Fix process Stop timeout error handling

pkg/internal/testing/process/process.go

@@ -18,6 +18,7 @@ package process
 
 import (
 	"crypto/tls"
+	"errors"
 	"fmt"
 	"io"
 	"net"
@@ -128,6 +129,8 @@ func (ps *State) Init(name string) error {
 
 type stopChannel chan struct{}
 
+var signalProcess = signalProcessImpl
+
 // CheckFlag checks the help output of this command for the presence of the given flag, specified
 // without the leading `--` (e.g. `CheckFlag("insecure-port")` checks for `--insecure-port`),
 // returning true if the flag is present.
@@ -266,10 +269,11 @@ func (ps *State) Stop() error {
 	case <-ps.waitDone:
 		break
 	case <-timedOut:
+		timeoutErr := fmt.Errorf("timeout waiting for process %s to stop", path.Base(ps.Path))
 		if err := signalProcess(ps.Cmd.Process, syscall.SIGKILL); err != nil {
-			return fmt.Errorf("unable to kill process %s: %w", ps.Path, err)
+			return errors.Join(timeoutErr, fmt.Errorf("unable to kill process %s: %w", ps.Path, err))
 		}
-		return fmt.Errorf("timeout waiting for process %s to stop", path.Base(ps.Path))
+		return timeoutErr
 	}
 	ps.ready = false
 	return nil

pkg/internal/testing/process/process_internal_test.go

@@ -0,0 +1,69 @@
+/*
+Copyright 2026 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package process
+
+import (
+	"errors"
+	"os"
+	"os/exec"
+	"strings"
+	"syscall"
+	"testing"
+	"time"
+)
+
+func TestStopReturnsTimeoutWhenKillAfterTimeoutFails(t *testing.T) {
+	process, err := os.FindProcess(os.Getpid())
+	if err != nil {
+		t.Fatalf("failed to find current process: %v", err)
+	}
+
+	originalSignalProcess := signalProcess
+	defer func() {
+		signalProcess = originalSignalProcess
+	}()
+
+	signalCount := 0
+	signalProcess = func(_ *os.Process, sig syscall.Signal) error {
+		signalCount++
+		if sig == syscall.SIGKILL {
+			return errors.New("kill failed")
+		}
+		return nil
+	}
+
+	ps := &State{
+		Cmd:         &exec.Cmd{Process: process},
+		Path:        "/path/to/test-process",
+		StopTimeout: time.Nanosecond,
+		waitDone:    make(chan struct{}),
+	}
+
+	err = ps.Stop()
+	if err == nil {
+		t.Fatal("expected Stop to return an error")
+	}
+	if !strings.Contains(err.Error(), "timeout waiting for process test-process to stop") {
+		t.Fatalf("expected timeout error, got %q", err.Error())
+	}
+	if !strings.Contains(err.Error(), "unable to kill process /path/to/test-process: kill failed") {
+		t.Fatalf("expected kill error to be included, got %q", err.Error())
+	}
+	if signalCount != 2 {
+		t.Fatalf("expected SIGTERM and SIGKILL to be sent, got %d signals", signalCount)
+	}
+}

pkg/internal/testing/process/signal_other.go

@@ -23,6 +23,6 @@ import (
 	"syscall"
 )
 
-func signalProcess(process *os.Process, sig syscall.Signal) error {
+func signalProcessImpl(process *os.Process, sig syscall.Signal) error {
 	return process.Signal(sig)
 }

pkg/internal/testing/process/signal_unix.go

@@ -23,6 +23,6 @@ import (
 	"syscall"
 )
 
-func signalProcess(process *os.Process, sig syscall.Signal) error {
+func signalProcessImpl(process *os.Process, sig syscall.Signal) error {
 	return syscall.Kill(-process.Pid, sig)
 }