transport: Add SetDefaultUserAgent

This adds a function to allow overriding the global default user agent.
The intention for this function is to allow clients to define the user
agent in main without needing to plumb through the user agent with an
option / reusable puller.

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)
+			}
+		})
+	}
+}