Handle draft pull requests in canMerge

Github now supports draft pull requests which are blocked from merging
by github. Currently those can end up in a gate loop if all
requirements for the gate pipeline are fulfilled except for
un-drafting it.  Zuul needs to handle those in the canMerge check in
order to handle them correctly.

Change-Id: Ie9164ad5127d19ca3d75660b9718979da7cc344c

tests/base.py

@@ -1503,7 +1503,7 @@ class FakeGithubPullRequest(object):
 
     def __init__(self, github, number, project, branch,
                  subject, upstream_root, files=[], number_of_commits=1,
-                 writers=[], body=None):
+                 writers=[], body=None, draft=False):
         """Creates a new PR with several commits.
         Sends an event about opened PR."""
         self.github = github
@@ -1513,6 +1513,7 @@ class FakeGithubPullRequest(object):
         self.branch = branch
         self.subject = subject
         self.body = body
+        self.draft = draft
         self.number_of_commits = 0
         self.upstream_root = upstream_root
         self.files = []
@@ -1887,11 +1888,11 @@ class FakeGithubConnection(githubconnection.GithubConnection):
         self.zuul_web_port = port
 
     def openFakePullRequest(self, project, branch, subject, files=[],
-                            body=None):
+                            body=None, draft=False):
         self.pr_number += 1
         pull_request = FakeGithubPullRequest(
             self, self.pr_number, project, branch, subject, self.upstream_root,
-            files=files, body=body)
+            files=files, body=body, draft=draft)
         self.pull_requests[self.pr_number] = pull_request
         return pull_request
 

tests/fakegithub.py

@@ -19,6 +19,7 @@ import re
 import time
 
 from requests import HTTPError
+from requests.structures import CaseInsensitiveDict
 
 FAKE_BASE_URL = 'https://example.com/api/v3/'
 
@@ -380,6 +381,7 @@ class FakePull(object):
                 },
                 'ref': pr.branch,
             },
+            'draft': pr.draft,
             'mergeable': True,
             'state': pr.state,
             'head': {
@@ -421,6 +423,7 @@ class FakeGithubSession(object):
 
     def __init__(self, data):
         self._data = data
+        self.headers = CaseInsensitiveDict()
 
     def build_url(self, *args):
         fakepath = '/'.join(args)

tests/unit/test_github_driver.py

@@ -613,6 +613,18 @@ class TestGithubDriver(ZuulTestCase):
         self.assertEqual(len(D.comments), 1)
         self.assertEqual(D.comments[0], 'Merge failed')
 
+    @simple_layout('layouts/dependent-github.yaml', driver='github')
+    def test_draft_pr(self):
+        # pipeline merges the pull request on success
+        A = self.fake_github.openFakePullRequest('org/project', 'master',
+                                                 'PR title', draft=True)
+        self.fake_github.emitEvent(A.addLabel('merge'))
+        self.waitUntilSettled()
+
+        # A draft pull request must not enter the gate
+        self.assertFalse(A.is_merged)
+        self.assertHistory([])
+
     @simple_layout('layouts/reporting-multiple-github.yaml', driver='github')
     def test_reporting_multiple_github(self):
         project = 'org/project1'

zuul/driver/github/githubconnection.py

@@ -45,6 +45,7 @@ from zuul.driver.github.githubmodel import PullRequest, GithubTriggerEvent
 
 GITHUB_BASE_URL = 'https://api.github.com'
 PREVIEW_JSON_ACCEPT = 'application/vnd.github.machine-man-preview+json'
+PREVIEW_DRAFT_ACCEPT = 'application/vnd.github.shadow-cat-preview+json'
 
 
 def _sign_request(body, secret):
@@ -868,6 +869,15 @@ class GithubConnection(BaseConnection):
         if app_key:
             self.app_key = app_key
 
+    @staticmethod
+    def _append_accept_header(github, value):
+        old_header = github.session.headers.get('Accept', None)
+        if old_header:
+            new_value = '%s,%s' % (old_header, value)
+        else:
+            new_value = value
+        github.session.headers['Accept'] = new_value
+
     def _get_app_auth_headers(self):
         now = datetime.datetime.now(utc)
         expiry = now + datetime.timedelta(minutes=5)
@@ -1414,10 +1424,25 @@ class GithubConnection(BaseConnection):
         # conflicts which would block merging finally will be detected by
         # the zuul-mergers anyway.
 
+        log = get_annotated_logger(self.log, event)
+
         github = self.getGithubClient(change.project.name, zuul_event_id=event)
+
+        # Append accept header so we get the draft status
+        self._append_accept_header(github, PREVIEW_DRAFT_ACCEPT)
+
         owner, proj = change.project.name.split('/')
         pull = github.pull_request(owner, proj, change.number)
 
+        # If the PR is a draft it cannot be merged.
+        # TODO: This uses the dict instead of the pull object since github3.py
+        # has no support for draft PRs yet. Replace this with pull.draft when
+        # support has been added.
+        # https://github.com/sigmavirus24/github3.py/issues/926
+        if pull.as_dict().get('draft', False):
+            log.debug('Change %s can not merge because it is a draft', change)
+            return False
+
         protection = self._getBranchProtection(
             change.project.name, change.branch, zuul_event_id=event)