transport: Add SetDefaultUserAgent

pkg/v1/remote/transport/useragent.go

@@ -25,12 +25,13 @@ var (
 	// -ldflags="-X 'github.com/google/go-containerregistry/pkg/v1/remote/transport.Version=$TAG'"
 	Version string
 
-	ggcrVersion = defaultUserAgent
+	defaultUserAgent string
+	ggcrVersion      = ggcrProduct
 )
 
 const (
-	defaultUserAgent = "go-containerregistry"
-	moduleName       = "github.com/google/go-containerregistry"
+	ggcrProduct = "go-containerregistry"
+	moduleName  = "github.com/google/go-containerregistry"
 )
 
 type userAgentTransport struct {
@@ -40,7 +41,7 @@ type userAgentTransport struct {
 
 func init() {
 	if v := version(); v != "" {
-		ggcrVersion = fmt.Sprintf("%s/%s", defaultUserAgent, v)
+		ggcrVersion = fmt.Sprintf("%s/%s", ggcrProduct, v)
 	}
 }
 
@@ -76,6 +77,10 @@ func version() string {
 //
 // User-Agent: crane/v0.1.4 go-containerregistry/v0.1.4
 func NewUserAgent(inner http.RoundTripper, ua string) http.RoundTripper {
+	if ua == "" {
+		ua = defaultUserAgent
+	}
+	// defaultUserAgent might not be set, so check this again.
 	if ua == "" {
 		ua = ggcrVersion
 	} else {
@@ -92,3 +97,11 @@ func (ut *userAgentTransport) RoundTrip(in *http.Request) (*http.Response, error
 	in.Header.Set("User-Agent", ut.ua)
 	return ut.inner.RoundTrip(in)
 }
+
+// SetDefaultUserAgent sets the global default user agent string.
+// Default user agent behavior follows that of [NewUserAgent], in that the resulting
+// user agent string will include both the provided user agent and the go-containerregistry
+// version.
+func SetDefaultUserAgent(ua string) {
+	defaultUserAgent = ua
+}

pkg/v1/remote/transport/useragent_test.go

@@ -0,0 +1,44 @@
+package transport
+
+import (
+	"testing"
+)
+
+func TestDefaultUserAgent(t *testing.T) {
+	for _, tc := range []struct {
+		defaultUA string
+		ua        string
+		want      string
+	}{
+		{
+			want: "go-containerregistry",
+		},
+		{
+			defaultUA: "foo",
+			want:      "foo go-containerregistry",
+		},
+		{
+			ua:   "bar",
+			want: "bar go-containerregistry",
+		},
+		{
+			defaultUA: "foo",
+			ua:        "bar",
+			want:      "bar go-containerregistry",
+		},
+	} {
+		t.Run("", func(t *testing.T) {
+			SetDefaultUserAgent(tc.defaultUA)
+			t.Cleanup(func() {
+				SetDefaultUserAgent("")
+			})
+			rt, ok := NewUserAgent(nil, tc.ua).(*userAgentTransport)
+			if !ok {
+				t.Fatalf("NewUserAgent returned a %T, want *userAgentTransport", rt)
+			}
+			if rt.ua != tc.want {
+				t.Errorf("want %q, got %q", tc.want, rt.ua)
+			}
+		})
+	}
+}