lib/curl/Request: require the caller to explicitly register the request

This allows constructing an instance in any thread, and register it
inside the IOThread later.

src/input/plugins/CurlInputPlugin.cxx

@@ -373,6 +373,8 @@ CurlInputStream::InitEasy()
 	request_headers.Clear();
 	request_headers.Append("Icy-Metadata: 1");
 	request->SetOption(CURLOPT_HTTPHEADER, request_headers.Get());
+
+	request->Start();
 }
 
 void

src/lib/curl/Request.cxx

@@ -62,8 +62,6 @@ CurlRequest::CurlRequest(CurlGlobal &_global, const char *url,
 	easy.SetOption(CURLOPT_NOSIGNAL, 1l);
 	easy.SetOption(CURLOPT_CONNECTTIMEOUT, 10l);
 	easy.SetOption(CURLOPT_URL, url);
-
-	global.Add(easy.Get(), *this);
 }
 
 CurlRequest::~CurlRequest()
@@ -71,19 +69,40 @@ CurlRequest::~CurlRequest()
 	FreeEasy();
 }
 
+void
+CurlRequest::Start()
+{
+	assert(!registered);
+
+	global.Add(easy.Get(), *this);
+	registered = true;
+}
+
+void
+CurlRequest::Stop()
+{
+	assert(registered);
+
+	global.Remove(easy.Get());
+	registered = false;
+}
+
 void
 CurlRequest::FreeEasy()
 {
 	if (!easy)
 		return;
 
-	global.Remove(easy.Get());
+	if (registered)
+		Stop();
 	easy = nullptr;
 }
 
 void
 CurlRequest::Resume()
 {
+	assert(registered);
+
 	curl_easy_pause(easy.Get(), CURLPAUSE_CONT);
 
 	if (IsCurlOlderThan(0x072000))

src/lib/curl/Request.hxx

@@ -58,7 +58,12 @@ class CurlRequest {
 	/** error message provided by libcurl */
 	char error_buffer[CURL_ERROR_SIZE];
 
+	bool registered = false;
+
 public:
+	/**
+	 * To start sending the request, call Start().
+	 */
 	CurlRequest(CurlGlobal &_global, const char *url,
 		    CurlResponseHandler &_handler);
 	~CurlRequest();
@@ -66,6 +71,21 @@ public:
 	CurlRequest(const CurlRequest &) = delete;
 	CurlRequest &operator=(const CurlRequest &) = delete;
 
+	/**
+	 * Register this request via CurlGlobal::Add(), which starts
+	 * the request.
+	 *
+	 * This method must be called in the event loop thread.
+	 */
+	void Start();
+
+	/**
+	 * Unregister this request via CurlGlobal::Remove().
+	 *
+	 * This method must be called in the event loop thread.
+	 */
+	void Stop();
+
 	CURL *Get() {
 		return easy.Get();
 	}