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Fix race condition in NetworkChannel #8005

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Merged
merged 2 commits into from
Jan 12, 2025
Merged

Fix race condition in NetworkChannel #8005

merged 2 commits into from
Jan 12, 2025

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@dwickern
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@dwickern dwickern commented Jan 11, 2025

Fixes #6864

NetworkChannel starts two threads which may reference outputBuffer before the field is initialized, resulting in a NPE:

Exception in thread "sbt-networkchannel-0" java.lang.NullPointerException: Cannot enter synchronized block because "this.sbt$internal$server$NetworkChannel$$outputBuffer" is null
        at sbt.internal.server.NetworkChannel.sbt$internal$server$NetworkChannel$$doFlush(NetworkChannel.scala:672)
        at sbt.internal.server.NetworkChannel.shutdown(NetworkChannel.scala:568)
        at sbt.internal.server.NetworkChannel.shutdown(NetworkChannel.scala:554)
        at sbt.internal.server.NetworkChannel$$anon$3.run(NetworkChannel.scala:190)

It's easier to see in the disassembly:

public NetworkChannel(...) {
    ...
    this.thread = new Thread(...); // calls shutdown
    this.thread().start();
    this.writeThread = new Thread(...); // also calls shutdown
    this.writeThread.setDaemon(true);
    this.writeThread.start();
    this.sbt$internal$server$NetworkChannel$$outputBuffer = new LinkedBlockingQueue();
    this.sbt$internal$server$NetworkChannel$$flushExecutor = Executors.newSingleThreadScheduledExecutor(...);
}

This PR moves the thread start to the end of the constructor.

eed3si9n
eed3si9n approved these changes Jan 12, 2025
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Thanks!

@eed3si9n eed3si9n merged commit d99163b into sbt:1.10.x Jan 12, 2025
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@dwickern dwickern deleted the fix-npe branch January 12, 2025 05:26
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"sbt-networkchannel-0" java.lang.NullPointerException

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