fix(plugin): capture stderr during invocation

Capture process stderr in plugin invocations via executeCmd.

Closes #31843

Signed-off-by: Maksym Kondratenko <m.kondratenko.ua@gmail.com>

internal/plugin/error.go

@@ -15,15 +15,21 @@ limitations under the License.
 
 package plugin
 
+import "strconv"
+
 // InvokeExecError is returned when a plugin invocation returns a non-zero status/exit code
 // - subprocess plugin: child process exit code
 // - extism plugin: wasm function return code
 type InvokeExecError struct {
 	ExitCode int    // Exit code from plugin code execution
 	Err      error  // Underlying error
+	Stderr   []byte // Captured stderr output
 }
 
 // Error implements the error interface
 func (e *InvokeExecError) Error() string {
+	if len(e.Stderr) > 0 {
+		return e.Err.Error() + ": " + strconv.Quote(string(e.Stderr))
+	}
 	return e.Err.Error()
 }

internal/plugin/runtime_subprocess.go

@@ -166,17 +166,26 @@ func (r *SubprocessPluginRuntime) InvokeHook(event string) error {
 // right now we implement status and error return in 3 slightly different ways in this file
 // then replace the other three with a call to this func
 func executeCmd(prog *exec.Cmd, pluginName string) error {
+	var stderrBuf bytes.Buffer
+	if prog.Stderr != nil {
+		prog.Stderr = io.MultiWriter(prog.Stderr, &stderrBuf)
+	} else {
+		prog.Stderr = &stderrBuf
+	}
 	if err := prog.Run(); err != nil {
 		if eerr, ok := errors.AsType[*exec.ExitError](err); ok {
+			stderr := bytes.TrimSpace(stderrBuf.Bytes())
 			slog.Debug(
 				"plugin execution failed",
 				slog.String("pluginName", pluginName),
 				slog.String("error", err.Error()),
 				slog.Int("exitCode", eerr.ExitCode()),
-				slog.String("stderr", string(bytes.TrimSpace(eerr.Stderr))))
+				slog.String("stderr", string(stderr)),
+			)
 			return &InvokeExecError{
 				Err:      fmt.Errorf("plugin %q exited with error", pluginName),
 				ExitCode: eerr.ExitCode(),
+				Stderr:   stderr,
 			}
 		}
 

internal/plugin/runtime_subprocess_test.go

@@ -16,8 +16,10 @@ limitations under the License.
 package plugin
 
 import (
+	"bytes"
 	"fmt"
 	"os"
+	"os/exec"
 	"path/filepath"
 	"testing"
 
@@ -82,3 +84,30 @@ func TestSubprocessPluginRuntime(t *testing.T) {
 
 	assert.Nil(t, output)
 }
+
+func TestExecuteCmdCapturesStderr(t *testing.T) {
+	cmd := exec.Command("sh", "-c", `echo "plugin error" >&2; exit 1`)
+	err := executeCmd(cmd, "test-plugin")
+
+	require.Error(t, err)
+	var ieerr *InvokeExecError
+	require.ErrorAs(t, err, &ieerr)
+	assert.Equal(t, 1, ieerr.ExitCode)
+	assert.Equal(t, []byte("plugin error"), ieerr.Stderr)
+	assert.Equal(t, `plugin "test-plugin" exited with error: "plugin error"`, ieerr.Error())
+}
+
+func TestExecuteCmdTeesStderr(t *testing.T) {
+	var existing bytes.Buffer
+	cmd := exec.Command("sh", "-c", `echo "plugin error" >&2; exit 1`)
+	cmd.Stderr = &existing
+
+	err := executeCmd(cmd, "test-plugin")
+
+	require.Error(t, err)
+	var ieerr *InvokeExecError
+	require.ErrorAs(t, err, &ieerr)
+	assert.Equal(t, 1, ieerr.ExitCode)
+	assert.Equal(t, []byte("plugin error"), ieerr.Stderr)
+	assert.Equal(t, "plugin error\n", existing.String())
+}

pkg/cmd/load_plugins.go

@@ -124,7 +124,7 @@ func loadCLIPlugins(baseCmd *cobra.Command, out io.Writer) {
 				execErr := &plugin.InvokeExecError{}
 				if errors.As(err, &execErr) {
 					return CommandError{
-						error:    execErr.Err,
+						error:    execErr,
 						ExitCode: execErr.ExitCode,
 					}
 				}