Trigger postponed jobs on running_ec if that is available

Currently, any postponed job triggered from a non-ruby thread gets sent
to the main thread, but if the main thread is sleeping it won't be
checking ints. Instead, we should try and interrupt running_ec if that's
possible, and only fall back to the main thread if it's not.

[Bug #20197]

1 files changed, 16 insertions, 0 deletions

diff --git a/ractor.c b/ractor.c
index 4d47e75a6c..f7b251a8dc 100644
--- a/ractor.c
+++ b/ractor.c
@@ -2481,6 +2481,22 @@ rb_ractor_terminate_all(void)
 rb_execution_context_t *
 rb_vm_main_ractor_ec(rb_vm_t *vm)
 {
+    /* This code needs to carefully work around two bugs:
+     *   - Bug #20016: When M:N threading is enabled, running_ec is NULL if no thread is
+     *     actually currently running (as opposed to without M:N threading, when
+     *     running_ec will still point to the _last_ thread which ran)
+     *   - Bug #20197: If the main thread is sleeping, setting its postponed job
+     *     interrupt flag is pointless; it won't look at the flag until it stops sleeping
+     *     for some reason. It would be better to set the flag on the running ec, which
+     *     will presumably look at it soon.
+     *
+     *  Solution: use running_ec if it's set, otherwise fall back to the main thread ec.
+     *  This is still susceptible to some rare race conditions (what if the last thread
+     *  to run just entered a long-running sleep?), but seems like the best balance of
+     *  robustness and complexity.
+     */
+    rb_execution_context_t *running_ec = vm->ractor.main_ractor->threads.running_ec;
+    if (running_ec) { return running_ec; }
     return vm->ractor.main_thread->ec;
 }