[Backport 2.3-maintainence] Fix "unexpected EOF" errors on macOS #9495
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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)
This was referenced Nov 30, 2023
IIRC this took multiple attempts to fix, so maybe we need more and/or different commits. |
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Motivation
Backport of legendary bugfix #8049 to 2.3.
Doesn't work yet, I am not sure why.
Context
This is needed so I can do #5650 which backports a number of test suite improvements that make it more robust --- and also more likely to catch the bugs that this catches. That is in turn needed so we can do better cross-version daemon testing to ensure protocol backwards compatibility works.
Priorities
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