Auto merge of #83121 - the8472:env-rwlock-2, r=joshtriplett

use RWlock when accessing os::env (take 2)

This reverts commit acdca31 (#82877) i.e. redoes #81850 since the invalid unlock attempts in the child process have been fixed in #82949

r? `@joshtriplett`

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

@@ -22,6 +22,7 @@ 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};
@@ -490,20 +491,20 @@ pub unsafe fn environ() -> *mut *const *const c_char {
     extern "C" {
         static mut environ: *const *const c_char;
     }
-    &mut environ
+    ptr::addr_of_mut!(environ)
 }
 
-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()
+static ENV_LOCK: StaticRWLock = StaticRWLock::new();
+
+pub fn env_read_lock() -> RWLockReadGuard {
+    ENV_LOCK.read_with_guard()
 }
 
 /// 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_lock();
+        let _guard = env_read_lock();
         let mut environ = *environ();
         let mut result = Vec::new();
         if !environ.is_null() {
@@ -540,7 +541,7 @@ pub fn getenv(k: &OsStr) -> io::Result<Option<OsString>> {
     // always None as well
     let k = CString::new(k.as_bytes())?;
     unsafe {
-        let _guard = env_lock();
+        let _guard = env_read_lock();
         let s = libc::getenv(k.as_ptr()) as *const libc::c_char;
         let ret = if s.is_null() {
             None
@@ -556,7 +557,7 @@ pub fn setenv(k: &OsStr, v: &OsStr) -> io::Result<()> {
     let v = CString::new(v.as_bytes())?;
 
     unsafe {
-        let _guard = env_lock();
+        let _guard = ENV_LOCK.write_with_guard();
         cvt(libc::setenv(k.as_ptr(), v.as_ptr(), 1)).map(drop)
     }
 }
@@ -565,7 +566,7 @@ pub fn unsetenv(n: &OsStr) -> io::Result<()> {
     let nbuf = CString::new(n.as_bytes())?;
 
     unsafe {
-        let _guard = env_lock();
+        let _guard = ENV_LOCK.write_with_guard();
         cvt(libc::unsetenv(nbuf.as_ptr())).map(drop)
     }
 }

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

@@ -48,7 +48,7 @@ impl Command {
         // a lock any more because the parent won't do anything and the child is
         // in its own process. Thus the parent drops the lock guard while the child
         // forgets it to avoid unlocking it on a new thread, which would be invalid.
-        let (env_lock, result) = unsafe { (sys::os::env_lock(), cvt(libc::fork())?) };
+        let (env_lock, result) = unsafe { (sys::os::env_read_lock(), cvt(libc::fork())?) };
 
         let pid = unsafe {
             match result {
@@ -127,7 +127,7 @@ 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_lock();
+                    let _lock = sys::os::env_read_lock();
 
                     let Err(e) = self.do_exec(theirs, envp.as_ref());
                     e
@@ -407,7 +407,7 @@ 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_lock();
+            let _env_lock = sys::os::env_read_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

@@ -86,3 +86,62 @@ 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() }
+    }
+}