Router PRs builds to last queue where a build was executed

Currently, every time a PR is triggered to be built we are building it under our
build:large queue. This is because we have no data about this version and we
build them on build:large.

This commit checks where the last build was built when a PR is triggered to be
built and use that queue to build the PR.

Closes #7815

Author: humitos

readthedocs/builds/tasks.py

@@ -15,6 +15,7 @@
 )
 from readthedocs.builds.constants import (
     BRANCH,
+    EXTERNAL,
     BUILD_STATUS_FAILURE,
     BUILD_STATUS_PENDING,
     BUILD_STATUS_SUCCESS,
@@ -38,10 +39,11 @@ class TaskRouter:
     Celery tasks router.
 
     It allows us to decide which queue is where we want to execute the task
-    based on project's settings but also in queue availability.
+    based on project's settings.
 
     1. the project is using conda
     2. new project with less than N successful builds
+    3. version to be built is external
 
     It ignores projects that have already set ``build_queue`` attribute.
 
@@ -92,6 +94,16 @@ def route_for_task(self, task, args, kwargs, **__):
                 )
                 return self.BUILD_LARGE_QUEUE
 
+        # Use last queue used for external versions
+        if version.type == EXTERNAL:
+            for build in last_builds.iterator():
+                if not build.builder:
+                    continue
+
+                if 'default' in build.builder:
+                    return self.BUILD_DEFAULT_QUEUE
+                return self.BUILD_LARGE_QUEUE
+
         # We do not have enough builds for this version yet
         if queryset.count() < self.N_BUILDS:
             log.info(

readthedocs/builds/tests/test_celery_task_router.py

@@ -3,6 +3,7 @@
 from django.test import TestCase
 from django.test.utils import override_settings
 
+from readthedocs.builds.constants import EXTERNAL
 from readthedocs.builds.models import Build, Version
 from readthedocs.builds.tasks import TaskRouter
 from readthedocs.projects.models import Project
@@ -67,3 +68,23 @@ def test_no_build_pk(self):
         self.assertIsNone(
             self.router.route_for_task(self.task, self.args, {}),
         )
+
+    def test_external_version(self):
+        self.version.type = EXTERNAL
+        self.version.save()
+
+        self.build.builder = 'build-default-a1b2c3'
+        self.build.save()
+
+        self.assertEqual(
+            self.router.route_for_task(self.task, self.args, self.kwargs),
+            TaskRouter.BUILD_DEFAULT_QUEUE,
+        )
+
+        self.build.builder = 'build-large-a1b2c3'
+        self.build.save()
+
+        self.assertEqual(
+            self.router.route_for_task(self.task, self.args, self.kwargs),
+            TaskRouter.BUILD_LARGE_QUEUE,
+        )