired session entries to prevent unbounded memory growth."""
+    now = time.time()
+    for token in [t for t, exp in _sessions.items() if now > exp]:
+        _sessions.pop(token, None)
+
+
 def verify_session(cookie_value) -> bool:
     """Verify a signed session cookie. Returns True if valid and not expired."""
     if not cookie_value or '.' not in cookie_value:
         return False
+    _prune_expired_sessions()  # lazy cleanup on every verification attempt
     token, sig = cookie_value.rsplit('.', 1)
     expected_sig = hmac.new(_signing_key(), token.encode(), hashlib.sha256).hexdigest()[:32]
     if not hmac.compare_digest(sig, expected_sig):

tests/test_auth_sessions.py

@@ -0,0 +1,134 @@
+"""
+Tests for auth session lifecycle — session creation, verification, expiry,
+and lazy pruning of expired entries.
+"""
+import time
+import unittest
+from pathlib import Path
+import tempfile
+import os
+
+# Isolate state dir so we don't touch real sessions
+_TEST_STATE = Path(tempfile.mkdtemp())
+os.environ["HERMES_WEBUI_STATE_DIR"] = str(_TEST_STATE)
+
+import sys
+sys.path.insert(0, str(Path(__file__).parent.parent))
+
+import importlib
+
+# Force re-import of auth module so it picks up our TEST_STATE_DIR
+auth = importlib.import_module("api.auth")
+
+
+class TestSessionPruning(unittest.TestCase):
+    """Verify expired session cleanup works correctly."""
+
+    def setUp(self):
+        # Clear any leftover sessions from other tests
+        auth._sessions.clear()
+
+    def test_session_created_valid(self):
+        """A fresh session token should verify as valid."""
+        token = auth.create_session()
+        self.assertTrue(auth.verify_session(token))
+
+    def test_expired_session_pruned(self):
+        """Manually inserting an expired entry should be pruned on next verify_session call."""
+        # Insert sessions that have already expired
+        auth._sessions["fake_token"] = time.time() - 100
+        auth._sessions["another_fake"] = time.time() - 50
+        # Insert one valid session (far future)
+        auth._sessions["good_token"] = time.time() + 3600
+
+        # _sessions has 3 entries, 2 expired
+        self.assertEqual(len(auth._sessions), 3)
+
+        # Call verify_session — this triggers _prune_expired_sessions()
+        # Cookie format is token.signature, so we need a dot to pass the early check
+        auth.verify_session("fake_token.fake_sig")
+
+        # After verification, only the valid session should remain
+        self.assertEqual(len(auth._sessions), 1)
+        self.assertIn("good_token", auth._sessions)
+        self.assertNotIn("fake_token", auth._sessions)
+        self.assertNotIn("another_fake", auth._sessions)
+
+    def test_prune_does_not_remove_valid_sessions(self):
+        """_prune_expired_sessions should never remove sessions that are still active."""
+        auth._sessions["active_1"] = time.time() + 86400  # 24 hours from now
+        auth._sessions["active_2"] = time.time() + 7200    # 2 hours from now
+        auth._sessions["expired_1"] = time.time() - 10
+
+        auth._prune_expired_sessions()
+
+        self.assertEqual(len(auth._sessions), 2)
+        self.assertIn("active_1", auth._sessions)
+        self.assertIn("active_2", auth._sessions)
+        self.assertNotIn("expired_1", auth._sessions)
+
+    def test_verify_session_prunes_before_verification(self):
+        """verify_session should prune expired entries before checking the target token.
+
+        This ensures that _prune_expired_sessions() is called at the very top
+        of verify_session(), so cleanup happens on every auth check.
+        """
+        auth._sessions["expired_for_test"] = time.time() - 999
+
+        # verify_session with an invalid cookie triggers the full path:
+        # _prune_expired_sessions -> signature check -> return False
+        result = auth.verify_session("nonexistent.bad_sig")
+        self.assertFalse(result)
+
+        # The expired entry should have been cleaned up
+        self.assertNotIn("expired_for_test", auth._sessions)
+
+    def test_prune_handles_empty_dict(self):
+        """_prune_expired_sessions should be safe on an empty dict."""
+        auth._sessions.clear()
+        auth._prune_expired_sessions()
+        self.assertEqual(len(auth._sessions), 0)
+
+    def test_session_ttl_is_24_hours(self):
+        """Newly created sessions should have the expected 24-hour TTL."""
+        auth._sessions.clear()
+        token_hex = auth.create_session().split(".")[0]
+        # The _sessions dict stores token -> expiry_time
+        # We can check the expiry is approximately SESSION_TTL seconds from now
+        # by looking up the raw entry via the token
+        from api.auth import _sessions, SESSION_TTL
+        # find our entry
+        for t, exp in _sessions.items():
+            if t == token_hex:
+                # expiry should be within 5 seconds of now + SESSION_TTL
+                expected = time.time() + SESSION_TTL
+                self.assertAlmostEqual(exp, expected, delta=5)
+                break
+        else:
+            self.fail("Session token not found in _sessions")
+
+
+class TestSessionInvalidation(unittest.TestCase):
+    """Test session logout / invalidation."""
+
+    def setUp(self):
+        auth._sessions.clear()
+
+    def test_invalidate_session_removes_token(self):
+        """Calling invalidate_session should remove the token from _sessions."""
+        token = auth.create_session()
+        self.assertTrue(auth.verify_session(token))
+
+        auth.invalidate_session(token)
+        # Token should be gone
+        self.assertFalse(auth.verify_session(token))
+
+    def test_invalidate_unknown_token_is_safe(self):
+        """Invalidating a non-existent token should not raise."""
+        auth._sessions.clear()
+        auth.invalidate_session("nonexistent_token")
+        # Should not raise
+
+
+if __name__ == "__main__":
+    unittest.main()