Fix replica failing when upstream is missing plugins

fixes: #4509

CHANGES/4509.bugfix

@@ -0,0 +1 @@
+Fixed replication failing when the upstream Pulp doesn't have all the same plugins as the downstream.

pulpcore/app/replica.py

@@ -46,6 +46,7 @@ class Replicator:
     remote_serializer_name = None
     app_label = None
     sync_task = None
+    required_version = None  # A PEP-440 compatible version range required in the Upstream.
 
     def __init__(self, pulp_ctx, task_group, tls_settings, server):
         """

pulpcore/app/tasks/replica.py

@@ -8,6 +8,7 @@
 from pulpcore.app.replica import ReplicaContext
 
 from pulp_glue.common import __version__ as pulp_glue_version
+from pulp_glue.common.context import PluginRequirement
 
 
 def user_agent():
@@ -67,8 +68,10 @@ def replicate_distributions(server_pk):
     for config in pulp_plugin_configs():
         if config.replicator_classes:
             for replicator_class in config.replicator_classes:
-                replicator = replicator_class(ctx, task_group, tls_settings, server)
-                supported_replicators.append(replicator)
+                req = PluginRequirement(config.label, specifier=replicator_class.required_version)
+                if ctx.has_plugin(req):
+                    replicator = replicator_class(ctx, task_group, tls_settings, server)
+                    supported_replicators.append(replicator)
 
     for replicator in supported_replicators:
         distros = replicator.upstream_distributions(q=server.q_select)