Stop builders gracefully on SIGTERM

When SIGTERM is received by the process that is building a build, we only log a
message instead of passing it to the child processes (`pip` running inside the
Docker container, for example).

Otherwise, the `pip` process also receives the SIGTERM and it kills itself,
producing a `exit_status != 0`, registering this as a BuildCommand and making
the whole build to fail. After that, celery "stops gracefully" with a failed build.

This is useful combined with supervisor to make our builders to stop gracefully
in this scenario:

1. supervisorctl stop build
2. celery receives the SIGTERM and logs the warning message
3. supervisor waits for `stopwaitsecs` before sending SIGKILL

readthedocs/projects/tasks.py

@@ -404,6 +404,15 @@ def run(self, version_pk):  # pylint: disable=arguments-differ
     default_retry_delay=7 * 60,
 )
 def update_docs_task(self, version_pk, *args, **kwargs):
+    import signal
+
+    def sigterm_received(*args, **kwargs):
+        log.warning('SIGTERM received. Waiting for build to stop gracefully after it finishes.')
+
+    # Do not send the SIGTERM signal to childs (pip is automatically killed when
+    # receives SIGTERM and make the build to fail one command and stop build)
+    signal.signal(signal.SIGTERM, sigterm_received)
+
     try:
         step = UpdateDocsTaskStep(task=self)
         return step.run(version_pk, *args, **kwargs)