lib/curl/Adapter: catch and postpone exceptions in WriteFunction()

This fixes a std::terminate() crash in the CURL storage plugin when
PropfindOperation::OnHeaders() throws an exception after receiving a
non-207 status.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1559

NEWS

@@ -1,4 +1,6 @@
 ver 0.23.8 (not yet released)
+* storage
+  - curl: fix crash if web server does not understand WebDAV
 
 ver 0.23.7 (2022/05/09)
 * database

src/lib/curl/Adapter.cxx

@@ -85,6 +85,12 @@ CurlResponseHandlerAdapter::FinishBody()
 void
 CurlResponseHandlerAdapter::Done(CURLcode result) noexcept
 {
+	if (postponed_error) {
+		state = State::CLOSED;
+		handler.OnError(std::move(postponed_error));
+		return;
+	}
+
 	try {
 		if (result != CURLE_OK) {
 			StripRight(error_buffer);
@@ -172,6 +178,13 @@ CurlResponseHandlerAdapter::DataReceived(const void *ptr,
 		return received_size;
 	} catch (CurlResponseHandler::Pause) {
 		return CURL_WRITEFUNC_PAUSE;
+	} catch (...) {
+		/* from inside this libCURL callback function, we
+		   can't do much, so we remember the exception to be
+		   handled later by Done(), and return 0, causing the
+		   response to be aborted with CURLE_WRITE_ERROR */
+		postponed_error = std::current_exception();
+		return 0;
 	}
 
 }

src/lib/curl/Adapter.hxx

@@ -34,6 +34,7 @@
 #include <curl/curl.h>
 
 #include <cstddef>
+#include <exception>
 
 struct StringView;
 class CurlEasy;
@@ -46,6 +47,12 @@ class CurlResponseHandlerAdapter {
 
 	Curl::Headers headers;
 
+	/**
+	 * An exception caught from within the WriteFunction() which
+	 * will later be handled by Done().
+	 */
+	std::exception_ptr postponed_error;
+
 	/** error message provided by libcurl */
 	char error_buffer[CURL_ERROR_SIZE];
 

src/lib/curl/Handler.hxx

@@ -50,13 +50,19 @@ public:
 
 	/**
 	 * Status line and headers have been received.
+	 *
+	 * Exceptions thrown by this method will be passed to
+	 * OnError(), aborting the request.
 	 */
 	virtual void OnHeaders(unsigned status, Curl::Headers &&headers) = 0;
 
 	/**
 	 * Response body data has been received.
 	 *
-	 * May throw #Pause (but nothing else).
+	 * May throw #Pause.
+	 *
+	 * Other exceptions thrown by this method will be passed to
+	 * OnError(), aborting the request.
 	 */
 	virtual void OnData(ConstBuffer<void> data) = 0;