Today I Learned

    Shim a missing libc header on Alpine

    In an elixir:*-alpine image, mix deps.compile failed on a native dependency (picosat_elixir, the SAT-solver NIF that Ash uses for policies):

    c_src/picosat.c:8150:10: fatal error: sys/unistd.h: No such file or directory

    <sys/unistd.h> is a glibc quirk. All it does is include <unistd.h>. The latter is the POSIX header, so it’s the only one that musl provides.

    There’s no need to fork picosat_elixir to fix this. I just added the same forwarding header to the image:

    Dockerfile
    RUN echo "#include <unistd.h>" > /usr/include/sys/unistd.h

    The trick works for other missing glibc forwarding headers too, as long as musl has the header they forward to.

    Other options:

    • Use Debian. Phoenix’s own generated Dockerfile already does.
    • For Ash specifically, replace {:picosat_elixir, "~> 0.2"} with the pure-Elixir {:simple_sat, "~> 0.1"}. It’s a bit slower but avoids NIF-compilation issues entirely.
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