[Backport 7.77.x] fix(cluster-agent): Guard against re-admission for APM auto-instrumentation in image_volume mode (#46819)

Backport 2d33331 from #46743.

 ___

### What does this PR do?
Avoid double-injection by returning early if the pod already has image_volume mode's init containers. 
Init_container mode was already guarded by checking for per-language init containers (e.g. datadog-lib-python-init). This change adds the same style of guard for image_volume mode by checking for the datadog-apm-inject-preload init container.

### Motivation
The webhook may be run twice, but we do not want to inject twice. CSI mode needs a guard in the future as well.

### Describe how you validated your changes
Tests in target_mutator_test.go were added for both re-admission cases: one for init_container mode and one for image_volume mode. The test asserts that the pod is not changed ("mutated") at all in the case that the representative init container(s) are present.

### Additional Notes

Co-authored-by: sabrina.lu <sabrina.lu@datadoghq.com>

Author: dd-octo-sts[bot]

pkg/clusteragent/admission/mutate/autoinstrumentation/target_mutator.go

@@ -23,6 +23,7 @@ import (
 	"github.com/DataDog/datadog-agent/pkg/clusteragent/admission/metrics"
 	"github.com/DataDog/datadog-agent/pkg/clusteragent/admission/mutate/autoinstrumentation/annotation"
 	"github.com/DataDog/datadog-agent/pkg/clusteragent/admission/mutate/autoinstrumentation/imageresolver"
+	"github.com/DataDog/datadog-agent/pkg/clusteragent/admission/mutate/autoinstrumentation/libraryinjection"
 	mutatecommon "github.com/DataDog/datadog-agent/pkg/clusteragent/admission/mutate/common"
 	"github.com/DataDog/datadog-agent/pkg/util/log"
 )
@@ -185,12 +186,18 @@ func (m *TargetMutator) MutatePod(pod *corev1.Pod, ns string, _ dynamic.Interfac
 	log.Debugf("Mutating pod in target mutator %q", mutatecommon.PodString(pod))
 
 	// The admission can be re-run for the same pod. Fast return if we injected the library already.
+	// Check for the init_container mode's per-language init containers.
 	for _, lang := range supportedLanguages {
 		if containsInitContainer(pod, initContainerName(lang)) {
 			log.Debugf("Init container %q already exists in pod %q", initContainerName(lang), mutatecommon.PodString(pod))
 			return false, nil
 		}
 	}
+	// Check for the image_volume mode's init container.
+	if containsInitContainer(pod, libraryinjection.InjectLDPreloadInitContainerName) {
+		log.Debugf("Init container %q already exists in pod %q", libraryinjection.InjectLDPreloadInitContainerName, mutatecommon.PodString(pod))
+		return false, nil
+	}
 
 	// Get the target to inject. If there is not target, we should not mutate the pod.
 	target := m.getTarget(pod)

pkg/clusteragent/admission/mutate/autoinstrumentation/target_mutator_test.go

@@ -26,6 +26,7 @@ import (
 	"github.com/DataDog/datadog-agent/pkg/clusteragent/admission/common"
 	"github.com/DataDog/datadog-agent/pkg/clusteragent/admission/mutate/autoinstrumentation/annotation"
 	"github.com/DataDog/datadog-agent/pkg/clusteragent/admission/mutate/autoinstrumentation/imageresolver"
+	"github.com/DataDog/datadog-agent/pkg/clusteragent/admission/mutate/autoinstrumentation/libraryinjection"
 	mutatecommon "github.com/DataDog/datadog-agent/pkg/clusteragent/admission/mutate/common"
 	configmock "github.com/DataDog/datadog-agent/pkg/config/mock"
 	"github.com/DataDog/datadog-agent/pkg/languagedetection/languagemodels"
@@ -142,6 +143,34 @@ func TestMutatePod(t *testing.T) {
 			},
 			expectNoChange: true,
 		},
+		// Re-admission guard: when the webhook runs again on an already-injected pod we must not
+		// mutate further (e.g. must not append to LD_PRELOAD or add duplicate init containers).
+		"re-admission with init_container mode init container already present does not mutate": {
+			configPath: "testdata/filter_simple_namespace.yaml",
+			in: mutatecommon.FakePodSpec{
+				NS: "application",
+				InitContainers: []corev1.Container{
+					{Name: "datadog-lib-python-init", Image: "registry/dd-lib-python-init:v3"},
+				},
+			}.Create(),
+			namespaces: []workloadmeta.KubernetesMetadata{
+				newTestNamespace("application", nil),
+			},
+			expectNoChange: true,
+		},
+		"re-admission with image_volume mode init container already present does not mutate": {
+			configPath: "testdata/filter_simple_namespace.yaml",
+			in: mutatecommon.FakePodSpec{
+				NS: "application",
+				InitContainers: []corev1.Container{
+					{Name: libraryinjection.InjectLDPreloadInitContainerName, Image: "registry/apm-inject:0"},
+				},
+			}.Create(),
+			namespaces: []workloadmeta.KubernetesMetadata{
+				newTestNamespace("application", nil),
+			},
+			expectNoChange: true,
+		},
 		"tracer configs get applied": {
 			configPath: "testdata/filter_simple_configs.yaml",
 			in: mutatecommon.WithLabels(

releasenotes/notes/guard-readmission-ssi-imgvol-e9dfef7c4b5b8b0d.yaml

@@ -0,0 +1,3 @@
+fixes:
+  - |
+    Fixes a bug in the admission controller webhook that allowed admission to re-run for pods that already had APM injection in image-volume mode.