Disable heartbeats in jl_print_task_backtraces()

`jl_print_task_backtraces()` can take long enough that there can
be heartbeat loss, which can trigger printing task backtraces
again, unless it is called from the heartbeat thread which takes
care of that possible problem.

src/stackwalk.c

@@ -1166,10 +1166,20 @@ JL_DLLEXPORT void jl_print_backtrace(void) JL_NOTSAFEPOINT
 }
 
 extern int gc_first_tid;
+extern int jl_inside_heartbeat_thread(void);
+extern int jl_heartbeat_pause(void);
+extern int jl_heartbeat_resume(void);
 
-// Print backtraces for all live tasks, for all threads, to jl_safe_printf stderr
+// Print backtraces for all live tasks, for all threads, to jl_safe_printf
+// stderr. This can take a _long_ time!
 JL_DLLEXPORT void jl_print_task_backtraces(int show_done) JL_NOTSAFEPOINT
 {
+    // disable heartbeats to prevent heartbeat loss while running this,
+    // unless this is called from the heartbeat thread
+    if (!jl_inside_heartbeat_thread()) {
+        jl_heartbeat_pause();
+    }
+
     size_t nthreads = jl_atomic_load_acquire(&jl_n_threads);
     jl_ptls_t *allstates = jl_atomic_load_relaxed(&jl_all_tls_states);
     int ctid = -1;
@@ -1232,6 +1242,10 @@ JL_DLLEXPORT void jl_print_task_backtraces(int show_done) JL_NOTSAFEPOINT
         jl_safe_printf("thread (%d) ==== End thread %d\n", ctid, ptls2->tid + 1);
     }
     jl_safe_printf("thread (%d) ++++ Done\n", ctid);
+
+    if (!jl_inside_heartbeat_thread()) {
+        jl_heartbeat_resume();
+    }
 }
 
 #ifdef __cplusplus