Rollup merge of rust-lang#87236 - sunfishcode:avoid-locking-args, r=joshtriplett

Simplify command-line argument initialization on unix

Simplify Rust's command-line argument initialization code on unix:
 - The cleanup code isn't needed, because it was just zeroing out non-owning variables at runtime cleanup time. After 91c3eee, Rust's command-line initialization code on unix no longer allocates `CString`s and a `Vec` at startup time.
 - The `Mutex` isn't needed; if there's somehow a call to `args()` before argument initialization has happened, the code returns return an empty list, which we can do with a null check.

With these changes, a simple cdylib that doesn't use threads avoids getting `pthread_mutex_lock`/`pthread_mutex_unlock` in its symbol table.

Author: GuillaumeGomez

library/std/src/sys/unix/args.rs

@@ -14,11 +14,6 @@ pub unsafe fn init(argc: isize, argv: *const *const u8) {
     imp::init(argc, argv)
 }
 
-/// One-time global cleanup.
-pub unsafe fn cleanup() {
-    imp::cleanup()
-}
-
 /// Returns the command line arguments
 pub fn args() -> Args {
     imp::args()
@@ -82,16 +77,10 @@ mod imp {
     use crate::ptr;
     use crate::sync::atomic::{AtomicIsize, AtomicPtr, Ordering};
 
-    use crate::sys_common::mutex::StaticMutex;
-
     static ARGC: AtomicIsize = AtomicIsize::new(0);
     static ARGV: AtomicPtr<*const u8> = AtomicPtr::new(ptr::null_mut());
-    // We never call `ENV_LOCK.init()`, so it is UB to attempt to
-    // acquire this mutex reentrantly!
-    static LOCK: StaticMutex = StaticMutex::new();
 
     unsafe fn really_init(argc: isize, argv: *const *const u8) {
-        let _guard = LOCK.lock();
         ARGC.store(argc, Ordering::Relaxed);
         ARGV.store(argv as *mut _, Ordering::Relaxed);
     }
@@ -127,21 +116,16 @@ mod imp {
         init_wrapper
     };
 
-    pub unsafe fn cleanup() {
-        let _guard = LOCK.lock();
-        ARGC.store(0, Ordering::Relaxed);
-        ARGV.store(ptr::null_mut(), Ordering::Relaxed);
-    }
-
     pub fn args() -> Args {
         Args { iter: clone().into_iter() }
     }
 
     fn clone() -> Vec<OsString> {
         unsafe {
-            let _guard = LOCK.lock();
-            let argc = ARGC.load(Ordering::Relaxed);
+            // Load ARGC and ARGV without a lock. If the store to either ARGV or
+            // ARGC isn't visible yet, we'll return an empty argument list.
             let argv = ARGV.load(Ordering::Relaxed);
+            let argc = if argv.is_null() { 0 } else { ARGC.load(Ordering::Relaxed) };
             (0..argc)
                 .map(|i| {
                     let cstr = CStr::from_ptr(*argv.offset(i) as *const libc::c_char);
@@ -159,8 +143,6 @@ mod imp {
 
     pub unsafe fn init(_argc: isize, _argv: *const *const u8) {}
 
-    pub fn cleanup() {}
-
     #[cfg(target_os = "macos")]
     pub fn args() -> Args {
         use crate::os::unix::prelude::*;

library/std/src/sys/unix/mod.rs

@@ -123,7 +123,6 @@ pub unsafe fn init(argc: isize, argv: *const *const u8) {
 // SAFETY: must be called only once during runtime cleanup.
 // NOTE: this is not guaranteed to run, for example when the program aborts.
 pub unsafe fn cleanup() {
-    args::cleanup();
     stack_overflow::cleanup();
 }