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Fix: on startup, NewGRF scan could case race-condition #9382

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merged 1 commit into from Jun 17, 2021

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@TrueBrain TrueBrain commented Jun 17, 2021

Motivation / Problem

NewGRF scan could trigger ThreadSanitizer (rightfully), because it tried to access a variable that wasn't fully prepared to be accessed yet.

Description

Creating a thread was not thread-safe. The irony.

The video-driver has a function GameLoopPause() which first checks
if the thread is the game-thread or not. For this it needs access
to this->game_thread. This variable is set in StartNewThread().

However, due to timing, it is well possible GameLoopPause() is
called from the thread well before this->game_thread is assigned.

And so we have a race-condition!

Simply solve this by preventing a thread to start till we are
done with our bookkeeping.

Limitations

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')
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Creating a thread was not thread-safe. The irony.

The video-driver has a function GameLoopPause() which first checks
if the thread is the game-thread or not. For this it needs access
to this->game_thread. This variable is set in StartNewThread().

However, due to timing, it is well possible GameLoopPause() is
called from the thread well before this->game_thread is assigned.

And so we have a race-condition!

Simply solve this by preventing a thread to start till we are
done with our bookkeeping.
@TrueBrain TrueBrain force-pushed the thread-thread-safety branch from 6b1bfce to 084c19c Jun 17, 2021
@TrueBrain TrueBrain merged commit b45c006 into OpenTTD:master Jun 17, 2021
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@TrueBrain TrueBrain deleted the thread-thread-safety branch Jun 17, 2021
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2 participants