feat(discovery): parse Compose-managed labels

internal/discovery/labels.go

@@ -0,0 +1,71 @@
+package discovery
+
+import (
+	"fmt"
+	"strings"
+)
+
+// ComposeLabels captures the four Compose-managed labels we need
+// to drive a `docker compose pull`/`up -d` against the right stack.
+type ComposeLabels struct {
+	Project     string
+	Service     string
+	WorkingDir  string
+	ConfigFiles []string
+}
+
+const (
+	labelProject     = "com.docker.compose.project"
+	labelService     = "com.docker.compose.service"
+	labelWorkingDir  = "com.docker.compose.project.working_dir"
+	labelConfigFiles = "com.docker.compose.project.config_files"
+)
+
+// ParseComposeLabels extracts the Compose labels we need. Returns an
+// error naming the missing label if any required field is absent —
+// in practice this should only happen if the container was not
+// started by Compose.
+func ParseComposeLabels(labels map[string]string) (ComposeLabels, error) {
+	get := func(key string) (string, error) {
+		v, ok := labels[key]
+		if !ok || v == "" {
+			return "", fmt.Errorf("missing required label: %s", key)
+		}
+		return v, nil
+	}
+
+	project, err := get(labelProject)
+	if err != nil {
+		return ComposeLabels{}, err
+	}
+	service, err := get(labelService)
+	if err != nil {
+		return ComposeLabels{}, err
+	}
+	workingDir, err := get(labelWorkingDir)
+	if err != nil {
+		return ComposeLabels{}, err
+	}
+	configFilesRaw, err := get(labelConfigFiles)
+	if err != nil {
+		return ComposeLabels{}, err
+	}
+
+	files := strings.Split(configFilesRaw, ",")
+	for i, f := range files {
+		files[i] = strings.TrimSpace(f)
+	}
+
+	return ComposeLabels{
+		Project:     project,
+		Service:     service,
+		WorkingDir:  workingDir,
+		ConfigFiles: files,
+	}, nil
+}
+
+// HasOptIn reports whether the labels include the opt-in marker with
+// value "true" (exact, case-sensitive — anything else is excluded).
+func HasOptIn(labels map[string]string, key string) bool {
+	return labels[key] == "true"
+}