feat(snapshots): Run snapshot comparisons when uploads received out-of-order (#112474)

We discovered some cases where base is being uploaded after head. This
results in comparisons not being fired for the HEAD snapshots, which
ends up as a no base build found error.

This wires up snapshots to be triggered when a base is uploaded and has
a head that contains no comparison yet.

Resolves EME-1019

Author: rbro112

src/sentry/preprod/api/endpoints/preprod_artifact_snapshot.py

@@ -54,7 +54,10 @@
     PreprodSnapshotMetrics,
 )
 from sentry.preprod.snapshots.tasks import compare_snapshots
-from sentry.preprod.snapshots.utils import find_base_snapshot_artifact
+from sentry.preprod.snapshots.utils import (
+    find_base_snapshot_artifact,
+    find_head_snapshot_artifacts_awaiting_base,
+)
 from sentry.preprod.url_utils import get_preprod_artifact_url
 from sentry.preprod.vcs.status_checks.snapshots.tasks import (
     create_preprod_snapshot_status_check_task,
@@ -639,6 +642,51 @@ def post(self, request: Request, project: Project) -> Response:
                     },
                 )
 
+        # Trigger comparisons for any head artifacts that were uploaded before this base.
+        # Handles possible out-of-order uploads where heads arrive before their base build.
+        if commit_comparison is not None:
+            try:
+                waiting_heads = find_head_snapshot_artifacts_awaiting_base(
+                    organization_id=project.organization_id,
+                    base_sha=commit_comparison.head_sha,
+                    base_repo_name=commit_comparison.head_repo_name,
+                    project_id=project.id,
+                    app_id=artifact.app_id,
+                    build_configuration=artifact.build_configuration,
+                )
+                for head_artifact in waiting_heads:
+                    head_metrics = head_artifact.preprodsnapshotmetrics
+                    logger.info(
+                        "Found head artifact awaiting base, triggering snapshot comparison",
+                        extra={
+                            "head_artifact_id": head_artifact.id,
+                            "base_artifact_id": artifact.id,
+                            "base_sha": commit_comparison.head_sha,
+                        },
+                    )
+                    try:
+                        PreprodSnapshotComparison.objects.get_or_create(
+                            head_snapshot_metrics=head_metrics,
+                            base_snapshot_metrics=snapshot_metrics,
+                            defaults={"state": PreprodSnapshotComparison.State.PENDING},
+                        )
+                    except IntegrityError:
+                        pass
+
+                    compare_snapshots.apply_async(
+                        kwargs={
+                            "project_id": project.id,
+                            "org_id": project.organization_id,
+                            "head_artifact_id": head_artifact.id,
+                            "base_artifact_id": artifact.id,
+                        },
+                    )
+            except Exception:
+                logger.exception(
+                    "Failed to trigger comparisons for head artifacts awaiting base",
+                    extra={"base_artifact_id": artifact.id},
+                )
+
         return Response(
             {
                 "artifactId": str(artifact.id),

src/sentry/preprod/snapshots/utils.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 from sentry.preprod.models import PreprodArtifact, PreprodBuildConfiguration
+from sentry.preprod.snapshots.models import PreprodSnapshotComparison
 
 
 def find_base_snapshot_artifact(
@@ -26,3 +27,35 @@ def find_base_snapshot_artifact(
         .order_by("-date_added")
         .first()
     )
+
+
+def find_head_snapshot_artifacts_awaiting_base(
+    organization_id: int,
+    base_sha: str,
+    base_repo_name: str,
+    project_id: int,
+    app_id: str | None,
+    build_configuration: PreprodBuildConfiguration | None,
+) -> list[PreprodArtifact]:
+    """Find head snapshot artifacts that were uploaded before their base was available.
+
+    When a base artifact is uploaded, its commit_comparison.head_sha is the SHA that waiting
+    head artifacts have as their commit_comparison.base_sha. This finds those heads so
+    comparisons can be triggered retroactively.
+    """
+    return list(
+        PreprodArtifact.objects.filter(
+            commit_comparison__organization_id=organization_id,
+            commit_comparison__base_sha=base_sha,
+            commit_comparison__base_repo_name=base_repo_name,
+            project_id=project_id,
+            preprodsnapshotmetrics__isnull=False,
+            app_id=app_id,
+            build_configuration=build_configuration,
+        )
+        .exclude(
+            preprodsnapshotmetrics__snapshot_comparisons_head_metrics__state=PreprodSnapshotComparison.State.SUCCESS,
+        )
+        .select_related("preprodsnapshotmetrics")
+        .order_by("-date_added")
+    )

tests/sentry/preprod/api/endpoints/test_preprod_artifact_snapshot.py

@@ -5,7 +5,7 @@
 
 from sentry.models.commitcomparison import CommitComparison
 from sentry.preprod.models import PreprodArtifact
-from sentry.preprod.snapshots.models import PreprodSnapshotMetrics
+from sentry.preprod.snapshots.models import PreprodSnapshotComparison, PreprodSnapshotMetrics
 from sentry.testutils.cases import APITestCase
 
 
@@ -250,6 +250,88 @@ def test_snapshot_invalid_sha_format(self) -> None:
 
         assert response.status_code == 400
 
+    @patch("sentry.preprod.api.endpoints.preprod_artifact_snapshot.get_preprod_session")
+    @patch("sentry.preprod.api.endpoints.preprod_artifact_snapshot.compare_snapshots")
+    def test_base_upload_triggers_comparison_for_waiting_head(
+        self, mock_compare_snapshots, mock_get_session
+    ) -> None:
+        """
+        When a head snapshot is uploaded before its base, uploading the base should
+        retroactively trigger a comparison for the waiting head.
+        """
+        head_sha = "a" * 40
+        base_sha = "b" * 40
+        repo_name = "owner/repo"
+        app_id = "com.example.app"
+
+        # Simulate a head artifact that was uploaded before its base was available.
+        # It has a commit_comparison with base_sha pointing to the not-yet-uploaded base.
+        head_commit_comparison = CommitComparison.objects.create(
+            organization_id=self.org.id,
+            head_repo_name=repo_name,
+            head_sha=head_sha,
+            base_sha=base_sha,
+            provider="github",
+            head_ref="feature-branch",
+            base_repo_name=repo_name,
+        )
+        head_artifact = PreprodArtifact.objects.create(
+            project=self.project,
+            state=PreprodArtifact.ArtifactState.UPLOADED,
+            app_id=app_id,
+            commit_comparison=head_commit_comparison,
+        )
+        head_metrics = PreprodSnapshotMetrics.objects.create(
+            preprod_artifact=head_artifact,
+            image_count=1,
+            extras={
+                "manifest_key": f"{self.org.id}/{self.project.id}/{head_artifact.id}/manifest.json"
+            },
+        )
+
+        # No comparison exists yet — the base was missing when the head was uploaded.
+        assert not PreprodSnapshotComparison.objects.filter(
+            head_snapshot_metrics=head_metrics
+        ).exists()
+
+        # Upload the base snapshot. Its head_sha matches the head artifact's base_sha.
+        url = self._get_create_url()
+        data = {
+            "app_id": app_id,
+            "head_sha": base_sha,
+            "provider": "github",
+            "head_repo_name": repo_name,
+            "head_ref": "main",
+            "images": {
+                "img1": {"display_name": "Screen 1", "width": 375, "height": 812},
+            },
+        }
+
+        with self.feature("organizations:preprod-snapshots"):
+            response = self.client.post(url, data, format="json")
+
+        assert response.status_code == 200
+
+        base_artifact = PreprodArtifact.objects.get(id=response.data["artifactId"])
+        base_metrics = PreprodSnapshotMetrics.objects.get(preprod_artifact=base_artifact)
+
+        # A pending comparison record should have been created linking head to base.
+        comparison = PreprodSnapshotComparison.objects.get(
+            head_snapshot_metrics=head_metrics,
+            base_snapshot_metrics=base_metrics,
+        )
+        assert comparison.state == PreprodSnapshotComparison.State.PENDING
+
+        # The comparison task should have been queued for the waiting head.
+        mock_compare_snapshots.apply_async.assert_called_once_with(
+            kwargs={
+                "project_id": self.project.id,
+                "org_id": self.org.id,
+                "head_artifact_id": head_artifact.id,
+                "base_artifact_id": base_artifact.id,
+            }
+        )
+
 
 class ProjectPreprodSnapshotGetTest(APITestCase):
     def setUp(self) -> None: