> The signal handler calls Py_AddPendingCall() which blocks on acquiring 
> "pending_lock".

It blocks in taking the mutex, not on waiting for the condition variable. Otherwise it wouldn't block (microseconds = 0).

I think the solution is to protect signal_handler() against reentrancy: 

diff --git a/Modules/signalmodule.c b/Modules/signalmodule.c
--- a/Modules/signalmodule.c
+++ b/Modules/signalmodule.c
@@ -185,10 +185,12 @@ signal_handler(int sig_num)
         Handlers[sig_num].tripped = 1;
         /* Set is_tripped after setting .tripped, as it gets
            cleared in PyErr_CheckSignals() before .tripped. */
-        is_tripped = 1;
-        Py_AddPendingCall(checksignals_witharg, NULL);
-        if (wakeup_fd != -1)
-            write(wakeup_fd, "\0", 1);
+        if (!is_tripped) {
+            is_tripped = 1;
+            Py_AddPendingCall(checksignals_witharg, NULL);
+            if (wakeup_fd != -1)
+                write(wakeup_fd, "\0", 1);
+        }
     }
 
 #ifndef HAVE_SIGACTION