Update buildbot.tac and master.cfg for all autogen masters

buildbot.tac as the entrypoint for a buildbot master should use relative
paths based on the deployment directory structure. Furthermore, the
basedir should be set to <srcdir> so that all imports will work relative
to <srcdir>, not the location of buildbot.tac or master.cfg.

buildbot.tac

@@ -1,30 +1,41 @@
+# -*- python -*-
+# ex: set filetype=python:
 import os
 
 from twisted.application import service
+from twisted.python.log import FileLogObserver, ILogObserver
+from twisted.python.logfile import LogFile
 
 from buildbot.master import BuildMaster
 
-basedir = "."
+# This buildbot.tac file is a basis for all 'autogen' masters.
+# The folder structure for autogen masters is:
+#
+# autogen
+# └── aarch64-master-0
+#     ├── buildbot.tac
+#     ├── master.cfg
+#     ├── master-config.yaml
+#     └── master-private.cfg
+#
+# Thus basedir is two levels below this file's position.
+buildbot_tac_dir = os.path.abspath(os.path.dirname(__file__))
+basedir = os.path.abspath(f'{buildbot_tac_dir}/../../')
+
+# Hard coded as it runs in containers.
+# TODO(cvicentiu) this should come as an environment variable.
 log_basedir = "/var/log/buildbot/"
 
 rotateLength = 20000000
 maxRotatedFiles = 30
-configfile = "master.cfg"
+configfile = os.path.join(buildbot_tac_dir, 'master.cfg')
 
 # Default umask for server
 umask = None
 
-# if this is a relocatable tac file, get the directory containing the TAC
-if basedir == ".":
-    import os
-
-    basedir = os.path.abspath(os.path.dirname(__file__))
-
 # note: this line is matched against to check that this is a buildmaster
 # directory; do not edit it.
 application = service.Application('buildmaster')  # fmt: skip
-from twisted.python.log import FileLogObserver, ILogObserver
-from twisted.python.logfile import LogFile
 
 logfile = LogFile.fromFullPath(
     os.path.join(log_basedir, "%s"),

master.cfg

@@ -36,8 +36,11 @@ from utils import (
     uploadDebArtifacts,
 )
 
-with open("master-config.yaml", "r") as f:
-    master_config = yaml.safe_load(f)
+cfg_dir = os.path.abspath(os.path.dirname(__file__))
+# Autogen master, see buildbot.tac for why this is the case.
+base_dir = os.path.abspath(f'{cfg_dir}/../../')
+with open(os.path.join(cfg_dir, "master-config.yaml"), "r") as file:
+    master_config = yaml.safe_load(file)
 
 # This is the dictionary that the buildmaster pays attention to. We also use
 # a shorter alias to save typing.
@@ -46,7 +49,8 @@ c = BuildmasterConfig = {}
 # Load the slave, database passwords and 3rd-party tokens from an external
 # private file, so that the rest of the configuration can be public.
 config = {"private": {}}
-exec(open("master-private.cfg").read(), config, {})
+with open(os.path.join(cfg_dir, "master-private.cfg"), "r") as file:
+    exec(file.read(), config, {})
 
 #######
 # BUILDBOT SERVICES
@@ -69,7 +73,9 @@ gs = reporters.GitHubStatusPush(
 )
 c["services"].append(gs)
 c["secretsProviders"] = [
-    secrets.SecretInAFile(dirname="/srv/buildbot/master/master-credential-provider")
+    # SecretsInAFile requires absolute path.
+    secrets.SecretInAFile(dirname=os.path.join(base_dir,
+                                               'master-credential-provider'))
 ]
 
 ####### PROJECT IDENTITY