feat(cli): identify tap in the HTTP user agent

cmd/tap/tap.go

@@ -19,6 +19,8 @@ var licenseText string
 
 func main() {
 	cli.LicenseText = licenseText
+	// Hubs list signed-in clients by user agent; name tap rather than Go.
+	cli.InstallUserAgent()
 
 	ctx := context.Background()
 	// Signal handling is intentionally not registered at the entrypoint.

pkg/cli/user_agent.go

@@ -0,0 +1,31 @@
+package cli
+
+import (
+	"net/http"
+	"runtime"
+)
+
+// UserAgent identifies tap to the hubs it talks to, so a hub's list of
+// signed-in clients can say which tap, and on what platform, holds a login.
+func UserAgent() string {
+	return "tap/" + Version + " (" + runtime.GOOS + "/" + runtime.GOARCH + ")"
+}
+
+// userAgentTransport sets UserAgent on requests that do not set their own.
+type userAgentTransport struct{ base http.RoundTripper }
+
+func (t userAgentTransport) RoundTrip(r *http.Request) (*http.Response, error) {
+	if r.Header.Get("User-Agent") == "" {
+		r = r.Clone(r.Context())
+		r.Header.Set("User-Agent", UserAgent())
+	}
+	return t.base.RoundTrip(r)
+}
+
+// InstallUserAgent makes every request sent through http.DefaultTransport,
+// which tap's hub clients use, carry UserAgent. Call it once from main.
+func InstallUserAgent() {
+	if _, done := http.DefaultTransport.(userAgentTransport); !done {
+		http.DefaultTransport = userAgentTransport{base: http.DefaultTransport}
+	}
+}

pkg/cli/user_agent_test.go

@@ -0,0 +1,31 @@
+package cli
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"runtime"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestUserAgentTransport(t *testing.T) {
+	t.Parallel()
+	var got []string
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		got = append(got, r.Header.Get("User-Agent"))
+	}))
+	defer srv.Close()
+	client := &http.Client{Transport: userAgentTransport{base: http.DefaultTransport}}
+
+	_, err := client.Get(srv.URL)
+	require.NoError(t, err)
+	req, err := http.NewRequest(http.MethodGet, srv.URL, nil)
+	require.NoError(t, err)
+	req.Header.Set("User-Agent", "custom/1")
+	_, err = client.Do(req)
+	require.NoError(t, err)
+
+	require.Equal(t, []string{"tap/" + Version + " (" + runtime.GOOS + "/" + runtime.GOARCH + ")", "custom/1"}, got,
+		"tap's agent is the default; a request's own agent wins")
+}