perf(informer): add TransformFuncs to reduce cache memory usage

Add cache transform functions for the Repository and PipelineRun
informers, stripping large unnecessary fields before objects enter
the informer cache. Inspired by tektoncd/pipeline#9316.

For Repository objects, ManagedFields, Annotations and Status are
stripped. The reconciler never reads Repository annotations or
Status from the lister; Status is always fetched fresh via direct
API call before updates.

For PipelineRun objects, ManagedFields and large Spec and Status
fields are stripped. The watcher only needs Annotations, Spec.Status
(pending check), Status.Conditions, and timing fields. All other
data is fetched directly from the API when needed.

Benchmark results with production-realistic objects show an 89% JSON
size reduction for Repository objects (5.6KB to 600B) and 94% for
PipelineRun objects (10.7KB to 677B), with corresponding 8-10x
reductions in heap allocation per cached object.

Signed-off-by: Akshay Pant <akpant@redhat.com>
Asisted-by: Claude <noreply@anthropic.com>

Author: theakshaypant

pkg/informer/transform/transform.go

@@ -0,0 +1,97 @@
+// Package transform provides cache transform functions for reducing memory
+// usage in the PAC watcher informer caches.
+//
+// Transform functions are applied to objects before they are stored in the
+// informer cache, allowing us to strip large, unnecessary fields while
+// preserving the data needed for reconciliation.
+//
+// DEVELOPER WARNING:
+// If you add new reconciliation logic that reads a field from cached objects
+// (via listers), you MUST verify that field is not stripped by these transforms.
+// Fields stripped from cached objects will be nil/empty even though they exist
+// in etcd. If you need a stripped field, fetch the full object via the API
+// client instead of the lister.
+package transform
+
+import (
+	pacv1alpha1 "github.com/openshift-pipelines/pipelines-as-code/pkg/apis/pipelinesascode/v1alpha1"
+	tektonv1 "github.com/tektoncd/pipeline/pkg/apis/pipeline/v1"
+	"k8s.io/client-go/tools/cache"
+)
+
+// RepositoryForCache strips fields from Repository objects before they are
+// stored in the informer cache to reduce memory usage.
+//
+// Fields stripped:
+//   - ManagedFields: written by the API server, not needed for reconciliation
+//   - Annotations: no reconciler logic reads Repository annotations from the
+//     lister; the largest annotation is kubectl.kubernetes.io/last-applied-configuration
+//   - Status: the reconciler always fetches Repository.Status via a direct API
+//     call before updating it; it is never read from the lister
+func RepositoryForCache(obj any) (any, error) {
+	if tombstone, ok := obj.(cache.DeletedFinalStateUnknown); ok {
+		transformed, err := RepositoryForCache(tombstone.Obj)
+		if err != nil {
+			return obj, nil //nolint:nilerr // return original on error for graceful degradation
+		}
+		return cache.DeletedFinalStateUnknown{Key: tombstone.Key, Obj: transformed}, nil
+	}
+
+	repo, ok := obj.(*pacv1alpha1.Repository)
+	if !ok {
+		return obj, nil
+	}
+
+	repo.ManagedFields = nil
+	repo.Annotations = nil
+	repo.Status = nil
+
+	return repo, nil
+}
+
+// PipelineRunForCache strips fields from PipelineRun objects before they are
+// stored in the informer cache to reduce memory usage.
+//
+// Fields the PAC watcher reads from cached PipelineRuns:
+//   - ObjectMeta: name, namespace, labels, annotations (PAC state/repo keys),
+//     finalizers, deletionTimestamp
+//   - Spec.Status: checked for PipelineRunSpecStatusPending
+//   - Status.Conditions: checked for completion state and reason
+//   - Status.StartTime, Status.CompletionTime: used for metrics
+//
+// All other Spec and Status fields are stripped. When the reconciler needs
+// the full object (e.g. postFinalStatus, GetStatusFromTaskStatusOrFromAsking),
+// it fetches it directly from the API server.
+func PipelineRunForCache(obj any) (any, error) {
+	if tombstone, ok := obj.(cache.DeletedFinalStateUnknown); ok {
+		transformed, err := PipelineRunForCache(tombstone.Obj)
+		if err != nil {
+			return obj, nil //nolint:nilerr // return original on error for graceful degradation
+		}
+		return cache.DeletedFinalStateUnknown{Key: tombstone.Key, Obj: transformed}, nil
+	}
+
+	pr, ok := obj.(*tektonv1.PipelineRun)
+	if !ok {
+		return obj, nil
+	}
+
+	pr.ManagedFields = nil
+
+	// Strip large Spec fields — watcher only checks Spec.Status (pending state)
+	pr.Spec.PipelineRef = nil
+	pr.Spec.PipelineSpec = nil
+	pr.Spec.Params = nil
+	pr.Spec.Workspaces = nil
+	pr.Spec.TaskRunSpecs = nil
+	pr.Spec.TaskRunTemplate = tektonv1.PipelineTaskRunTemplate{}
+	pr.Spec.Timeouts = nil
+
+	// Strip large Status fields — watcher only reads Conditions, StartTime, CompletionTime
+	pr.Status.PipelineSpec = nil
+	pr.Status.ChildReferences = nil
+	pr.Status.Provenance = nil
+	pr.Status.SpanContext = nil
+
+	return pr, nil
+}