MockWebhook: track shutdown status more gracefully

test/cli/acceptance/cli_test.go

@@ -422,7 +422,7 @@ receivers:
 		Tolerance: 1 * time.Second,
 	})
 	co := at.Collector("webhook")
-	wh := NewWebhook(co)
+	wh := NewWebhook(t, co)
 
 	amc := at.AlertmanagerCluster(fmt.Sprintf(conf, wh.Address()), 1)
 	require.NoError(t, amc.Start())
@@ -475,7 +475,7 @@ receivers:
 		Tolerance: 1 * time.Second,
 	})
 	co := at.Collector("webhook")
-	wh := NewWebhook(co)
+	wh := NewWebhook(t, co)
 
 	amc := at.AlertmanagerCluster(fmt.Sprintf(conf, wh.Address()), 1)
 	require.NoError(t, amc.Start())

test/testutils/acceptance.go

@@ -383,7 +383,7 @@ func (amc *AlertmanagerCluster) Terminate() {
 // data.
 func (am *Alertmanager) Terminate() {
 	am.T.Helper()
-	if am.cmd.Process != nil {
+	if am.cmd != nil && am.cmd.Process != nil {
 		if err := syscall.Kill(am.cmd.Process.Pid, syscall.SIGTERM); err != nil {
 			am.T.Logf("Error sending SIGTERM to Alertmanager process: %v", err)
 		}

test/testutils/mock.go

@@ -16,10 +16,12 @@ package testutils
 import (
 	"encoding/json"
 	"fmt"
+	"io"
 	"maps"
 	"net/http"
 	"net/http/httptest"
 	"reflect"
+	"sync/atomic"
 	"testing"
 	"time"
 
@@ -182,6 +184,7 @@ type MockWebhook struct {
 	opts      *AcceptanceOpts
 	collector *Collector
 	addr      string
+	closing   atomic.Bool
 
 	// Func is called early on when retrieving a notification by an
 	// Alertmanager. If Func returns true, the given notification is dropped.
@@ -202,6 +205,7 @@ func NewWebhook(t *testing.T, c *Collector) *MockWebhook {
 	wh.addr = server.Listener.Addr().String()
 
 	t.Cleanup(func() {
+		wh.closing.Store(true)
 		server.Close()
 	})
 
@@ -222,6 +226,10 @@ func (ws *MockWebhook) ServeHTTP(w http.ResponseWriter, req *http.Request) {
 
 	var v webhook.Message
 	if err := dec.Decode(&v); err != nil {
+		// During shutdown, ignore EOF errors from interrupted connections
+		if ws.closing.Load() && (err == io.EOF || err.Error() == "EOF") {
+			return
+		}
 		panic(err)
 	}