Subject: Re: bug#64819: 30.0.50; condition-wait not interruptible

From: Helmut Eller <[email-masked]>
Date: Tue, 25 Jul 2023 10:06:40 +0200

On Mon, Jul 24 2023, Eli Zaretskii wrote:

[...]
> So for this to work, the C-g handler will have to release some thread.

Here is a patch that works good enough for me:

>From 4de3198c10c4efaeaffdf43ba5e5b0f1729a7f09 Mon Sep 17 00:00:00 2001
From: Helmut Eller <[email-masked]>
Date: Tue, 25 Jul 2023 10:03:53 +0200
Subject: [PATCH] Make condition-wait interruptible by C-g

Code like

  (let* ((mutex (make-mutex))
         (cvar (make-condition-variable mutex)))
    (with-mutex mutex
      (condition-wait cvar)))

will block in pthread_cond_wait.  The problem is that
pthread_cond_wait may or may not return when it gets interrupted
by a signal (SIGIO).  On Linux it doesn't return and so even if a
signal handler sets pending_signals=true nobody processes those
pending signals.

The patch modifies the signal handler so that it will force a
spurious wakeup when the current thread is blocked in
condition-wait.

* src/keyboard.c (handle_input_available_signal)
(handle_interrupt): Call maybe_awake_current_thread.
* src/thread.c (maybe_awake_current_thread): New.
* src/thread.h (maybe_awake_current_thread): New prototype.
---
 src/keyboard.c |  8 +++++++-
 src/thread.c   | 16 ++++++++++++++++
 src/thread.h   |  1 +
 3 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/src/keyboard.c b/src/keyboard.c
index 41cda2e65de..f45bafa96c0 100644
--- a/src/keyboard.c
+++ b/src/keyboard.c
@@ -7745,6 +7745,10 @@ handle_input_available_signal (int sig)
 
   if (input_available_clear_time)
     *input_available_clear_time = make_timespec (0, 0);
+
+#ifdef THREADS_ENABLED
+  maybe_awake_current_thread ();
+#endif
 }
 
 static void
@@ -11556,8 +11560,10 @@ handle_interrupt (bool in_signal_handler)
   /* If we were called from a signal handler, we must be in the main
      thread, see deliver_process_signal.  So we must make sure the
      main thread holds the global lock.  */
-  if (in_signal_handler)
+  if (in_signal_handler) {
     maybe_reacquire_global_lock ();
+    maybe_awake_current_thread();
+  }
 #endif
   if (waiting_for_input && !echoing)
     quit_throw_to_read_char (in_signal_handler);
diff --git a/src/thread.c b/src/thread.c
index b8ca56fd372..0bd949f5779 100644
--- a/src/thread.c
+++ b/src/thread.c
@@ -172,6 +172,22 @@ maybe_reacquire_global_lock (void)
     }
 }
 
+/* This is called from keyboard.c when it sets pending_signals=true.
+   If the current thread is waiting, we create a spurious wakeup by
+   broadcasting on wait_condvar.  This is necessary because
+   pthread_cond_wait may or may not return if it was interrupted by a
+   signal (SIGIO).  Without the wakeup, nobody would process a
+   potential C-g.
+*/
+void
+maybe_awake_current_thread (void)
+{
+  if (current_thread->wait_condvar != NULL)
+    {
+      sys_cond_broadcast (current_thread->wait_condvar);
+    }
+}
+
 
 
 static void
diff --git a/src/thread.h b/src/thread.h
index 9b14cc44f35..60f601a6248 100644
--- a/src/thread.h
+++ b/src/thread.h
@@ -311,6 +311,7 @@ extern void finalize_one_thread (struct thread_state *state);
 extern void finalize_one_mutex (struct Lisp_Mutex *);
 extern void finalize_one_condvar (struct Lisp_CondVar *);
 extern void maybe_reacquire_global_lock (void);
+extern void maybe_awake_current_thread (void);
 
 extern void init_threads (void);
 extern void syms_of_threads (void);
-- 
2.39.2