fix(postrenderer): don't panic on a post-renderer plugin's malformed output

postRendererPlugin.Run type-asserted output.Message with the unchecked,
single-value form (output.Message.(schema.OutputMessagePostRendererV1)),
unlike the equivalent, already-guarded assertion in getterPlugin.Get
(pkg/getter/plugingetter.go), and then called
outputMessage.Manifests.Bytes() without checking whether Manifests was
nil. A plugin whose Invoke() returns a message with a nil Manifests
field (which is exactly what json.Unmarshal produces for a
WASM/extism/v1 plugin's output when its JSON response omits the
"manifests" key or sets it to null) crashes the whole helm process with
an unrecovered nil-pointer-dereference panic instead of a clean error.

This is a distinct, unrecovered-panic edge case of the already-known,
still-open "WASM post-renderer plugins receive empty manifests" defect
(#31832, attempted fix in stale/closed #31834): that report's own
echo-plugin reproducer round-trips {"manifests":{}} rather than
{"manifests":null}/omitted, which unmarshals to a non-nil empty Buffer
and therefore only hits the existing graceful "produced empty output"
error, not this panic.

Add the same ok-checked type assertion getterPlugin.Get already uses,
and treat a nil Manifests the same as an empty one.

Signed-off-by: zanarelli <zanarelli.dev@gmail.com>

pkg/postrenderer/postrenderer.go

@@ -72,11 +72,15 @@ func (r *postRendererPlugin) Run(renderedManifests *bytes.Buffer) (*bytes.Buffer
 		return nil, fmt.Errorf("failed to invoke post-renderer plugin %q: %w", r.plugin.Metadata().Name, err)
 	}
 
-	outputMessage := output.Message.(schema.OutputMessagePostRendererV1)
+	outputMessage, ok := output.Message.(schema.OutputMessagePostRendererV1)
+	if !ok {
+		return nil, fmt.Errorf("invalid output message type from plugin %q", r.plugin.Metadata().Name)
+	}
 
-	// If the binary returned almost nothing, it's likely that it didn't
-	// successfully render anything
-	if len(bytes.TrimSpace(outputMessage.Manifests.Bytes())) == 0 {
+	// If the binary returned almost nothing (including no Manifests buffer at
+	// all, which a malformed or misbehaving plugin can produce), it's likely
+	// that it didn't successfully render anything.
+	if outputMessage.Manifests == nil || len(bytes.TrimSpace(outputMessage.Manifests.Bytes())) == 0 {
 		return nil, fmt.Errorf("post-renderer %q produced empty output", r.plugin.Metadata().Name)
 	}
 

pkg/postrenderer/postrenderer_test.go

@@ -18,15 +18,47 @@ package postrenderer
 
 import (
 	"bytes"
+	"context"
 	"runtime"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 
+	"helm.sh/helm/v4/internal/plugin"
+	"helm.sh/helm/v4/internal/plugin/schema"
 	"helm.sh/helm/v4/pkg/cli"
 )
 
+// nilManifestPlugin simulates a plugin.Plugin whose Invoke() returns a
+// well-typed OutputMessagePostRendererV1 with a nil Manifests field. This is
+// exactly what internal/plugin's ExtismV1PluginRuntime.Invoke produces (via
+// json.Unmarshal into a freshly reflect.New'd struct) when a wasm/extism
+// post-renderer plugin's JSON output omits the "manifests" key or sets it to
+// null -- e.g. because the plugin errored internally but still exited 0, or
+// is simply buggy/malformed. No handcrafted wasm binary is required to prove
+// the bug: the defect is in postRendererPlugin.Run's handling of the output
+// message, not in how a specific Runtime constructs it.
+type nilManifestPlugin struct{}
+
+func (nilManifestPlugin) Dir() string { return "" }
+func (nilManifestPlugin) Metadata() plugin.Metadata {
+	return plugin.Metadata{Name: "nil-manifest-plugin", Type: "postrenderer/v1"}
+}
+
+func (nilManifestPlugin) Invoke(_ context.Context, _ *plugin.Input) (*plugin.Output, error) {
+	return &plugin.Output{
+		Message: schema.OutputMessagePostRendererV1{Manifests: nil},
+	}, nil
+}
+
+func TestPostRendererPluginRunWithNilManifestsDoesNotPanic(t *testing.T) {
+	r := &postRendererPlugin{plugin: nilManifestPlugin{}}
+
+	_, err := r.Run(bytes.NewBufferString("apiVersion: v1\nkind: ConfigMap\n"))
+	require.Error(t, err, "a plugin returning nil Manifests should produce a clean error, not a panic")
+}
+
 func TestNewPostRenderPluginRunWithNoOutput(t *testing.T) {
 	if runtime.GOOS == "windows" {
 		// the actual Run test uses a basic sed example, so skip this test on windows