Fix crashes upon C-g on Posix TTY frames

* src/thread.h (struct thread_state): New member not_holding_lock.
(maybe_reacquire_global_lock): Add prototype.
* src/thread.c: Include syssignal.h.
(maybe_reacquire_global_lock): New function.
(really_call_select): Set the not_holding_lock member of the
thread state before releasing the lock, and rest it after
re-acquiring the lock when the select function returns.  Block
SIGINT while doing this to make sure we are not interrupted on TTY
frames.
* src/sysdep.c (block_interrupt_signal, restore_signal_mask): New
functions.
* src/syssignal.h (block_interrupt_signal, restore_signal_mask):
Add prototypes.
* src/keyboard.c (read_char) [THREADS_ENABLED]: Call
maybe_reacquire_global_lock.  (Bug#25178)

1 files changed, 8 insertions, 0 deletions

diff --git a/src/thread.h b/src/thread.h
index e6084b13c22..7dee67d6595 100644
--- a/src/thread.h
+++ b/src/thread.h
@@ -171,6 +171,13 @@ struct thread_state
      interrupter should broadcast to this condition.  */
   sys_cond_t *wait_condvar;
 
+  /* This thread might have released the global lock.  If so, this is
+     non-zero.  When a thread runs outside thread_select with this
+     flag non-zero, it means it has been interrupted by SIGINT while
+     in thread_select, and didn't have a chance of acquiring the lock.
+     It must do so ASAP.  */
+  int not_holding_lock;
+
   /* Threads are kept on a linked list.  */
   struct thread_state *next_thread;
 };
@@ -224,6 +231,7 @@ extern void unmark_threads (void);
 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 init_threads_once (void);
 extern void init_threads (void);