Fix up attach filtering

pkg/compose/up.go

@@ -246,10 +246,7 @@ func (s *composeService) Up(ctx context.Context, project *types.Project, options
 	}
 
 	monitor.withListener(func(event api.ContainerEvent) {
-		if event.Type != api.ContainerEventStarted {
-			return
-		}
-		if slices.Contains(attached, event.ID) && !event.Restarting {
+		if !shouldFollowStartEvent(event, attached, options.Start.AttachTo) {
 			return
 		}
 		eg.Go(func() error {
@@ -302,3 +299,16 @@ func (s *composeService) Up(ctx context.Context, project *types.Project, options
 	}
 	return err
 }
+
+func shouldFollowStartEvent(event api.ContainerEvent, attached []string, attachTo []string) bool {
+	if event.Type != api.ContainerEventStarted {
+		return false
+	}
+	if len(attachTo) > 0 && !slices.Contains(attachTo, event.Service) {
+		return false
+	}
+	if slices.Contains(attached, event.ID) && !event.Restarting {
+		return false
+	}
+	return true
+}

pkg/compose/up_test.go

@@ -0,0 +1,104 @@
+/*
+   Copyright 2020 Docker Compose CLI 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 compose
+
+import (
+	"testing"
+
+	"gotest.tools/v3/assert"
+
+	"github.com/docker/compose/v5/pkg/api"
+)
+
+func TestShouldFollowStartEvent(t *testing.T) {
+	tests := []struct {
+		name     string
+		event    api.ContainerEvent
+		attached []string
+		attachTo []string
+		want     bool
+	}{
+		{
+			name: "ignores non-start events",
+			event: api.ContainerEvent{
+				Type:    api.ContainerEventExited,
+				Service: "validator",
+			},
+			attachTo: []string{"validator"},
+			want:     false,
+		},
+		{
+			name: "ignores services outside explicit attach selection",
+			event: api.ContainerEvent{
+				Type:    api.ContainerEventStarted,
+				Service: "event-bus-validator",
+				ID:      "event-bus-validator-1",
+			},
+			attachTo: []string{"validator"},
+			want:     false,
+		},
+		{
+			name: "ignores containers already attached unless restarting",
+			event: api.ContainerEvent{
+				Type:    api.ContainerEventStarted,
+				Service: "validator",
+				ID:      "validator-1",
+			},
+			attached: []string{"validator-1"},
+			attachTo: []string{"validator"},
+			want:     false,
+		},
+		{
+			name: "follows restarts for attached service",
+			event: api.ContainerEvent{
+				Type:       api.ContainerEventStarted,
+				Service:    "validator",
+				ID:         "validator-1",
+				Restarting: true,
+			},
+			attached: []string{"validator-1"},
+			attachTo: []string{"validator"},
+			want:     true,
+		},
+		{
+			name: "follows selected service when not already attached",
+			event: api.ContainerEvent{
+				Type:    api.ContainerEventStarted,
+				Service: "validator",
+				ID:      "validator-2",
+			},
+			attachTo: []string{"validator"},
+			want:     true,
+		},
+		{
+			name: "follows service when no explicit attach filter exists",
+			event: api.ContainerEvent{
+				Type:    api.ContainerEventStarted,
+				Service: "validator",
+				ID:      "validator-2",
+			},
+			want: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := shouldFollowStartEvent(tt.event, tt.attached, tt.attachTo)
+			assert.Equal(t, got, tt.want)
+		})
+	}
+}