Revert "use RWlock when accessing os::env #81850"

This reverts commit 354f19cf24, reversing
changes made to 0cfba2fd09.

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

@@ -22,7 +22,6 @@ use crate::str;
 use crate::sys::cvt;
 use crate::sys::fd;
 use crate::sys_common::mutex::{StaticMutex, StaticMutexGuard};
-use crate::sys_common::rwlock::{RWLockReadGuard, StaticRWLock};
 use crate::vec;
 
 use libc::{c_char, c_int, c_void};
@@ -491,20 +490,20 @@ pub unsafe fn environ() -> *mut *const *const c_char {
     extern "C" {
         static mut environ: *const *const c_char;
     }
-    ptr::addr_of_mut!(environ)
+    &mut environ
 }
 
-static ENV_LOCK: StaticRWLock = StaticRWLock::new();
-
-pub fn env_read_lock() -> RWLockReadGuard {
-    ENV_LOCK.read_with_guard()
+pub unsafe fn env_lock() -> StaticMutexGuard {
+    // It is UB to attempt to acquire this mutex reentrantly!
+    static ENV_LOCK: StaticMutex = StaticMutex::new();
+    ENV_LOCK.lock()
 }
 
 /// Returns a vector of (variable, value) byte-vector pairs for all the
 /// environment variables of the current process.
 pub fn env() -> Env {
     unsafe {
-        let _guard = env_read_lock();
+        let _guard = env_lock();
         let mut environ = *environ();
         let mut result = Vec::new();
         if !environ.is_null() {
@@ -541,7 +540,7 @@ pub fn getenv(k: &OsStr) -> io::Result<Option<OsString>> {
     // always None as well
     let k = CString::new(k.as_bytes())?;
     unsafe {
-        let _guard = env_read_lock();
+        let _guard = env_lock();
         let s = libc::getenv(k.as_ptr()) as *const libc::c_char;
         let ret = if s.is_null() {
             None
@@ -557,7 +556,7 @@ pub fn setenv(k: &OsStr, v: &OsStr) -> io::Result<()> {
     let v = CString::new(v.as_bytes())?;
 
     unsafe {
-        let _guard = ENV_LOCK.write_with_guard();
+        let _guard = env_lock();
         cvt(libc::setenv(k.as_ptr(), v.as_ptr(), 1)).map(drop)
     }
 }
@@ -566,7 +565,7 @@ pub fn unsetenv(n: &OsStr) -> io::Result<()> {
     let nbuf = CString::new(n.as_bytes())?;
 
     unsafe {
-        let _guard = ENV_LOCK.write_with_guard();
+        let _guard = env_lock();
         cvt(libc::unsetenv(nbuf.as_ptr())).map(drop)
     }
 }

library/std/src/sys/unix/process/process_unix.rs

@@ -47,7 +47,7 @@ impl Command {
         // a lock any more because the parent won't do anything and the child is
         // in its own process.
         let result = unsafe {
-            let _env_lock = sys::os::env_read_lock();
+            let _env_lock = sys::os::env_lock();
             cvt(libc::fork())?
         };
 
@@ -124,7 +124,7 @@ impl Command {
                     // Similar to when forking, we want to ensure that access to
                     // the environment is synchronized, so make sure to grab the
                     // environment lock before we try to exec.
-                    let _lock = sys::os::env_read_lock();
+                    let _lock = sys::os::env_lock();
 
                     let Err(e) = self.do_exec(theirs, envp.as_ref());
                     e
@@ -404,7 +404,7 @@ impl Command {
             cvt_nz(libc::posix_spawnattr_setflags(attrs.0.as_mut_ptr(), flags as _))?;
 
             // Make sure we synchronize access to the global `environ` resource
-            let _env_lock = sys::os::env_read_lock();
+            let _env_lock = sys::os::env_lock();
             let envp = envp.map(|c| c.as_ptr()).unwrap_or_else(|| *sys::os::environ() as *const _);
             cvt_nz(libc::posix_spawnp(
                 &mut p.pid,