std: Re-enable at_exit()

The new semantics of this function are that the callbacks are run when the *main
thread* exits, not when all threads have exited. This implies that other threads
may still be running when the `at_exit` callbacks are invoked and users need to
be prepared for this situation.

Users in the standard library have been audited in accordance to these new rules
as well.

Closes #20012

1 files changed, 9 insertions, 2 deletions

diff --git a/src/libstd/sys/unix/timer.rs b/src/libstd/sys/unix/timer.rs
index fe393b81e3d..1ababbc0d85 100644
--- a/src/libstd/sys/unix/timer.rs
+++ b/src/libstd/sys/unix/timer.rs
@@ -100,7 +100,7 @@ pub fn now() -> u64 {
 fn helper(input: libc::c_int, messages: Receiver<Req>, _: ()) {
     let mut set: c::fd_set = unsafe { mem::zeroed() };
 
-    let mut fd = FileDesc::new(input, true);
+    let fd = FileDesc::new(input, true);
     let mut timeout: libc::timeval = unsafe { mem::zeroed() };
 
     // active timers are those which are able to be selected upon (and it's a
@@ -168,8 +168,15 @@ fn helper(input: libc::c_int, messages: Receiver<Req>, _: ()) {
             1 => {
                 loop {
                     match messages.try_recv() {
+                        // Once we've been disconnected it means the main thread
+                        // is exiting (at_exit has run). We could still have
+                        // active timers for other threads, so we're just going
+                        // to drop them all on the floor. This is all we can
+                        // really do, however, to prevent resource leakage. The
+                        // remaining timers will likely start panicking quickly
+                        // as they attempt to re-use this thread but are
+                        // disallowed to do so.
                         Err(comm::Disconnected) => {
-                            assert!(active.len() == 0);
                             break 'outer;
                         }