std: Add a helper for symbols that may not exist

Right now we only attempt to call one symbol which my not exist everywhere,
__pthread_get_minstack, but this pattern will come up more often as we start to
bind newer functionality of systems like Linux.

Take a similar strategy as the Windows implementation where we use `dlopen` to
lookup whether a symbol exists or not.

src/liblibc

@@ -1 +1 @@
-Subproject commit 30f70baa6cc1ba3ddebb55b988fafbad0c0cc810
+Subproject commit a64ee24718c0289b82a77d692cf56f8a1226de51

src/libstd/sys/unix/mod.rs

@@ -27,6 +27,9 @@ use ops::Neg;
 #[cfg(target_os = "openbsd")]   pub use os::openbsd as platform;
 #[cfg(target_os = "solaris")]   pub use os::solaris as platform;
 
+#[macro_use]
+pub mod weak;
+
 pub mod backtrace;
 pub mod condvar;
 pub mod ext;

src/libstd/sys/unix/thread.rs

@@ -317,37 +317,12 @@ pub mod guard {
 // storage.  We need that information to avoid blowing up when a small stack
 // is created in an application with big thread-local storage requirements.
 // See #6233 for rationale and details.
-//
-// Use dlsym to get the symbol value at runtime, both for
-// compatibility with older versions of glibc, and to avoid creating
-// dependencies on GLIBC_PRIVATE symbols.  Assumes that we've been
-// dynamically linked to libpthread but that is currently always the
-// case.  We previously used weak linkage (under the same assumption),
-// but that caused Debian to detect an unnecessarily strict versioned
-// dependency on libc6 (#23628).
 #[cfg(target_os = "linux")]
 #[allow(deprecated)]
 fn min_stack_size(attr: *const libc::pthread_attr_t) -> usize {
-    use dynamic_lib::DynamicLibrary;
-    use sync::Once;
+    weak!(fn __pthread_get_minstack(*const libc::pthread_attr_t) -> libc::size_t);
 
-    type F = unsafe extern "C" fn(*const libc::pthread_attr_t) -> libc::size_t;
-    static INIT: Once = Once::new();
-    static mut __pthread_get_minstack: Option<F> = None;
-
-    INIT.call_once(|| {
-        let lib = match DynamicLibrary::open(None) {
-            Ok(l) => l,
-            Err(..) => return,
-        };
-        unsafe {
-            if let Ok(f) = lib.symbol("__pthread_get_minstack") {
-                __pthread_get_minstack = Some(mem::transmute::<*const (), F>(f));
-            }
-        }
-    });
-
-    match unsafe { __pthread_get_minstack } {
+    match __pthread_get_minstack.get() {
         None => libc::PTHREAD_STACK_MIN as usize,
         Some(f) => unsafe { f(attr) as usize },
     }

src/libstd/sys/unix/weak.rs

@@ -0,0 +1,81 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! 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).
+
+use libc;
+
+use ffi::CString;
+use marker;
+use mem;
+use sync::atomic::{AtomicUsize, Ordering};
+
+macro_rules! weak {
+    (fn $name:ident($($t:ty),*) -> $ret:ty) => (
+        static $name: ::sys::weak::Weak<unsafe extern fn($($t),*) -> $ret> =
+            ::sys::weak::Weak::new(stringify!($name));
+    )
+}
+
+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: 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);
+            }
+            mem::transmute::<&AtomicUsize, Option<&F>>(&self.addr)
+        }
+    }
+}
+
+unsafe fn fetch(name: &str) -> usize {
+    let name = match CString::new(name) {
+        Ok(cstr) => cstr,
+        Err(..) => return 0,
+    };
+    let lib = libc::dlopen(0 as *const _, libc::RTLD_LAZY);
+    if lib.is_null() {
+        return 0
+    }
+    let ret = libc::dlsym(lib, name.as_ptr()) as usize;
+    libc::dlclose(lib);
+    return ret
+}