Parallelize prime_installation_map

During zuul startup when using Github app mechanism zuul iterates
single threaded over all installations and queries the projects of the
installation. In large zuul installations with many projects and
organizations in Github this blocks zuul startup for quite some time.

We can easily parallelize this by using a thread pool. This brings
down the time spent from 18s to 2.5s in our deployment.

Change-Id: I16b940488cd0b3705c8f10b4c17fabfc77db368a

zuul/driver/github/githubconnection.py

@@ -13,6 +13,7 @@
 # under the License.
 
 import collections
+import concurrent.futures
 import datetime
 import logging
 import hmac
@@ -668,6 +669,23 @@ class GithubConnection(BaseConnection):
 
         return token
 
+    def _get_repos_of_installation(self, inst_id, headers):
+        url = '%s/installation/repositories?per_page=100' % self.base_url
+        project_names = []
+        while url:
+            self.log.debug("Fetching repos for install %s" % inst_id)
+            response = requests.get(url, headers=headers)
+            response.raise_for_status()
+            repos = response.json()
+
+            for repo in repos.get('repositories'):
+                project_name = repo.get('full_name')
+                project_names.append(project_name)
+
+            # check if we need to do further paged calls
+            url = response.links.get('next', {}).get('url')
+        return project_names
+
     def _prime_installation_map(self):
         """Walks each app install for the repos to prime install IDs"""
 
@@ -690,27 +708,35 @@ class GithubConnection(BaseConnection):
             url = response.links.get(
                 'next', {}).get('url')
 
-        for install in installations:
-            inst_id = install.get('id')
-            token = self._get_installation_key(
-                project=None, inst_id=inst_id)
-            headers = {'Accept': PREVIEW_JSON_ACCEPT,
-                       'Authorization': 'token %s' % token}
+        headers_per_inst = {}
+        with concurrent.futures.ThreadPoolExecutor() as executor:
 
-            url = '%s/installation/repositories?per_page=100' % self.base_url
-            while url:
-                self.log.debug("Fetching repos for install %s" % inst_id)
-                response = requests.get(url, headers=headers)
-                response.raise_for_status()
-                repos = response.json()
+            token_by_inst = {}
+            for install in installations:
+                inst_id = install.get('id')
+                token_by_inst[inst_id] = executor.submit(
+                    self._get_installation_key, project=None, inst_id=inst_id)
 
-                for repo in repos.get('repositories'):
-                    project_name = repo.get('full_name')
+            for inst_id, result in token_by_inst.items():
+                token = result.result()
+                headers_per_inst[inst_id] = {
+                    'Accept': PREVIEW_JSON_ACCEPT,
+                    'Authorization': 'token %s' % token
+                }
+
+            project_names_by_inst = {}
+            for install in installations:
+                inst_id = install.get('id')
+                headers = headers_per_inst[inst_id]
+
+                project_names_by_inst[inst_id] = executor.submit(
+                    self._get_repos_of_installation, inst_id, headers)
+
+            for inst_id, result in project_names_by_inst.items():
+                project_names = result.result()
+                for project_name in project_names:
                     self.installation_map[project_name] = inst_id
 
-                # check if we need to do further paged calls
-                url = response.links.get('next', {}).get('url')
-
     def addEvent(self, data, event=None, delivery=None):
         return self.event_queue.put((time.time(), data, event, delivery))