Auto merge of #82877 - ehuss:revert-rwlock, r=m-ou-se

Revert switch of env locking to rwlock, to fix deadlock in process spawning

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

PR https://github.com/rust-lang/rust/pull/81850 switched the environment lock from a mutex to an rwlock. However, process spawning (when not able to use `posix_spawn`) locks the environment before forking, and unlocks it after forking (in both the parent and the child). With a mutex, this works (although probably not correct even with a mutex). With an rwlock, on at least some targets, unlocking in the child does not work correctly, resulting in a deadlock.

This has manifested as CI hangs on i686 Linux; that target doesn't use `posix_spawn` in the CI environment due to the age of the installed C library (currently glibc 2.23). (Switching to `posix_spawn` would just mask this issue, though, which would still arise in any case that can't use `posix_spawn`.)

Some additional cleanup of environment handling around process spawning may help, but for now, revert the PR and go back to a standard mutex.

Fixes #82221

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,

library/std/src/sys_common/rwlock.rs

@@ -86,62 +86,3 @@ impl RWLock {
         self.0.destroy()
     }
 }
-
-// the cfg annotations only exist due to dead code warnings. the code itself is portable
-#[cfg(unix)]
-pub struct StaticRWLock(RWLock);
-
-#[cfg(unix)]
-impl StaticRWLock {
-    pub const fn new() -> StaticRWLock {
-        StaticRWLock(RWLock::new())
-    }
-
-    /// Acquires shared access to the underlying lock, blocking the current
-    /// thread to do so.
-    ///
-    /// The lock is automatically unlocked when the returned guard is dropped.
-    #[inline]
-    pub fn read_with_guard(&'static self) -> RWLockReadGuard {
-        // SAFETY: All methods require static references, therefore self
-        // cannot be moved between invocations.
-        unsafe {
-            self.0.read();
-        }
-        RWLockReadGuard(&self.0)
-    }
-
-    /// Acquires write access to the underlying lock, blocking the current thread
-    /// to do so.
-    ///
-    /// The lock is automatically unlocked when the returned guard is dropped.
-    #[inline]
-    pub fn write_with_guard(&'static self) -> RWLockWriteGuard {
-        // SAFETY: All methods require static references, therefore self
-        // cannot be moved between invocations.
-        unsafe {
-            self.0.write();
-        }
-        RWLockWriteGuard(&self.0)
-    }
-}
-
-#[cfg(unix)]
-pub struct RWLockReadGuard(&'static RWLock);
-
-#[cfg(unix)]
-impl Drop for RWLockReadGuard {
-    fn drop(&mut self) {
-        unsafe { self.0.read_unlock() }
-    }
-}
-
-#[cfg(unix)]
-pub struct RWLockWriteGuard(&'static RWLock);
-
-#[cfg(unix)]
-impl Drop for RWLockWriteGuard {
-    fn drop(&mut self) {
-        unsafe { self.0.write_unlock() }
-    }
-}