[Issue 20271] New: Handle forking in the GC

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20271

          Issue ID: 20271
           Summary: Handle forking in the GC
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P1
         Component: druntime
          Assignee: nobody at puremagic.com
          Reporter: dlang-bugzilla at thecybershadow.net

This problem currently never finishes execution:

////////////// test2817a.d //////////////
import core.stdc.stdlib : exit;
import core.sys.posix.sys.wait : waitpid;
import core.sys.posix.unistd : fork;
import core.thread : Thread;

void main()
{
    foreach (t; 0 .. 10)
        new Thread({
            foreach (n; 0 .. 100)
            {
                foreach (x; 0 .. 100)
                    new ubyte[x];
                auto f = fork();
                assert(f >= 0);
                if (f == 0)
                    exit(0);
                else
                    waitpid(f, null, 0);
            }
        }).start();
}
/////////////////////////////////////////

Here the exit function (not _exit) calls finalizers, which includes shutting
down the GC. However, if the GC lock was being held while a fork happened, that
leaves the GC in an inconsistent state in the fork.

PR: https://github.com/dlang/druntime/pull/2817

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