Fix unix fork locking to work with parking_lot

src/libstd/sys/unix/process/process_unix.rs

@@ -1,4 +1,5 @@
 use crate::io::{self, Error, ErrorKind};
+use crate::mem;
 use crate::ptr;
 use crate::sys::cvt;
 use crate::sys::process::process_common::*;
@@ -35,17 +36,16 @@ impl Command {
         // accessing the `environ` pointer ourselves). Make sure no other thread
         // is accessing the environment when we do the fork itself.
         //
-        // Note that as soon as we're done with the fork there's no need to hold
-        // 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_lock();
-            cvt(libc::fork())?
-        };
-
+        // Note that as soon as we're done with the fork the parent can stop
+        // holding the lock, because it won't do anything. And the child must
+        // forget it, keeping it locked. This is because parking_lot is
+        // not fork safe and the child process must not lock or unlock any
+        // locks before it gets to libc::execvp.
         let pid = unsafe {
-            match result {
+            let _env_lock = sys::os::env_lock();
+            match cvt(libc::fork())? {
                 0 => {
+                    mem::forget(_env_lock);
                     drop(input);
                     let err = self.do_exec(theirs, envp.as_ref());
                     let errno = err.raw_os_error().unwrap_or(libc::EINVAL) as u32;

src/test/run-pass/command-pre-exec.rs

@@ -17,8 +17,7 @@ fn main() {
     if let Some(arg) = env::args().nth(1) {
         match &arg[..] {
             "test1" => println!("hello2"),
-            "test2" => assert_eq!(env::var("FOO").unwrap(), "BAR"),
-            "test3" => assert_eq!(env::current_dir().unwrap().to_str().unwrap(), "/"),
+            "test2" => assert_eq!(env::current_dir().unwrap().to_str().unwrap(), "/"),
             "empty" => {}
             _ => panic!("unknown argument: {}", arg),
         }
@@ -44,20 +43,6 @@ fn main() {
     let output = unsafe {
         Command::new(&me)
             .arg("test2")
-            .pre_exec(|| {
-                env::set_var("FOO", "BAR");
-                Ok(())
-            })
-            .output()
-            .unwrap()
-    };
-    assert!(output.status.success());
-    assert!(output.stderr.is_empty());
-    assert!(output.stdout.is_empty());
-
-    let output = unsafe {
-        Command::new(&me)
-            .arg("test3")
             .pre_exec(|| {
                 env::set_current_dir("/").unwrap();
                 Ok(())