Leave a comment if failed to merge PR. (#191)

Closes python/miss-islington#188

Author: Mariatta

miss_islington/status_change.py

@@ -1,5 +1,7 @@
 import re
 
+import gidgethub
+
 from gidgethub import routing
 
 from . import util
@@ -107,29 +109,28 @@ async def merge_pr(gh, pr, sha, is_automerge=False):
     pr_number = pr["number"]
     async for commit in gh.getiter(f"/repos/python/cpython/pulls/{pr_number}/commits"):
         if commit["sha"] == sha:
+            pr_title = ""
+            pr_commit_msg = ""
+
             if is_automerge:
                 pr_commit_msg = util.normalize_message(pr["body"])
                 pr_title = f"{pr['title']} (GH-{pr_number})"
-                await gh.put(
-                    f"/repos/python/cpython/pulls/{pr_number}/merge",
-                    data={
-                        "commit_title": pr_title,
-                        "commit_message": pr_commit_msg,
-                        "sha": sha,
-                        "merge_method": "squash",
-                    },
-                )
             else:
                 pr_commit_msg = commit["commit"]["message"].split("\n")
-
-                cleaned_up_title = f"{pr_commit_msg[0]}"
+                pr_title = f"{pr_commit_msg[0]}"
+
+            data = {
+                "commit_title": pr_title,
+                "commit_message": pr_commit_msg,
+                "sha": sha,
+                "merge_method": "squash",
+            }
+            try:
                 await gh.put(
-                    f"/repos/python/cpython/pulls/{pr_number}/merge",
-                    data={
-                        "commit_title": cleaned_up_title,
-                        "commit_message": "\n".join(pr_commit_msg[1:]),
-                        "sha": sha,
-                        "merge_method": "squash",
-                    },
+                    f"/repos/python/cpython/pulls/{pr_number}/merge", data=data
+                )
+            except gidgethub.BadRequest as err:
+                await util.comment_on_pr(
+                    gh, pr_number, f"Sorry, I can't merge this PR. Reason: `{err}`."
                 )
             break

tests/test_status_change.py

@@ -8,13 +8,13 @@
 
 
 class FakeGH:
-    def __init__(self, *, getitem=None, getiter=None, put=None):
+    def __init__(self, *, getitem=None, getiter=None, put=None, post=None):
         self._getitem_return = getitem
         self._getiter_return = getiter
         self.getitem_url = None
         self.getiter_url = None
         self._put_return = put
-        self._post_return = put
+        self._post_return = post
 
     async def getitem(self, url):
         self.getitem_url = url
@@ -30,7 +30,11 @@ async def getiter(self, url):
     async def put(self, url, *, data):
         self.put_url = url
         self.put_data = data
-        return self._put_return
+        to_return = self._put_return
+        if isinstance(to_return, Exception):
+            raise to_return
+        else:
+            return to_return
 
     async def post(self, url, *, data):
         self.post_url = url
@@ -1217,3 +1221,74 @@ async def test_automerge_commit_not_found():
     await status_change.router.dispatch(event, gh)
     assert not hasattr(gh, "post_data")  # does not leave a comment
     assert not hasattr(gh, "put_data")  # does not merge
+
+
+async def test_automerge_failed():
+    sha = "f2393593c99dd2d3ab8bfab6fcc5ddee540518a9"
+    data = {
+        "action": "labeled",
+        "pull_request": {
+            "user": {"login": "Mariatta"},
+            "labels": [
+                {"name": "awaiting merge"},
+                {"name": AUTOMERGE_LABEL},
+                {"name": "CLA signed"},
+            ],
+            "head": {"sha": sha},
+            "number": 5547,
+            "title": "bpo-32720: Fixed the replacement field grammar documentation.",
+            "body": "\n\n`arg_name` and `element_index` are defined as `digit`+ instead of `integer`.",
+        },
+    }
+
+    event = sansio.Event(data, event="pull_request", delivery_id="1")
+
+    getitem = {
+        f"/repos/python/cpython/commits/{sha}/status": {
+            "state": "success",
+            "statuses": [
+                {
+                    "state": "success",
+                    "description": "Issue report skipped",
+                    "context": "bedevere/issue-number",
+                },
+                {
+                    "state": "success",
+                    "description": "The Travis CI build passed",
+                    "target_url": "https://travis-ci.org/python/cpython/builds/340259685?utm_source=github_status&utm_medium=notification",
+                    "context": "continuous-integration/travis-ci/pr",
+                },
+            ],
+        }
+    }
+
+    getiter = {
+        "/repos/python/cpython/pulls/5547/commits": [
+            {"sha": "5f007046b5d4766f971272a0cc99f8461215c1ec"},
+            {"sha": sha},
+        ]
+    }
+
+    gh = FakeGH(
+        getitem=getitem,
+        getiter=getiter,
+        put=gidgethub.BadRequest(status_code=http.HTTPStatus(400)),
+        post={
+            "html_url": f"https://github.com/python/cpython/pull/5547#issuecomment-401309376"
+        },
+    )
+
+    await status_change.router.dispatch(event, gh)
+
+    assert gh.put_data["sha"] == sha
+    assert gh.put_data["merge_method"] == "squash"
+    assert (
+        gh.put_data["commit_title"]
+        == "bpo-32720: Fixed the replacement field grammar documentation. (GH-5547)"
+    )
+    assert (
+        gh.put_data["commit_message"]
+        == "\n\n`arg_name` and `element_index` are defined as `digit`+ instead of `integer`."
+    )
+
+    assert "Sorry, I can't merge this PR" in gh.post_data["body"]