Phase 3: OpenSubsonic core (response envelope, error codes 42-44, getOpenSubsonicExtensions) and apiKeyAuthentication extension with tokenInfo, web API key management

internal/subsonic/formpost.go

@@ -0,0 +1,22 @@
+package subsonic
+
+// The OpenSubsonic formPost extension requires the server accept API
+// arguments via POST with an application/x-www-form-urlencoded body, in
+// addition to the classic GET query-string form. This needs no
+// dedicated handler: the router's dispatch (router.go) already calls
+// r.ParseForm() before building params, and net/http's ParseForm merges
+// URL query values and a url-encoded POST body into r.Form for any HTTP
+// method — GET, POST, or otherwise — so every endpoint above already
+// accepts either form for free.
+//
+// Precedence when the same key appears in both the query string and the
+// POST body: net/http's ParseForm populates r.Form with POST-body values
+// before query-string values, so params.Get (url.Values.Get, which
+// returns the first entry) resolves a body value over a query value for
+// the same key. This is deterministic — governed entirely by net/http's
+// single parse of one request, not by any proxying/multi-parser
+// ambiguity — and is asserted directly rather than silently relied upon;
+// see TestFormPostBodyTakesPrecedenceOverQuery.
+func init() {
+	registerExtension("formPost", 1)
+}