For what it's worth, your example (and the resulting symptom) seem to
fall squarely in this undefined territory from the vfork manpage / POSIX
the behavior is undefined if the process created by vfork() [..] calls
any other function before successfully calling [..] one of the exec(3)
family of functions
This doesn't solve your problem, but I wanted to make sure you're aware
of (and deliberately chose to accept) that.
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