Commits on Nov 12, 2021

1. Refactor weak symbols in std::sys::unix …

   This makes a few changes to the weak symbol macros in `sys::unix`:
   
   - `dlsym!` is added to keep the functionality for runtime `dlsym`
     lookups, like for `__pthread_get_minstack@GLIBC_PRIVATE` that we don't
     want to show up in ELF symbol tables.
   - `weak!` now uses `#[linkage = "extern_weak"]` symbols, so its runtime
     behavior is just a simple null check. This is also used by `syscall!`.
     - On non-ELF targets (macos/ios) where that linkage is not known to
       behave, `weak!` is just an alias to `dlsym!` for the old behavior.
   - `raw_syscall!` is added to always call `libc::syscall` on linux and
     android, for cases like `clone3` that have no known libc wrapper.
   
   The new `weak!` linkage does mean that you'll get versioned symbols if
   you build with a newer glibc, like `WEAK DEFAULT UND statx@GLIBC_2.28`.
   This might seem problematic, but old non-weak symbols can tie the build
   to new versions too, like `dlsym@GLIBC_2.34` from their recent library
   unification. If you build with an old glibc like `dist-x86_64-linux`
   does, you'll still get unversioned `WEAK DEFAULT UND statx`, which may
   be resolved based on the runtime glibc.
   
   I also found a few functions that don't need to be weak anymore:
   
   - Android can directly use `ftruncate64`, `pread64`, and `pwrite64`, as
     these were added in API 12, and our baseline is API 14.
   - Linux can directly use `splice`, added way back in glibc 2.5 and
     similarly old musl. Android only added it in API 21 though.

   

   cuviper committed Nov 12, 2021
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