From 6992f958fc5eb8309f204da953664181256e96ed Mon Sep 17 00:00:00 2001
From: "M. J. Fromberger" <fromberger@tailscale.com>
Date: Wed, 17 Sep 2025 08:39:29 -0700
Subject: [PATCH] util/eventbus: add an EqualTo helper for testing (#17178)

For a common case of events being simple struct types with some exported
fields, add a helper to check (reflectively) for equal values using cmp.Diff so
that a failed comparison gives a useful diff in the test output.

More complex uses will still want to provide their own comparisons; this
(intentionally) does not export diff options or other hooks from the cmp
package.

Updates #15160

Change-Id: I86bee1771cad7debd9e3491aa6713afe6fd577a6
Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>
---
 util/eventbus/eventbustest/eventbustest.go    | 14 ++++++++
 .../eventbustest/eventbustest_test.go         | 35 ++++++++++++++-----
 2 files changed, 40 insertions(+), 9 deletions(-)

diff --git a/util/eventbus/eventbustest/eventbustest.go b/util/eventbus/eventbustest/eventbustest.go
index d5cfe5395..c32e71140 100644
--- a/util/eventbus/eventbustest/eventbustest.go
+++ b/util/eventbus/eventbustest/eventbustest.go
@@ -10,6 +10,7 @@ import (
 	"testing"
 	"time"
 
+	"github.com/google/go-cmp/cmp"
 	"tailscale.com/util/eventbus"
 )
 
@@ -249,3 +250,16 @@ func Inject[T any](inj *Injector, event T) {
 	}
 	pub.(*eventbus.Publisher[T]).Publish(event)
 }
+
+// EqualTo returns an event-matching function for use with [Expect] and
+// [ExpectExactly] that matches on an event of the given type that is equal to
+// want by comparison with [cmp.Diff]. The expectation fails with an error
+// message including the diff, if present.
+func EqualTo[T any](want T) func(T) error {
+	return func(got T) error {
+		if diff := cmp.Diff(got, want); diff != "" {
+			return fmt.Errorf("wrong result (-got, +want):\n%s", diff)
+		}
+		return nil
+	}
+}
diff --git a/util/eventbus/eventbustest/eventbustest_test.go b/util/eventbus/eventbustest/eventbustest_test.go
index 351553cc8..f8b37eefe 100644
--- a/util/eventbus/eventbustest/eventbustest_test.go
+++ b/util/eventbus/eventbustest/eventbustest_test.go
@@ -5,6 +5,7 @@ package eventbustest_test
 
 import (
 	"fmt"
+	"strings"
 	"testing"
 	"time"
 
@@ -29,19 +30,17 @@ func TestExpectFilter(t *testing.T) {
 		name       string
 		events     []int
 		expectFunc any
-		wantErr    bool
+		wantErr    string // if non-empty, an error is expected containing this text
 	}{
 		{
 			name:       "single event",
 			events:     []int{42},
 			expectFunc: eventbustest.Type[EventFoo](),
-			wantErr:    false,
 		},
 		{
 			name:       "multiple events, single expectation",
 			events:     []int{42, 1, 2, 3, 4, 5},
 			expectFunc: eventbustest.Type[EventFoo](),
-			wantErr:    false,
 		},
 		{
 			name:   "filter on event with function",
@@ -52,7 +51,6 @@ func TestExpectFilter(t *testing.T) {
 				}
 				return false, nil
 			},
-			wantErr: false,
 		},
 		{
 			name:   "filter-with-nil-error",
@@ -73,7 +71,7 @@ func TestExpectFilter(t *testing.T) {
 				}
 				return nil
 			},
-			wantErr: true,
+			wantErr: "value > 10",
 		},
 		{
 			name:   "first event has to be func",
@@ -84,7 +82,18 @@ func TestExpectFilter(t *testing.T) {
 				}
 				return false, nil
 			},
-			wantErr: true,
+			wantErr: "expected 42, got 24",
+		},
+		{
+			name:       "equal-values",
+			events:     []int{23},
+			expectFunc: eventbustest.EqualTo(EventFoo{Value: 23}),
+		},
+		{
+			name:       "unequal-values",
+			events:     []int{37},
+			expectFunc: eventbustest.EqualTo(EventFoo{Value: 23}),
+			wantErr:    "wrong result (-got, +want)",
 		},
 		{
 			name:   "no events",
@@ -92,7 +101,7 @@ func TestExpectFilter(t *testing.T) {
 			expectFunc: func(event EventFoo) (bool, error) {
 				return true, nil
 			},
-			wantErr: true,
+			wantErr: "timed out waiting",
 		},
 	}
 
@@ -113,8 +122,16 @@ func TestExpectFilter(t *testing.T) {
 				updater.Publish(EventFoo{i})
 			}
 
-			if err := eventbustest.Expect(tw, tt.expectFunc); (err != nil) != tt.wantErr {
-				t.Errorf("ExpectFilter[EventFoo]: error = %v, wantErr %v", err, tt.wantErr)
+			if err := eventbustest.Expect(tw, tt.expectFunc); err != nil {
+				if tt.wantErr == "" {
+					t.Errorf("Expect[EventFoo]: unexpected error: %v", err)
+				} else if !strings.Contains(err.Error(), tt.wantErr) {
+					t.Errorf("Expect[EventFoo]: err = %v, want %q", err, tt.wantErr)
+				} else {
+					t.Logf("Got expected error: %v (OK)", err)
+				}
+			} else if tt.wantErr != "" {
+				t.Errorf("Expect[EventFoo]: unexpectedly succeeded, want error %q", tt.wantErr)
 			}
 		})
 	}