fix: add Navidrome UserAgent in all outgoing requests (#6020)

* There has been a report about navidrome hitting listenbrainz hard
and the listenbrainz guys wanting to be able to distinguish navidrome

* feat: apply Navidrome User-Agent to all outgoing HTTP requests

Add utils/httpclient, a shared http.Client factory whose transport sets
the User-Agent header (Navidrome/{version} - https://github.com/navidrome)
on any request that does not already have one, and use it at every place
the server builds an HTTP client: Last.fm, ListenBrainz and Deezer agents
and auth routers, insights collector, backgrounds handler, and the plugin
host HTTP service. Plugin-set User-Agent values are preserved. The
per-request header lines from the previous commit are superseded by the
transport.

---------

Co-authored-by: Deluan <deluan@navidrome.org>

utils/httpclient/httpclient.go

@@ -0,0 +1,35 @@
+// Package httpclient provides a shared http.Client factory that identifies
+// Navidrome via the User-Agent header on all outgoing requests.
+package httpclient
+
+import (
+	"net/http"
+	"time"
+
+	"github.com/navidrome/navidrome/consts"
+)
+
+type uaTransport struct {
+	base http.RoundTripper
+}
+
+func (t *uaTransport) RoundTrip(req *http.Request) (*http.Response, error) {
+	if _, ok := req.Header["User-Agent"]; !ok {
+		req = req.Clone(req.Context())
+		req.Header.Set("User-Agent", consts.HTTPUserAgent)
+	}
+	return t.base.RoundTrip(req)
+}
+
+// NewTransport wraps base (or http.DefaultTransport if nil) to set the
+// Navidrome User-Agent on requests that don't have one.
+func NewTransport(base http.RoundTripper) http.RoundTripper {
+	if base == nil {
+		base = http.DefaultTransport
+	}
+	return &uaTransport{base: base}
+}
+
+func New(timeout time.Duration) *http.Client {
+	return &http.Client{Timeout: timeout, Transport: NewTransport(nil)}
+}

utils/httpclient/httpclient_suite_test.go

@@ -0,0 +1,17 @@
+package httpclient_test
+
+import (
+	"testing"
+
+	"github.com/navidrome/navidrome/log"
+	"github.com/navidrome/navidrome/tests"
+	. "github.com/onsi/ginkgo/v2"
+	. "github.com/onsi/gomega"
+)
+
+func TestHTTPClient(t *testing.T) {
+	tests.Init(t, false)
+	log.SetLevel(log.LevelFatal)
+	RegisterFailHandler(Fail)
+	RunSpecs(t, "HTTPClient Suite")
+}

utils/httpclient/httpclient_test.go

@@ -0,0 +1,76 @@
+package httpclient_test
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"time"
+
+	"github.com/navidrome/navidrome/consts"
+	"github.com/navidrome/navidrome/utils/httpclient"
+	. "github.com/onsi/ginkgo/v2"
+	. "github.com/onsi/gomega"
+)
+
+var _ = Describe("httpclient", func() {
+	var server *httptest.Server
+	var receivedUA string
+
+	BeforeEach(func() {
+		server = httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			receivedUA = r.Header.Get("User-Agent")
+		}))
+		DeferCleanup(server.Close)
+	})
+
+	Describe("New", func() {
+		It("sets the Navidrome User-Agent when the request has none", func() {
+			c := httpclient.New(time.Second)
+			resp, err := c.Get(server.URL)
+			Expect(err).ToNot(HaveOccurred())
+			resp.Body.Close()
+			Expect(receivedUA).To(Equal(consts.HTTPUserAgent))
+		})
+
+		It("keeps a User-Agent already set by the caller", func() {
+			c := httpclient.New(time.Second)
+			req, err := http.NewRequest(http.MethodGet, server.URL, nil)
+			Expect(err).ToNot(HaveOccurred())
+			req.Header.Set("User-Agent", "CustomAgent/1.0")
+			resp, err := c.Do(req)
+			Expect(err).ToNot(HaveOccurred())
+			resp.Body.Close()
+			Expect(receivedUA).To(Equal("CustomAgent/1.0"))
+		})
+
+		It("applies the given timeout", func() {
+			c := httpclient.New(5 * time.Second)
+			Expect(c.Timeout).To(Equal(5 * time.Second))
+		})
+	})
+
+	Describe("NewTransport", func() {
+		It("uses the default transport when base is nil", func() {
+			c := &http.Client{Transport: httpclient.NewTransport(nil)}
+			resp, err := c.Get(server.URL)
+			Expect(err).ToNot(HaveOccurred())
+			resp.Body.Close()
+			Expect(receivedUA).To(Equal(consts.HTTPUserAgent))
+		})
+
+		It("does not modify the original request", func() {
+			c := &http.Client{Transport: httpclient.NewTransport(nil)}
+			req, err := http.NewRequest(http.MethodGet, server.URL, nil)
+			Expect(err).ToNot(HaveOccurred())
+			resp, err := c.Do(req)
+			Expect(err).ToNot(HaveOccurred())
+			resp.Body.Close()
+			Expect(req.Header).ToNot(HaveKey("User-Agent"))
+		})
+	})
+
+	Describe("HTTPUserAgent", func() {
+		It("identifies Navidrome with version and project URL", func() {
+			Expect(consts.HTTPUserAgent).To(Equal("Navidrome/" + consts.Version + " - https://github.com/navidrome"))
+		})
+	})
+})