Taught import to be able to use less than all-available workers.

IMPORT_WORKERS_PERCENT is configurable in settings. We will
document/expose this in a future PR, to keep this one maximally
backportable. Default behavior remains "all workers".

fixes pulp#4068.

(cherry picked from commit b5cb1d1)

CHANGES/4068.bugfix

@@ -0,0 +1 @@
+Taught pulp-import to be able to use a subset of available worker-threads.

pulpcore/app/settings.py

@@ -296,6 +296,10 @@
 
 DEFAULT_AUTO_FIELD = "django.db.models.AutoField"
 
+# What percentage of available-workers will pulpimport use at a time, max
+# By default, use all available workers.
+IMPORT_WORKERS_PERCENT = 100
+
 # HERE STARTS DYNACONF EXTENSION LOAD (Keep at the very bottom of settings.py)
 # Read more at https://dynaconf.readthedocs.io/en/latest/guides/django.html
 import dynaconf  # noqa

pulpcore/app/tasks/importer.py

@@ -8,6 +8,7 @@
 from gettext import gettext as _
 from logging import getLogger
 
+from django.conf import settings
 from django.core.files.storage import default_storage
 from django.db.models import F
 from naya.json import stream_array, tokenize
@@ -34,6 +35,7 @@
     ContentArtifactResource,
 )
 from pulpcore.constants import TASK_STATES
+from pulpcore.tasking.pulpcore_worker import Worker
 from pulpcore.tasking.tasks import dispatch
 
 log = getLogger(__name__)
@@ -482,6 +484,18 @@ def validate_and_assemble(toc_filename):
                             default_storage.save(base_path, f)
 
         # Now import repositories, in parallel.
+
+        # We want to be able to limit the number of available-workers that import will consume,
+        # so that pulp can continue to work while doing an import. We accomplish this by creating
+        # a reserved-resource string for each repo-import-task based on that repo's index in
+        # the dispatch loop, mod number-of-workers-to-consume.
+        #
+        # By default (setting is not-set), import will continue to use 100% of the available
+        # workers.
+        import_workers_percent = int(settings.get("IMPORT_WORKERS_PERCENT", 100))
+        total_workers = Worker.objects.online_workers().count()
+        import_workers = max(1, int(total_workers * (import_workers_percent / 100.0)))
+
         with open(os.path.join(temp_dir, REPO_FILE), "r") as repo_data_file:
             data = json.load(repo_data_file)
             gpr = GroupProgressReport(
@@ -493,7 +507,10 @@ def validate_and_assemble(toc_filename):
             )
             gpr.save()
 
-            for src_repo in data:
+            for index, src_repo in enumerate(data):
+                # pulpcore-worker limiter
+                worker_rsrc = f"import-worker-{index % import_workers}"
+                exclusive_resources = [worker_rsrc]
                 try:
                     dest_repo = _destination_repo(importer, src_repo["name"])
                 except Repository.DoesNotExist:
@@ -503,6 +520,8 @@ def validate_and_assemble(toc_filename):
                         )
                     )
                     continue
+                else:
+                    exclusive_resources.append(dest_repo)
 
                 dispatch(
                     import_repository_version,