Commits on Oct 31, 2023

1. Move pseudoterminal slave open to child …

   Hopefully this fixes "unexpected EOF" failures on macOS
   (#3137, #3605, #7242, #7702).
   
   The problem appears to be that under some circumstances, macOS
   discards the output written to the slave side of the
   pseudoterminal. Hence the parent never sees the "sandbox initialized"
   message from the child, even though it succeeded. The conditions are:
   
   * The child finishes very quickly. That's why this bug is likely to
     trigger in nix-env tests, since that uses a builtin builder. Adding
     a short sleep before the child exits makes the problem go away.
   
   * The parent has closed its duplicate of the slave file
     descriptor. This shouldn't matter, since the child has a duplicate
     as well, but it does. E.g. moving the close to the bottom of
     startBuilder() makes the problem go away. However, that's not a
     solution because it would make Nix hang if the child dies before
     sending the "sandbox initialized" message.
   
   * The system is under high load. E.g. "make installcheck -j16" makes
     the issue pretty reproducible, while it's very rare under "make
     installcheck -j1".
   
   As a fix/workaround, we now open the pseudoterminal slave in the
   child, rather than the parent. This removes the second condition
   (i.e. the parent no longer needs to close the slave fd) and I haven't
   been able to reproduce the "unexpected EOF" with this.
   
   (cherry picked from commit c536e00)

   

   edolstra authored and Ericson2314 committed Oct 31, 2023
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