26.1: ClientGameTest hangs upon disconnecting the world

[YaLTeR]

Reproducer on top of fabric-example-mod:

package com.example.test;

import net.fabricmc.fabric.api.client.gametest.v1.FabricClientGameTest;
import net.fabricmc.fabric.api.client.gametest.v1.context.ClientGameTestContext;
import net.fabricmc.fabric.api.client.gametest.v1.context.TestSingleplayerContext;

public class HangReproducerTest implements FabricClientGameTest {
    @Override
    public void runTest(ClientGameTestContext context) {
        // Simply creating and closing a singleplayer world hangs.
        try (TestSingleplayerContext world = context.worldBuilder().create()) {
            world.getClientLevel().waitForChunksDownload();
        }
    }
}

sourceSets {
        testmodClient {
                compileClasspath += main.compileClasspath + main.output + client.output
                runtimeClasspath += main.runtimeClasspath + main.output + client.output
        }
}

loom {
        runs {
                clientGametest {
                        client()
                        ideConfigGenerated true
                        name "Client Game Test"
                        source sourceSets.testmodClient
                        vmArg "-Dfabric.client.gametest"
                }
        }
}

Then, ./gradlew runClientGametest will do everything successfully, then hang when the world is supposed to be disconnected.

[deirn]

Seems to be related to network synchronizer, adding -Dfabric.client.gametest.disableNetworkSynchronizer=true vm arg makes it run just fine.

I guess this is why the Fabric API's own tests are working.

[modmuss50]

Ah yeah, I forgot I disabled that for FAPI to work around these kinds of issues. We either need to fix it or disable it by default. This code is far from trivial and I struggle to understand it.

[deirn]

It does seem flaky; I got this kind of error randomly.
#4415

[modmuss50]

I think I will disable it by default, I'm not convined that it solves a real problem and making these complex changes means the tests are running on something that behaves differently from a real server/client.

[YaLTeR]

I can confirm -Dfabric.client.gametest.disableNetworkSynchronizer=true fixes the hangs for me.