feat(server): add shared user avatar serving helper

ServeUserAvatar lives in server/imghttp so both server/subsonic and
server/jellyfin can call it without importing each other. It delegates
ETag/If-None-Match handling and Content-Type detection to
http.ServeContent instead of hand-parsing the header, avoiding a
false-304 on multi-valued or wildcard If-None-Match headers.

server/imghttp/avatar.go

@@ -0,0 +1,34 @@
+package imghttp
+
+import (
+	"net/http"
+	"os"
+
+	"github.com/navidrome/navidrome/log"
+	"github.com/navidrome/navidrome/model"
+)
+
+// ServeUserAvatar writes the user's uploaded avatar, reporting whether it answered the request;
+// when false the caller must fall back. http.ServeContent gives correct ETag/If-None-Match handling.
+func ServeUserAvatar(w http.ResponseWriter, r *http.Request, u *model.User) bool {
+	path := u.UploadedImagePath()
+	if path == "" {
+		return false
+	}
+	f, err := os.Open(path)
+	if err != nil {
+		log.Warn(r.Context(), "Could not open user avatar", "user", u.UserName, err)
+		return false
+	}
+	defer f.Close()
+	info, err := f.Stat()
+	if err != nil {
+		log.Warn(r.Context(), "Could not stat user avatar", "user", u.UserName, err)
+		return false
+	}
+
+	w.Header().Set("ETag", `"`+u.AvatarTag()+`"`)
+	w.Header().Set("Cache-Control", "private, no-cache")
+	http.ServeContent(w, r, info.Name(), info.ModTime(), f)
+	return true
+}

server/imghttp/avatar_test.go

@@ -0,0 +1,87 @@
+package imghttp_test
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"path/filepath"
+	"time"
+
+	"github.com/navidrome/navidrome/conf"
+	"github.com/navidrome/navidrome/conf/configtest"
+	"github.com/navidrome/navidrome/model"
+	"github.com/navidrome/navidrome/server/imghttp"
+	. "github.com/onsi/ginkgo/v2"
+	. "github.com/onsi/gomega"
+)
+
+var _ = Describe("ServeUserAvatar", func() {
+	var w *httptest.ResponseRecorder
+	var r *http.Request
+	var usr model.User
+
+	BeforeEach(func() {
+		DeferCleanup(configtest.SetupConfig())
+		conf.Server.DataFolder = conf.NewDir(GinkgoT().TempDir())
+		w = httptest.NewRecorder()
+		r = httptest.NewRequest("GET", "/avatar", nil)
+		usr = model.User{ID: "u1", UserName: "deluan", UpdatedAt: time.Unix(1000, 0)}
+	})
+
+	It("returns false when the user has no avatar", func() {
+		Expect(imghttp.ServeUserAvatar(w, r, &usr)).To(BeFalse())
+	})
+
+	It("returns false when the file is missing on disk", func() {
+		usr.UploadedImage = "u1_deluan.png"
+		Expect(imghttp.ServeUserAvatar(w, r, &usr)).To(BeFalse())
+	})
+
+	It("serves the file with a content type and an ETag", func() {
+		usr.UploadedImage = writeAvatar(usr, "png")
+		Expect(imghttp.ServeUserAvatar(w, r, &usr)).To(BeTrue())
+		Expect(w.Code).To(Equal(http.StatusOK))
+		Expect(w.Header().Get("Content-Type")).To(Equal("image/png"))
+		Expect(w.Header().Get("ETag")).To(Equal(`"` + usr.AvatarTag() + `"`))
+	})
+
+	It("answers 304 when the ETag matches", func() {
+		usr.UploadedImage = writeAvatar(usr, "png")
+		r.Header.Set("If-None-Match", `"`+usr.AvatarTag()+`"`)
+		Expect(imghttp.ServeUserAvatar(w, r, &usr)).To(BeTrue())
+		Expect(w.Code).To(Equal(http.StatusNotModified))
+	})
+
+	It("does not leak the absolute filesystem path in the response", func() {
+		usr.UploadedImage = writeAvatar(usr, "png")
+		Expect(imghttp.ServeUserAvatar(w, r, &usr)).To(BeTrue())
+		Expect(w.Body.String()).NotTo(ContainSubstring(conf.Server.DataFolder.String()))
+		for _, values := range w.Header() {
+			for _, v := range values {
+				Expect(v).NotTo(ContainSubstring(conf.Server.DataFolder.String()))
+			}
+		}
+	})
+
+	It("does not false-304 on an unrelated multi-value If-None-Match", func() {
+		usr.UploadedImage = writeAvatar(usr, "png")
+		r.Header.Set("If-None-Match", `"deadbeefdeadbeef", "cafecafecafecafe"`)
+		Expect(imghttp.ServeUserAvatar(w, r, &usr)).To(BeTrue())
+		Expect(w.Code).To(Equal(http.StatusOK))
+	})
+
+	It("treats If-None-Match: * as matching the current representation", func() {
+		usr.UploadedImage = writeAvatar(usr, "png")
+		r.Header.Set("If-None-Match", "*")
+		Expect(imghttp.ServeUserAvatar(w, r, &usr)).To(BeTrue())
+		Expect(w.Code).To(Equal(http.StatusNotModified))
+	})
+})
+
+func writeAvatar(u model.User, ext string) string {
+	name := u.ID + "_" + u.UserName + "." + ext
+	path := filepath.Join(conf.Server.DataFolder.String(), "avatar", name)
+	Expect(os.MkdirAll(filepath.Dir(path), 0755)).To(Succeed())
+	Expect(os.WriteFile(path, []byte{0x89, 'P', 'N', 'G'}, 0600)).To(Succeed())
+	return name
+}