Rename Weak to LazyPtr and move it to lazy.rs (#427)

Currently the `Weak` type is used only by the NetBSD backend, but in
future it may be used by other backends. To simplify code a bit and
prepare for potential use on Windows, `Weak` now accepts a "pointer
initialization function" instead of a function name, i.e. it now works
similarly to `LazyUsize` and `LazyBool`.

src/lazy.rs

@@ -1,4 +1,8 @@
-use core::sync::atomic::{AtomicUsize, Ordering::Relaxed};
+#![allow(dead_code)]
+use core::{
+    ffi::c_void,
+    sync::atomic::{AtomicPtr, AtomicUsize, Ordering},
+};
 
 // This structure represents a lazily initialized static usize value. Useful
 // when it is preferable to just rerun initialization instead of locking.
@@ -21,22 +25,22 @@ use core::sync::atomic::{AtomicUsize, Ordering::Relaxed};
 pub(crate) struct LazyUsize(AtomicUsize);
 
 impl LazyUsize {
+    // The initialization is not completed.
+    const UNINIT: usize = usize::max_value();
+
     pub const fn new() -> Self {
         Self(AtomicUsize::new(Self::UNINIT))
     }
 
-    // The initialization is not completed.
-    pub const UNINIT: usize = usize::max_value();
-
     // Runs the init() function at most once, returning the value of some run of
     // init(). Multiple callers can run their init() functions in parallel.
     // init() should always return the same value, if it succeeds.
     pub fn unsync_init(&self, init: impl FnOnce() -> usize) -> usize {
         // Relaxed ordering is fine, as we only have a single atomic variable.
-        let mut val = self.0.load(Relaxed);
+        let mut val = self.0.load(Ordering::Relaxed);
         if val == Self::UNINIT {
             val = init();
-            self.0.store(val, Relaxed);
+            self.0.store(val, Ordering::Relaxed);
         }
         val
     }
@@ -54,3 +58,53 @@ impl LazyBool {
         self.0.unsync_init(|| init() as usize) != 0
     }
 }
+
+// This structure represents a lazily initialized static pointer value.
+///
+/// It's intended to be used for weak linking of a C function that may
+/// or may not be present at runtime.
+///
+/// Based off of the DlsymWeak struct in libstd:
+/// https://github.com/rust-lang/rust/blob/1.61.0/library/std/src/sys/unix/weak.rs#L84
+/// except that the caller must manually cast self.ptr() to a function pointer.
+pub struct LazyPtr {
+    addr: AtomicPtr<c_void>,
+}
+
+impl LazyPtr {
+    /// A non-null pointer value which indicates we are uninitialized.
+    ///
+    /// This constant should ideally not be a valid pointer. However,
+    /// if by chance initialization function passed to the `unsync_init`
+    /// method does return UNINIT, there will not be undefined behavior.
+    /// The initialization function will just be called each time `get()`
+    /// is called. This would be inefficient, but correct.
+    const UNINIT: *mut c_void = !0usize as *mut c_void;
+
+    /// Construct new `LazyPtr` in uninitialized state.
+    pub const fn new() -> Self {
+        Self {
+            addr: AtomicPtr::new(Self::UNINIT),
+        }
+    }
+
+    // Runs the init() function at most once, returning the value of some run of
+    // init(). Multiple callers can run their init() functions in parallel.
+    // init() should always return the same value, if it succeeds.
+    pub fn unsync_init(&self, init: impl Fn() -> *mut c_void) -> *mut c_void {
+        // Despite having only a single atomic variable (self.addr), we still
+        // cannot always use Ordering::Relaxed, as we need to make sure a
+        // successful call to `init` is "ordered before" any data read through
+        // the returned pointer (which occurs when the function is called).
+        // Our implementation mirrors that of the one in libstd, meaning that
+        // the use of non-Relaxed operations is probably unnecessary.
+        match self.addr.load(Ordering::Acquire) {
+            Self::UNINIT => {
+                let addr = init();
+                self.addr.store(addr, Ordering::Release);
+                addr
+            }
+            addr => addr,
+        }
+    }
+}

src/lib.rs

@@ -306,6 +306,7 @@ cfg_if! {
         #[path = "solaris.rs"] mod imp;
     } else if #[cfg(target_os = "netbsd")] {
         mod util_libc;
+        mod lazy;
         #[path = "netbsd.rs"] mod imp;
     } else if #[cfg(target_os = "fuchsia")] {
         #[path = "fuchsia.rs"] mod imp;

src/netbsd.rs

@@ -1,9 +1,6 @@
 //! Implementation for NetBSD
-use crate::{
-    util_libc::{sys_fill_exact, Weak},
-    Error,
-};
-use core::{mem::MaybeUninit, ptr};
+use crate::{lazy::LazyPtr, util_libc::sys_fill_exact, Error};
+use core::{ffi::c_void, mem::MaybeUninit, ptr};
 
 fn kern_arnd(buf: &mut [MaybeUninit<u8>]) -> libc::ssize_t {
     static MIB: [libc::c_int; 2] = [libc::CTL_KERN, libc::KERN_ARND];
@@ -27,10 +24,18 @@ fn kern_arnd(buf: &mut [MaybeUninit<u8>]) -> libc::ssize_t {
 
 type GetRandomFn = unsafe extern "C" fn(*mut u8, libc::size_t, libc::c_uint) -> libc::ssize_t;
 
+// getrandom(2) was introduced in NetBSD 10.0
+static GETRANDOM: LazyPtr = LazyPtr::new();
+
+fn dlsym_getrandom() -> *mut c_void {
+    static NAME: &[u8] = b"getrandom\0";
+    let name_ptr = NAME.as_ptr() as *const libc::c_char;
+    unsafe { libc::dlsym(libc::RTLD_DEFAULT, name_ptr) }
+}
+
 pub fn getrandom_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
-    // getrandom(2) was introduced in NetBSD 10.0
-    static GETRANDOM: Weak = unsafe { Weak::new("getrandom\0") };
-    if let Some(fptr) = GETRANDOM.ptr() {
+    let fptr = GETRANDOM.unsync_init(dlsym_getrandom);
+    if !fptr.is_null() {
         let func: GetRandomFn = unsafe { core::mem::transmute(fptr) };
         return sys_fill_exact(dest, |buf| unsafe {
             func(buf.as_mut_ptr() as *mut u8, buf.len(), 0)

src/util_libc.rs

@@ -1,12 +1,6 @@
 #![allow(dead_code)]
 use crate::Error;
-use core::{
-    mem::MaybeUninit,
-    num::NonZeroU32,
-    ptr::NonNull,
-    sync::atomic::{fence, AtomicPtr, Ordering},
-};
-use libc::c_void;
+use core::{mem::MaybeUninit, num::NonZeroU32};
 
 cfg_if! {
     if #[cfg(any(target_os = "netbsd", target_os = "openbsd", target_os = "android"))] {
@@ -76,62 +70,6 @@ pub fn sys_fill_exact(
     Ok(())
 }
 
-// A "weak" binding to a C function that may or may not be present at runtime.
-// Used for supporting newer OS features while still building on older systems.
-// Based off of the DlsymWeak struct in libstd:
-// https://github.com/rust-lang/rust/blob/1.61.0/library/std/src/sys/unix/weak.rs#L84
-// except that the caller must manually cast self.ptr() to a function pointer.
-pub struct Weak {
-    name: &'static str,
-    addr: AtomicPtr<c_void>,
-}
-
-impl Weak {
-    // A non-null pointer value which indicates we are uninitialized. This
-    // constant should ideally not be a valid address of a function pointer.
-    // However, if by chance libc::dlsym does return UNINIT, there will not
-    // be undefined behavior. libc::dlsym will just be called each time ptr()
-    // is called. This would be inefficient, but correct.
-    // TODO: Replace with core::ptr::invalid_mut(1) when that is stable.
-    const UNINIT: *mut c_void = 1 as *mut c_void;
-
-    // Construct a binding to a C function with a given name. This function is
-    // unsafe because `name` _must_ be null terminated.
-    pub const unsafe fn new(name: &'static str) -> Self {
-        Self {
-            name,
-            addr: AtomicPtr::new(Self::UNINIT),
-        }
-    }
-
-    // Return the address of a function if present at runtime. Otherwise,
-    // return None. Multiple callers can call ptr() concurrently. It will
-    // always return _some_ value returned by libc::dlsym. However, the
-    // dlsym function may be called multiple times.
-    pub fn ptr(&self) -> Option<NonNull<c_void>> {
-        // Despite having only a single atomic variable (self.addr), we still
-        // cannot always use Ordering::Relaxed, as we need to make sure a
-        // successful call to dlsym() is "ordered before" any data read through
-        // the returned pointer (which occurs when the function is called).
-        // Our implementation mirrors that of the one in libstd, meaning that
-        // the use of non-Relaxed operations is probably unnecessary.
-        match self.addr.load(Ordering::Relaxed) {
-            Self::UNINIT => {
-                let symbol = self.name.as_ptr() as *const _;
-                let addr = unsafe { libc::dlsym(libc::RTLD_DEFAULT, symbol) };
-                // Synchronizes with the Acquire fence below
-                self.addr.store(addr, Ordering::Release);
-                NonNull::new(addr)
-            }
-            addr => {
-                let func = NonNull::new(addr)?;
-                fence(Ordering::Acquire);
-                Some(func)
-            }
-        }
-    }
-}
-
 // SAFETY: path must be null terminated, FD must be manually closed.
 pub unsafe fn open_readonly(path: &str) -> Result<libc::c_int, Error> {
     debug_assert_eq!(path.as_bytes().last(), Some(&0));