fix(slack-bot): return 400 on malformed request bodies (#254)

Bolt parse_body raises JSONDecodeError for empty or non-JSON form
payload fields, which turned probe POSTs into unhandled Lambda 500s.

Fixes AFTERHOURS-SHIFT-MANAGER-2

src/slack-bot/handler.py

@@ -1,5 +1,6 @@
 """Lambda handler — Slack Bolt app entry point."""
 
+import json
 import logging
 import os
 
@@ -33,4 +34,8 @@ def _get_handler() -> SlackRequestHandler:
 
 
 def handler(event, context):
-    return _get_handler().handle(event, context)
+    try:
+        return _get_handler().handle(event, context)
+    except json.JSONDecodeError:
+        logger.warning("rejecting malformed slack request body")
+        return {"statusCode": 400, "body": "invalid request"}

tests/slack_bot/conftest.py

@@ -24,6 +24,35 @@ def slackbot_app():
     return _load("slackbot_app", "src/slack-bot/app.py")
 
 
+@pytest.fixture
+def slackbot_handler(monkeypatch):
+    """Load handler.py; skip Slack auth.test so request parsing is the unit."""
+    from slack_bolt import App
+    from slack_bolt.adapter.aws_lambda import SlackRequestHandler
+
+    slack_bot_dir = str(_ROOT / "src/slack-bot")
+    monkeypatch.syspath_prepend(slack_bot_dir)
+    saved = {name: sys.modules.get(name) for name in ("app", "handler")}
+    for name in saved:
+        sys.modules.pop(name, None)
+    mod = _load("slackbot_handler", "src/slack-bot/handler.py")
+    bolt_app = App(
+        token="xoxb-test",
+        signing_secret="test-secret",
+        process_before_response=True,
+        token_verification_enabled=False,
+    )
+    wrapped = SlackRequestHandler(app=bolt_app)
+    monkeypatch.setattr(mod, "_get_handler", lambda: wrapped)
+    yield mod
+    mod._slack_handler = None
+    for name, previous in saved.items():
+        if previous is None:
+            sys.modules.pop(name, None)
+        else:
+            sys.modules[name] = previous
+
+
 @pytest.fixture
 def respond():
     return MagicMock(name="respond")

tests/slack_bot/test_handler.py

@@ -0,0 +1,76 @@
+"""Slack-bot Lambda handler: malformed bodies return 400, not 500."""
+
+import json
+from types import SimpleNamespace
+
+import pytest
+from slack_bolt.request.internals import parse_body
+
+
+def _context():
+    return SimpleNamespace(
+        function_name="afterhours-shift-manager",
+        invoked_function_arn="arn:aws:lambda:us-east-1:1:function:x",
+    )
+
+
+def _event(body, content_type, user_agent="Python-urllib/3.12"):
+    return {
+        "version": "2.0",
+        "routeKey": "POST /slack/events",
+        "rawPath": "/slack/events",
+        "headers": {
+            "content-type": content_type,
+            "user-agent": user_agent,
+        },
+        "requestContext": {"http": {"method": "POST", "path": "/slack/events"}},
+        "body": body,
+        "isBase64Encoded": False,
+        "queryStringParameters": {},
+    }
+
+
+def test_parse_body_empty_payload_raises():
+    with pytest.raises(json.JSONDecodeError):
+        parse_body("payload=", "application/x-www-form-urlencoded")
+
+
+def test_parse_body_non_json_payload_raises():
+    with pytest.raises(json.JSONDecodeError):
+        parse_body("payload=not-json", "application/x-www-form-urlencoded")
+
+
+def test_empty_form_payload_returns_400(slackbot_handler):
+    result = slackbot_handler.handler(
+        _event("payload=", "application/x-www-form-urlencoded"), _context()
+    )
+    assert result["statusCode"] == 400
+    assert result["body"] == "invalid request"
+
+
+def test_non_json_form_payload_returns_400(slackbot_handler):
+    result = slackbot_handler.handler(
+        _event("payload=not-json", "application/x-www-form-urlencoded"),
+        _context(),
+    )
+    assert result["statusCode"] == 400
+    assert result["body"] == "invalid request"
+
+
+def test_payload_substring_without_form_key_is_unsigned(slackbot_handler):
+    result = slackbot_handler.handler(
+        _event("hello payload world", "text/plain"), _context()
+    )
+    assert result["statusCode"] in (401, 403)
+
+
+def test_json_events_body_is_unsigned_not_jsondecode(slackbot_handler):
+    result = slackbot_handler.handler(
+        _event(
+            '{"type":"url_verification","challenge":"abc"}',
+            "application/json",
+            user_agent="Slackbot 1.0",
+        ),
+        _context(),
+    )
+    assert result["statusCode"] in (401, 403)