executor: add merge_jobs options to disable gearman merge jobs

This change adds a zuul.conf option to disable the global merge
jobs from running on an executor node.

Change-Id: Icd6374a6c97a404662b39de9df54f4b7c5ab36aa

doc/source/admin/components.rst

@@ -736,6 +736,15 @@ The following sections of ``zuul.conf`` are used by the executor:
             node-attributes:
               executor-zone: vpn
 
+   .. attr:: merge_jobs
+      :default: True
+
+      To disable global merge job, set it to false. This is useful for zoned
+      executors that are running on slow network where you don't want them to
+      perform merge operations for any events. The executor will still perform
+      the merge operations required for the build they are executing.
+
+
 .. attr:: merger
 
    .. attr:: git_user_email

releasenotes/notes/executor-merger-f853f2f717ea4640.yaml

@@ -0,0 +1,4 @@
+---
+features:
+  - |
+    A new config option enable executor to not performed global merge job.

zuul/executor/server.py

@@ -2416,20 +2416,23 @@ class ExecutorServer(object):
                 self.ansible_manager.install()
         self.ansible_manager.copyAnsibleFiles()
 
-        self.merger_jobs = {
-            'merger:merge': self.merge,
-            'merger:cat': self.cat,
-            'merger:refstate': self.refstate,
-            'merger:fileschanges': self.fileschanges,
-        }
-        self.merger_gearworker = ZuulGearWorker(
-            'Zuul Executor Merger',
-            'zuul.ExecutorServer',
-            'merger',
-            self.config,
-            self.merger_jobs,
-            worker_class=ExecutorMergeWorker,
-            worker_args=[self])
+        if get_default(self.config, 'executor', 'merge_jobs', True):
+            self.merger_jobs = {
+                'merger:merge': self.merge,
+                'merger:cat': self.cat,
+                'merger:refstate': self.refstate,
+                'merger:fileschanges': self.fileschanges,
+            }
+            self.merger_gearworker = ZuulGearWorker(
+                'Zuul Executor Merger',
+                'zuul.ExecutorServer',
+                'merger',
+                self.config,
+                self.merger_jobs,
+                worker_class=ExecutorMergeWorker,
+                worker_args=[self])
+        else:
+            self.merger_gearworker = None
 
         function_name = 'executor:execute'
         if self.zone:
@@ -2460,7 +2463,8 @@ class ExecutorServer(object):
         self._running = True
         self._command_running = True
 
-        self.merger_gearworker.start()
+        if self.merger_gearworker is not None:
+            self.merger_gearworker.start()
         self.executor_gearworker.start()
 
         self.log.debug("Starting command processor")
@@ -2512,7 +2516,8 @@ class ExecutorServer(object):
         self.governor_stop_event.set()
         self.governor_thread.join()
         # Stop accepting new jobs
-        self.merger_gearworker.gearman.setFunctions([])
+        if self.merger_gearworker is not None:
+            self.merger_gearworker.gearman.setFunctions([])
         self.executor_gearworker.gearman.setFunctions([])
         # Tell the executor worker to abort any jobs it just accepted,
         # and grab the list of currently running job workers.
@@ -2542,7 +2547,8 @@ class ExecutorServer(object):
 
         # All job results should have been sent by now, shutdown the
         # gearman workers.
-        self.merger_gearworker.stop()
+        if self.merger_gearworker is not None:
+            self.merger_gearworker.stop()
         self.executor_gearworker.stop()
 
         if self.statsd:
@@ -2559,7 +2565,8 @@ class ExecutorServer(object):
         self.governor_thread.join()
         for update_thread in self.update_threads:
             update_thread.join()
-        self.merger_gearworker.join()
+        if self.merger_gearworker is not None:
+            self.merger_gearworker.join()
         self.executor_gearworker.join()
 
     def pause(self):