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, 27 insertions, 0 deletions

diff --git a/src/thread.c b/src/thread.c
index e8cb430119f..bf2cf1b06c8 100644
--- a/src/thread.c
+++ b/src/thread.c
@@ -24,6 +24,7 @@ along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.  */
 #include "buffer.h"
 #include "process.h"
 #include "coding.h"
+#include "syssignal.h"
 
 static struct thread_state primary_thread;
 
@@ -100,6 +101,23 @@ acquire_global_lock (struct thread_state *self)
   post_acquire_global_lock (self);
 }
 
+/* This is called from keyboard.c when it detects that SIGINT
+   interrupted thread_select before the current thread could acquire
+   the lock.  We must acquire the lock to prevent a thread from
+   running without holding the global lock, and to avoid repeated
+   calls to sys_mutex_unlock, which invokes undefined behavior.  */
+void
+maybe_reacquire_global_lock (void)
+{
+  if (current_thread->not_holding_lock)
+    {
+      struct thread_state *self = current_thread;
+
+      acquire_global_lock (self);
+      current_thread->not_holding_lock = 0;
+    }
+}
+
 
 
 static void
@@ -493,11 +511,20 @@ really_call_select (void *arg)
 {
   struct select_args *sa = arg;
   struct thread_state *self = current_thread;
+  sigset_t oldset;
 
+  block_interrupt_signal (&oldset);
+  self->not_holding_lock = 1;
   release_global_lock ();
+  restore_signal_mask (&oldset);
+
   sa->result = (sa->func) (sa->max_fds, sa->rfds, sa->wfds, sa->efds,
                            sa->timeout, sa->sigmask);
+
+  block_interrupt_signal (&oldset);
   acquire_global_lock (self);
+  self->not_holding_lock = 0;
+  restore_signal_mask (&oldset);
 }
 
 int