1 files changed, 101 insertions, 0 deletions
diff --git a/library/std/src/sys/unix/weak.rs b/library/std/src/sys/unix/weak.rs
new file mode 100644
index 00000000000..f4b33a00f7c
--- /dev/null
+++ b/library/std/src/sys/unix/weak.rs
@@ -0,0 +1,101 @@
+//! Support for "weak linkage" to symbols on Unix
+//!
+//! Some I/O operations we do in libstd require newer versions of OSes but we
+//! need to maintain binary compatibility with older releases for now. In order
+//! to use the new functionality when available we use this module for
+//! detection.
+//!
+//! One option to use here is weak linkage, but that is unfortunately only
+//! really workable on Linux. Hence, use dlsym to get the symbol value at
+//! runtime. This is also done for compatibility with older versions of glibc,
+//! and to avoid creating dependencies on GLIBC_PRIVATE symbols. It assumes that
+//! we've been dynamically linked to the library the symbol comes from, but that
+//! is currently always the case for things like libpthread/libc.
+//!
+//! A long time ago this used weak linkage for the __pthread_get_minstack
+//! symbol, but that caused Debian to detect an unnecessarily strict versioned
+//! dependency on libc6 (#23628).
+
+// There are a variety of `#[cfg]`s controlling which targets are involved in
+// each instance of `weak!` and `syscall!`. Rather than trying to unify all of
+// that, we'll just allow that some unix targets don't use this module at all.
+#![allow(dead_code, unused_macros)]
+
+use crate::ffi::CStr;
+use crate::marker;
+use crate::mem;
+use crate::sync::atomic::{AtomicUsize, Ordering};
+
+macro_rules! weak {
+    (fn $name:ident($($t:ty),*) -> $ret:ty) => (
+        static $name: crate::sys::weak::Weak<unsafe extern fn($($t),*) -> $ret> =
+            crate::sys::weak::Weak::new(concat!(stringify!($name), '\0'));
+    )
+}
+
+pub struct Weak<F> {
+    name: &'static str,
+    addr: AtomicUsize,
+    _marker: marker::PhantomData<F>,
+}
+
+impl<F> Weak<F> {
+    pub const fn new(name: &'static str) -> Weak<F> {
+        Weak { name, addr: AtomicUsize::new(1), _marker: marker::PhantomData }
+    }
+
+    pub fn get(&self) -> Option<F> {
+        assert_eq!(mem::size_of::<F>(), mem::size_of::<usize>());
+        unsafe {
+            if self.addr.load(Ordering::SeqCst) == 1 {
+                self.addr.store(fetch(self.name), Ordering::SeqCst);
+            }
+            match self.addr.load(Ordering::SeqCst) {
+                0 => None,
+                addr => Some(mem::transmute_copy::<usize, F>(&addr)),
+            }
+        }
+    }
+}
+
+unsafe fn fetch(name: &str) -> usize {
+    let name = match CStr::from_bytes_with_nul(name.as_bytes()) {
+        Ok(cstr) => cstr,
+        Err(..) => return 0,
+    };
+    libc::dlsym(libc::RTLD_DEFAULT, name.as_ptr()) as usize
+}
+
+#[cfg(not(target_os = "linux"))]
+macro_rules! syscall {
+    (fn $name:ident($($arg_name:ident: $t:ty),*) -> $ret:ty) => (
+        unsafe fn $name($($arg_name: $t),*) -> $ret {
+            use super::os;
+
+            weak! { fn $name($($t),*) -> $ret }
+
+            if let Some(fun) = $name.get() {
+                fun($($arg_name),*)
+            } else {
+                os::set_errno(libc::ENOSYS);
+                -1
+            }
+        }
+    )
+}
+
+#[cfg(target_os = "linux")]
+macro_rules! syscall {
+    (fn $name:ident($($arg_name:ident: $t:ty),*) -> $ret:ty) => (
+        unsafe fn $name($($arg_name:$t),*) -> $ret {
+            // This looks like a hack, but concat_idents only accepts idents
+            // (not paths).
+            use libc::*;
+
+            syscall(
+                concat_idents!(SYS_, $name),
+                $($arg_name as c_long),*
+            ) as $ret
+        }
+    )
+}