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Fix #9501: [Network] crash when more than one game-info query was pending #9502

merged 1 commit into from Aug 23, 2021


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@TrueBrain TrueBrain commented Aug 21, 2021

Motivation / Problem

When you have more than one query-for-game-info pending, the game crashes. This is easiest reproduced by starting a local server, and adding and to your server-list. Close the game, restart it, click Multiplayer. Now two queries will be send out at the exact same time.


We piggy-backed on the client code to, after establishing a TCP connection, query the server. The client code only has a very weird and annoying to follow singleton called my_client. As it turns out, when it is being asked to query two different servers at the same time, it panics.

To resolve the situation in a more robust way, I cut out the game-info query part from the client code, and moved it to network_query. This part is now responsible solely for querying the game-info of a server, and can have as many parallel connection as needed. It self-registers and self-destroys.

As an additional bonus, this means we can also handle errors from servers in a bit nicer way for when a query fails. This is best seen about the code removed from the client code.

As the protocol reuses other packages to indicate issues, the server can send back 4 different packets depending on the situation on a game-info request:

  • game-info, what we are after.
  • error, when something weird went wrong. For example, when it never saw your game-info request packet .. it will hit a timeout and close your connection. But this is really unlikely to ever happen.
  • banned, when your IP is banned (you won't be able to query the server in that case either; only seems fair).
  • full, which is already a bug on our bugtracker, but currently at a very early moment in time we validate if the server has room for you. If not, you get this packet back, and refresh fails. This of course has to be resolved, but as for now, this is still happening.


Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

  • The bug fix is important enough to be backported? (label: 'backport requested')
  • This PR affects the save game format? (label 'savegame upgrade')
  • This PR affects the GS/AI API? (label 'needs review: Script API')
    • ai_changelog.hpp, gs_changelog.hpp need updating.
    • The compatibility wrappers (compat_*.nut) need updating.
  • This PR affects the NewGRF API? (label 'needs review: NewGRF')

src/network/network_query.cpp Outdated Show resolved Hide resolved
src/network/network_query.cpp Outdated Show resolved Hide resolved
#include "network_internal.h"

/** Class for handling the client side of quering a game server. */
class QueryNetworkGameSocketHandler : public ZeroedMemoryAllocator, public NetworkGameSocketHandler {
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@rubidium42 rubidium42 Aug 22, 2021

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Is the ZeroedMemoryAllocator really needed here? Only the ClientNetworkGameSocketHandler uses it but not the ServerNetworkGameSocketHandler so it is not something the NetworkGameSocketHandler intrinsicly needs. Since this has only one instance variable that gets in the constructor, I think there is no use for it.

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@TrueBrain TrueBrain Aug 23, 2021

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ServerNetworkGameSocketHandler also zero's the object, but this is done by the pool (Tzero is set to true). So there too Calloc is used. In other words: it currently is consistent in zero'ing the object on creation.

Of course these days we can just set the initial values of variables, and possibly get ride of this whole ZeroedMemoryAllocator, but that might be best for another PR. For now, I suggest we leave it consistent, and zero the object for all children using NetworkGameSocketHandler.

@TrueBrain TrueBrain changed the title Fix #9501: [Network] crash when more than one game-info query was pen… Fix #9501: [Network] crash when more than one game-info query was pending Aug 23, 2021
@TrueBrain TrueBrain merged commit b2f0491 into OpenTTD:master Aug 23, 2021
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@TrueBrain TrueBrain deleted the fix-multiple-servers branch Aug 23, 2021
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3 participants