lib/curl/Request: move exception handling out of the WRITEFUNCTION

libcurl's WRITEFUNCTION is pretty fragile; if we destroy the CURL*
instance or even unregister it using curl_multi_remove_handle(),
libcurl will crash instantly.  But still we need to be able to handle
exceptions from inside the WRITEFUNCTION, and call
CurlResponseHandler::OnError(), which may destroy the whole thing.  As
a workaround, I use DeferredMonitor to postpone the OnError() call
into a stack frame which is allowed to destroy the request.

src/lib/curl/Request.cxx

@@ -46,7 +46,8 @@
 
 CurlRequest::CurlRequest(CurlGlobal &_global, const char *url,
 			 CurlResponseHandler &_handler)
-	:global(_global), handler(_handler)
+	:DeferredMonitor(_global.GetEventLoop()),
+	 global(_global), handler(_handler)
 {
 	error_buffer[0] = 0;
 
@@ -226,8 +227,11 @@ CurlRequest::DataReceived(const void *ptr, size_t received_size)
 		return CURL_WRITEFUNC_PAUSE;
 	} catch (...) {
 		state = State::CLOSED;
-		handler.OnError(std::current_exception());
-		return 0;
+		/* move the CurlResponseHandler::OnError() call into a
+		   "safe" stack frame */
+		postponed_error = std::current_exception();
+		DeferredMonitor::Schedule();
+		return CURL_WRITEFUNC_PAUSE;
 	}
 
 }
@@ -243,3 +247,11 @@ CurlRequest::WriteFunction(void *ptr, size_t size, size_t nmemb, void *stream)
 
 	return c.DataReceived(ptr, size);
 }
+
+void
+CurlRequest::RunDeferred()
+{
+	assert(postponed_error);
+
+	handler.OnError(postponed_error);
+}