feat(discovery): add SwarmDiscovery to find opted-in Swarm services by image

Verified the Docker SDK v28.5.2+incompatible ServiceList signature via go doc
before implementing: the options type is swarm.ServiceListOptions, not
types.ServiceListOptions as the brief assumed. Everything else (Service.ID,
Service.Spec via embedded Annotations for Name/Labels, TaskTemplate.ContainerSpec.Image)
matched the brief exactly.

Reuses discovery.ImagesMatch and discovery.HasOptIn rather than duplicating
matching/opt-in logic. Opt-in label is read from the service spec's own
labels since Swarm services have no local compose file to anchor a
STACKS_ROOT path check against (unlike Compose mode).

internal/discovery/swarm.go

@@ -0,0 +1,60 @@
+package discovery
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/docker/docker/api/types/swarm"
+)
+
+// SwarmDockerClient is the subset of the Docker SDK SwarmDiscovery depends
+// on. Defined as an interface so tests can supply a fake.
+type SwarmDockerClient interface {
+	ServiceList(ctx context.Context, opts swarm.ServiceListOptions) ([]swarm.Service, error)
+}
+
+// SwarmDiscovery orchestrates "given an image, which Swarm services should
+// we update?". Unlike Discovery, it has no filesystem path to check — the
+// opt-in label on the service itself is the only gate, since Swarm services
+// have no local compose file to anchor a STACKS_ROOT check against.
+type SwarmDiscovery struct {
+	cli        SwarmDockerClient
+	optInLabel string
+}
+
+// NewSwarm returns a SwarmDiscovery bound to the given Docker client and
+// opt-in label.
+func NewSwarm(cli SwarmDockerClient, optInLabel string) *SwarmDiscovery {
+	return &SwarmDiscovery{cli: cli, optInLabel: optInLabel}
+}
+
+// FindJobs lists Swarm services, filters by image match + opt-in label, and
+// returns one Job per matching service. Signature matches Discovery.FindJobs
+// so both satisfy api.Finder.
+func (d *SwarmDiscovery) FindJobs(ctx context.Context, image string) ([]Job, error) {
+	services, err := d.cli.ServiceList(ctx, swarm.ServiceListOptions{})
+	if err != nil {
+		return nil, fmt.Errorf("docker service list: %w", err)
+	}
+
+	var jobs []Job
+	for _, svc := range services {
+		if svc.Spec.TaskTemplate.ContainerSpec == nil {
+			continue
+		}
+		if !ImagesMatch(image, svc.Spec.TaskTemplate.ContainerSpec.Image) {
+			continue
+		}
+		if !HasOptIn(svc.Spec.Labels, d.optInLabel) {
+			continue
+		}
+
+		jobs = append(jobs, Job{
+			Service:   svc.Spec.Name,
+			ServiceID: svc.ID,
+			Image:     image,
+		})
+	}
+
+	return jobs, nil
+}

internal/discovery/swarm_test.go

@@ -0,0 +1,86 @@
+package discovery_test
+
+import (
+	"context"
+	"errors"
+	"testing"
+
+	"github.com/docker/docker/api/types/swarm"
+	"github.com/shcizo/package-updater/internal/discovery"
+	"github.com/stretchr/testify/require"
+)
+
+type fakeSwarmDockerClient struct {
+	services []swarm.Service
+	err      error
+}
+
+func (f *fakeSwarmDockerClient) ServiceList(_ context.Context, _ swarm.ServiceListOptions) ([]swarm.Service, error) {
+	return f.services, f.err
+}
+
+func mkService(id, name, image string, labels map[string]string) swarm.Service {
+	return swarm.Service{
+		ID: id,
+		Spec: swarm.ServiceSpec{
+			Annotations: swarm.Annotations{Name: name, Labels: labels},
+			TaskTemplate: swarm.TaskSpec{
+				ContainerSpec: &swarm.ContainerSpec{Image: image},
+			},
+		},
+	}
+}
+
+func TestSwarmFindJobs_MatchAndOptIn(t *testing.T) {
+	fake := &fakeSwarmDockerClient{services: []swarm.Service{
+		mkService("svc-myapp", "myapp_web", "registry.example.com/myapp:v1", map[string]string{
+			"se.shcizo.auto-update": "true",
+		}),
+		mkService("svc-other", "other_web", "registry.example.com/other:v1", map[string]string{
+			"se.shcizo.auto-update": "true",
+		}),
+	}}
+	d := discovery.NewSwarm(fake, "se.shcizo.auto-update")
+	jobs, err := d.FindJobs(context.Background(), "registry.example.com/myapp")
+	require.NoError(t, err)
+	require.Len(t, jobs, 1)
+	require.Equal(t, "myapp_web", jobs[0].Service)
+	require.Equal(t, "svc-myapp", jobs[0].ServiceID)
+	require.Equal(t, "registry.example.com/myapp", jobs[0].Image)
+	require.False(t, jobs[0].Refused)
+	require.Empty(t, jobs[0].WorkingDir)
+	require.Empty(t, jobs[0].ConfigFiles)
+}
+
+func TestSwarmFindJobs_SkipsWithoutOptIn(t *testing.T) {
+	fake := &fakeSwarmDockerClient{services: []swarm.Service{
+		mkService("svc-myapp", "myapp_web", "registry.example.com/myapp:v1", nil),
+	}}
+	d := discovery.NewSwarm(fake, "se.shcizo.auto-update")
+	jobs, err := d.FindJobs(context.Background(), "registry.example.com/myapp")
+	require.NoError(t, err)
+	require.Empty(t, jobs)
+}
+
+func TestSwarmFindJobs_DockerError(t *testing.T) {
+	fake := &fakeSwarmDockerClient{err: errors.New("connection refused")}
+	d := discovery.NewSwarm(fake, "se.shcizo.auto-update")
+	_, err := d.FindJobs(context.Background(), "registry.example.com/myapp")
+	require.Error(t, err)
+}
+
+func TestSwarmFindJobs_NoContainerSpecIsSkipped(t *testing.T) {
+	fake := &fakeSwarmDockerClient{services: []swarm.Service{
+		{
+			ID: "svc-weird",
+			Spec: swarm.ServiceSpec{
+				Annotations:  swarm.Annotations{Name: "weird", Labels: map[string]string{"se.shcizo.auto-update": "true"}},
+				TaskTemplate: swarm.TaskSpec{ContainerSpec: nil},
+			},
+		},
+	}}
+	d := discovery.NewSwarm(fake, "se.shcizo.auto-update")
+	jobs, err := d.FindJobs(context.Background(), "registry.example.com/myapp")
+	require.NoError(t, err)
+	require.Empty(t, jobs)
+}