vibe/http/server.d(875): AssertionError "A void body was already written!"

[dcarp]

This occurs because HTTPServerResponse.writeJsonBody must not be called more than once per instance.

Anyhow in the following code section (vibe/web/rest.d:540-561) HTTPServerResponse.writeJsonBody is called second time in the catch block, when an exception is thrown during the first call in the try block.

        try {
            import vibe.internal.meta.funcattr;

            auto handler = createAttributedFunction!Func(req, res);

            static if (is(RT == void)) {
                handler(&__traits(getMember, inst, method), params);
                res.writeJsonBody(Json.emptyObject);
            } else {
                auto ret = handler(&__traits(getMember, inst, method), params);
                res.writeJsonBody(ret);
            }
        } catch (HTTPStatusException e) {
            res.writeJsonBody([ "statusMessage": e.msg ], e.status);
        } catch (Exception e) {
            // TODO: better error description!
            logDebug("REST handler exception: %s", e.toString());
            res.writeJsonBody(
                [ "statusMessage": e.msg, "statusDebugMessage": sanitizeUTF8(cast(ubyte[])e.toString()) ],
                HTTPStatus.internalServerError
            );
        }

[dcarp]

A solution could be to add a HTTPServerResponse.reset method, that resets the state of the object. The reset method will then be called in the catch block. I can produce a PR for such a solution.

[s-ludwig]

That would in general corrupt the HTTP protocol. The only safe solution would be to add an if (!res.headerWritten) and only write an error response at all when that passes.

[dcarp]

Does res.headerWritten == true mean, that the header has already been sent over to the client?

What will happen if the suggested condition is added and res.headerWritten == true?

[dcarp]

This is the exception thrown on the first call of HTTPServerResponse.writeJsonBody:

object.Exception@include/vibed/vibe/core/drivers/libevent2_tcp.d(367): Connection error while writing to TCPConnection.
----------------
./build/release/AnnouncementMonitor(void vibe.core.drivers.libevent2_tcp.Libevent2TCPConnection.write(const(ubyte[]))+0x70) [0x993844]
./build/release/AnnouncementMonitor(void vibe.core.stream.OutputStream.write(const(char[]))+0x5d) [0xa0e139]
./build/release/AnnouncementMonitor(void vibe.http.server.HTTPServerResponse.writeHeader()+0x50d) [0xbe8885]
./build/release/AnnouncementMonitor(@property vibe.core.stream.OutputStream vibe.http.server.HTTPServerResponse.bodyWriter()+0x6d8) [0xbe6098]
./build/release/AnnouncementMonitor(void vibe.http.server.HTTPServerResponse.writeJsonBody!(monitor.model.Announcement.Announcement[]).writeJsonBody(monitor.model.Announcement.Announcement[], int, immutable(char)[])+0x91) [0x74fd11]
./build/release/AnnouncementMonitor(void vibe.web.rest.__T17jsonMethodHandlerTC7monitor8protocol3Api3ApiVAyaa24_6765745370656369666963416e6e6f756e63656d656e7473S133_D7monitor8protocol3Api3Api24getSpecificAnnouncementsMFAyaAyaAyaAyaAyaAyaAyaAyaAyaAyaAyaZAS7monitor5model12Announcement12AnnouncementZ.jsonMethodHandler(monitor.protocol.Api.Api).handler(vibe.http.server.HTTPServerRequest, vibe.http.server.HTTPServerResponse)+0x1b2e) [0x75b4c6]
./build/release/AnnouncementMonitor(void vibe.http.router.URLRouter.handleRequest(vibe.http.server.HTTPServerRequest, vibe.http.server.HTTPServerResponse).void __lambda3!(ulong, immutable(char)[][]).__lambda3(ulong, scope immutable(char)[][])+0x275) [0xbb7ddd]
./build/release/AnnouncementMonitor(void vibe.http.router.MatchTree!(vibe.http.router.Route).MatchTree.match(immutable(char)[], scope void delegate(ulong, scope immutable(char)[][]))+0x175) [0xbb83bd]
./build/release/AnnouncementMonitor(void vibe.http.router.URLRouter.handleRequest(vibe.http.server.HTTPServerRequest, vibe.http.server.HTTPServerResponse)+0x1b7) [0xbb7aff]
./build/release/AnnouncementMonitor(bool vibe.http.server.handleRequest(vibe.core.stream.Stream, vibe.core.net.TCPConnection, vibe.http.server.HTTPServerListener, ref vibe.http.server.HTTPServerSettings, ref bool)+0x3b87) [0xbef257]
./build/release/AnnouncementMonitor(void vibe.http.server.handleHTTPConnection(vibe.core.net.TCPConnection, vibe.http.server.HTTPServerListener)+0x23b) [0xbeb48b]
./build/release/AnnouncementMonitor(void vibe.http.server.listenHTTPPlain(vibe.http.server.HTTPServerSettings).doListen(vibe.http.server.HTTPServerSettings, vibe.http.server.HTTPServerListener, immutable(char)[]).__lambda4(vibe.core.net.TCPConnection)+0x28) [0xbe3350]
./build/release/AnnouncementMonitor(void vibe.core.drivers.libevent2_tcp.onConnect(int, short, void*).ClientTask.execute()+0x5b0) [0x9951ac]
./build/release/AnnouncementMonitor(_D4vibe4core4core12__T7runTaskZ7runTaskFDFZvZS4vibe4core4task4Task12callDelegateFC4vibe4core4core8CoreTaskZv+0x24) [0x7115bc]
./build/release/AnnouncementMonitor(void vibe.core.core.CoreTask.run()+0x2a4) [0x91d6fc]
./build/release/AnnouncementMonitor(void core.thread.Fiber.run()+0x2a) [0x102b28a]
./build/release/AnnouncementMonitor(fiber_entryPoint+0x61) [0x102b195]
[(nil)]

For this case, it will be OK to just log the error and give-up the HTTPServerResponse processing.

The clean solution would be that at vibe/core/drivers/libevent2_tcp.d(367) and co., instead of the generic Exception, a specialized ConnectionException is thrown. In this way, when you catch ConnectionException, you don't even try to call HTTPServerResponse.writeJsonBody anymore.