GetCoverArt returns placeholder if id is missing

This mimics Subsonic behaviour, even if it contradicts the API documentation, which states `id` is required

Fixes #1139

server/subsonic/media_retrieval.go

@@ -58,10 +58,7 @@ func (c *MediaRetrievalController) getPlaceHolderAvatar(w http.ResponseWriter, r
 }
 
 func (c *MediaRetrievalController) GetCoverArt(w http.ResponseWriter, r *http.Request) (*responses.Subsonic, error) {
-	id, err := requiredParamString(r, "id")
+	id := utils.ParamStringDefault(r, "id", "non-existent")
-	if err != nil {
-		return nil, err
-	}
 	size := utils.ParamInt(r, "size", 0)
 
 	w.Header().Set("cache-control", "public, max-age=315360000")

server/subsonic/media_retrieval_test.go

@@ -37,11 +37,12 @@ var _ = Describe("MediaRetrievalController", func() {
 			Expect(w.Body.String()).To(Equal(artwork.data))
 		})
 
-		It("should fail if missing id parameter", func() {
+		It("should return placeholder if id parameter is missing (mimicking Subsonic)", func() {
 			r := newGetRequest()
 			_, err := controller.GetCoverArt(w, r)
 
-			Expect(err).To(MatchError("required 'id' parameter is missing"))
+			Expect(err).To(BeNil())
+			Expect(w.Body.String()).To(Equal(artwork.data))
 		})
 
 		It("should fail when the file is not found", func() {

utils/request_helpers.go

@@ -11,6 +11,14 @@ func ParamString(r *http.Request, param string) string {
 	return r.URL.Query().Get(param)
 }
 
+func ParamStringDefault(r *http.Request, param, def string) string {
+	v := ParamString(r, param)
+	if v == "" {
+		return def
+	}
+	return v
+}
+
 func ParamStrings(r *http.Request, param string) []string {
 	return r.URL.Query()[param]
 }

utils/request_helpers_test.go

@@ -27,6 +27,20 @@ var _ = Describe("Request Helpers", func() {
 		})
 	})
 
+	Describe("ParamStringDefault", func() {
+		BeforeEach(func() {
+			r = httptest.NewRequest("GET", "/ping?a=123", nil)
+		})
+
+		It("returns default string if param does not exist", func() {
+			Expect(ParamStringDefault(r, "xx", "default_value")).To(Equal("default_value"))
+		})
+
+		It("returns param as string", func() {
+			Expect(ParamStringDefault(r, "a", "default_value")).To(Equal("123"))
+		})
+	})
+
 	Describe("ParamStrings", func() {
 		BeforeEach(func() {
 			r = httptest.NewRequest("GET", "/ping?a=123&a=456", nil)