fix(dynamic-sampling): enusre that at leas one project has count (#81727

)

Make sure that at least one project with counts is present before doing
a Project Rebalancing run

Closes #81556

src/sentry/dynamic_sampling/tasks/boost_low_volume_projects.py

@@ -401,6 +401,10 @@ def adjust_sample_rates_of_projects(
     projects_with_counts = {
         project_id: count_per_root for project_id, count_per_root, _, _ in projects_with_tx_count
     }
+    # The rebalancing will not work (or would make sense) when we have only projects with zero-counts.
+    if not any(projects_with_counts.values()):
+        return
+
     # Since we don't mind about strong consistency, we query a replica of the main database with the possibility of
     # having out of date information. This is a trade-off we accept, since we work under the assumption that eventually
     # the projects of an org will be replicated consistently across replicas, because no org should continue to create

tests/sentry/dynamic_sampling/tasks/test_boost_low_volume_projects.py

@@ -1,5 +1,6 @@
 from datetime import timedelta
 from typing import cast
+from unittest.mock import patch
 
 from django.utils import timezone
 
@@ -183,6 +184,22 @@ def test_project_mode_sampling_with_query(self):
 
         assert get_guarded_project_sample_rate(org1, p1) == 0.2
 
+    @with_feature(["organizations:dynamic-sampling", "organizations:dynamic-sampling-custom"])
+    def test_project_mode_sampling_with_query_zero_metrics(self):
+        organization = self.create_organization("test-org")
+        project = self.create_project(organization=organization)
+
+        organization.update_option("sentry:sampling_mode", DynamicSamplingMode.PROJECT)
+        project.update_option("sentry:target_sample_rate", 0.2)
+
+        # make sure that no rebalancing is actually run
+        with patch(
+            "sentry.dynamic_sampling.models.projects_rebalancing.ProjectsRebalancingModel._run"
+        ) as mock_run:
+            with self.tasks():
+                boost_low_volume_projects.delay()
+            assert not mock_run.called
+
     def test_complex(self):
         context = TaskContext("rebalancing", 20)
         org1 = self.create_organization("test-org1")