feat(discovery): report available content pack versions in metadata

pkg/proxy/discovery/metadata_test.go

@@ -0,0 +1,56 @@
+package discovery
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+// pack_versions must list every linux-inventory-v<N>.yaml in pack_dir,
+// ascending, ignoring unrelated files — the server picks a version from this
+// list to pin in discovery_inventory requests, since content_pack_version is
+// a required request param with no "latest" default.
+func TestCollectMetadata_ReportsPackVersions(t *testing.T) {
+	dir := t.TempDir()
+	for _, name := range []string{
+		"linux-inventory-v2.yaml",
+		"linux-inventory-v10.yaml",
+		"linux-inventory-vX.yaml", // non-numeric — ignored
+		"windows-inventory-v1.yaml",
+		"README.md",
+	} {
+		if err := os.WriteFile(filepath.Join(dir, name), []byte("x"), 0o600); err != nil {
+			t.Fatalf("writing %s: %v", name, err)
+		}
+	}
+
+	p, _ := newTestProxy(t, map[string]any{"pack_dir": dir}, nil)
+
+	meta, err := p.CollectMetadata(context.Background())
+	if err != nil {
+		t.Fatalf("CollectMetadata: %v", err)
+	}
+	versions, ok := meta["pack_versions"].([]int)
+	if !ok {
+		t.Fatalf("pack_versions missing or wrong type: %#v", meta["pack_versions"])
+	}
+	if len(versions) != 2 || versions[0] != 2 || versions[1] != 10 {
+		t.Fatalf("pack_versions = %v, want [2 10]", versions)
+	}
+}
+
+// Without a pack_dir there is nothing to report — the key must be absent,
+// not an empty list, so the server treats it the same as an older agent
+// that predates the field.
+func TestCollectMetadata_NoPackDirOmitsPackVersions(t *testing.T) {
+	p, _ := newTestProxy(t, map[string]any{}, nil)
+
+	meta, err := p.CollectMetadata(context.Background())
+	if err != nil {
+		t.Fatalf("CollectMetadata: %v", err)
+	}
+	if _, present := meta["pack_versions"]; present {
+		t.Fatalf("pack_versions should be absent without pack_dir, got %#v", meta["pack_versions"])
+	}
+}

pkg/proxy/discovery/proxy.go

@@ -10,6 +10,9 @@ import (
 	"net/netip"
 	"os"
 	"path/filepath"
+	"regexp"
+	"sort"
+	"strconv"
 	"strings"
 	"sync"
 	"time"
@@ -658,9 +661,60 @@ func (p *Proxy) Actions() []string {
 }
 
 // CollectMetadata satisfies proxy.MetadataCollector so the supported actions
-// reach the server alongside the datasource inventory.
+// reach the server alongside the datasource inventory. pack_versions lists
+// the content pack versions present in pack_dir so the server can pick a
+// version to pin in discovery_inventory requests (content_pack_version is a
+// required request param with no "latest" default). Presence means the file
+// exists — signature verification still happens at execution time.
 func (p *Proxy) CollectMetadata(ctx context.Context) (map[string]any, error) {
-	return map[string]any{"actions": p.Actions()}, nil
+	meta := map[string]any{"actions": p.Actions()}
+	if versions := p.packVersions(); len(versions) > 0 {
+		meta["pack_versions"] = versions
+	}
+	return meta, nil
+}
+
+// packVersionPattern matches cached pack filenames (linux-inventory-v<N>.yaml),
+// mirroring the path resolvePack reads.
+var packVersionPattern = regexp.MustCompile(`^linux-inventory-v(\d+)\.yaml$`)
+
+// packVersions lists content pack versions available in pack_dir, ascending.
+func (p *Proxy) packVersions() []int {
+	p.mu.RLock()
+	packDir := p.cfg.PackDir
+	p.mu.RUnlock()
+	if packDir == "" {
+		return nil
+	}
+
+	entries, err := os.ReadDir(packDir)
+	if err != nil {
+		// A configured-but-not-yet-created pack_dir is a normal fresh-install
+		// state (packs arrive via the distribution pipeline) — don't warn on
+		// every metadata cycle for it.
+		if !os.IsNotExist(err) {
+			p.logger.Warn("discovery: cannot list pack_dir for metadata", "err", err)
+		}
+		return nil
+	}
+
+	var versions []int
+	for _, e := range entries {
+		if e.IsDir() {
+			continue
+		}
+		m := packVersionPattern.FindStringSubmatch(e.Name())
+		if m == nil {
+			continue
+		}
+		v, err := strconv.Atoi(m[1])
+		if err != nil || v <= 0 {
+			continue
+		}
+		versions = append(versions, v)
+	}
+	sort.Ints(versions)
+	return versions
 }
 
 func (p *Proxy) Close() error {