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.

1 files changed, 2 insertions, 88 deletions

diff --git a/library/std/src/sys/unix/android.rs b/library/std/src/sys/unix/android.rs
index 6a46525f682..73ff10ab8a2 100644
--- a/library/std/src/sys/unix/android.rs
+++ b/library/std/src/sys/unix/android.rs
@@ -18,11 +18,9 @@
 
 #![cfg(target_os = "android")]
 
-use libc::{c_int, c_void, sighandler_t, size_t, ssize_t};
-use libc::{ftruncate, pread, pwrite};
+use libc::{c_int, sighandler_t};
 
-use super::{cvt, cvt_r, weak::weak};
-use crate::io;
+use super::weak::weak;
 
 // The `log2` and `log2f` functions apparently appeared in android-18, or at
 // least you can see they're not present in the android-17 header [1] and they
@@ -81,87 +79,3 @@ pub unsafe fn signal(signum: c_int, handler: sighandler_t) -> sighandler_t {
     let f = f.expect("neither `signal` nor `bsd_signal` symbols found");
     f(signum, handler)
 }
-
-// The `ftruncate64` symbol apparently appeared in android-12, so we do some
-// dynamic detection to see if we can figure out whether `ftruncate64` exists.
-//
-// If it doesn't we just fall back to `ftruncate`, generating an error for
-// too-large values.
-#[cfg(target_pointer_width = "32")]
-pub fn ftruncate64(fd: c_int, size: u64) -> io::Result<()> {
-    weak!(fn ftruncate64(c_int, i64) -> c_int);
-
-    unsafe {
-        match ftruncate64.get() {
-            Some(f) => cvt_r(|| f(fd, size as i64)).map(drop),
-            None => {
-                if size > i32::MAX as u64 {
-                    Err(io::Error::new_const(io::ErrorKind::InvalidInput, &"cannot truncate >2GB"))
-                } else {
-                    cvt_r(|| ftruncate(fd, size as i32)).map(drop)
-                }
-            }
-        }
-    }
-}
-
-#[cfg(target_pointer_width = "64")]
-pub fn ftruncate64(fd: c_int, size: u64) -> io::Result<()> {
-    unsafe { cvt_r(|| ftruncate(fd, size as i64)).map(drop) }
-}
-
-#[cfg(target_pointer_width = "32")]
-pub unsafe fn cvt_pread64(
-    fd: c_int,
-    buf: *mut c_void,
-    count: size_t,
-    offset: i64,
-) -> io::Result<ssize_t> {
-    use crate::convert::TryInto;
-    weak!(fn pread64(c_int, *mut c_void, size_t, i64) -> ssize_t);
-    pread64.get().map(|f| cvt(f(fd, buf, count, offset))).unwrap_or_else(|| {
-        if let Ok(o) = offset.try_into() {
-            cvt(pread(fd, buf, count, o))
-        } else {
-            Err(io::Error::new_const(io::ErrorKind::InvalidInput, &"cannot pread >2GB"))
-        }
-    })
-}
-
-#[cfg(target_pointer_width = "32")]
-pub unsafe fn cvt_pwrite64(
-    fd: c_int,
-    buf: *const c_void,
-    count: size_t,
-    offset: i64,
-) -> io::Result<ssize_t> {
-    use crate::convert::TryInto;
-    weak!(fn pwrite64(c_int, *const c_void, size_t, i64) -> ssize_t);
-    pwrite64.get().map(|f| cvt(f(fd, buf, count, offset))).unwrap_or_else(|| {
-        if let Ok(o) = offset.try_into() {
-            cvt(pwrite(fd, buf, count, o))
-        } else {
-            Err(io::Error::new_const(io::ErrorKind::InvalidInput, &"cannot pwrite >2GB"))
-        }
-    })
-}
-
-#[cfg(target_pointer_width = "64")]
-pub unsafe fn cvt_pread64(
-    fd: c_int,
-    buf: *mut c_void,
-    count: size_t,
-    offset: i64,
-) -> io::Result<ssize_t> {
-    cvt(pread(fd, buf, count, offset))
-}
-
-#[cfg(target_pointer_width = "64")]
-pub unsafe fn cvt_pwrite64(
-    fd: c_int,
-    buf: *const c_void,
-    count: size_t,
-    offset: i64,
-) -> io::Result<ssize_t> {
-    cvt(pwrite(fd, buf, count, offset))
-}