Enable faulthandler around the test runs to get a notice on shard crashes.

runtests.py

@@ -2501,7 +2501,33 @@ def runtests(options, cmd_args, coverage=None):
     except AttributeError:
         pass  # not available on PyPy
 
-    result = test_runner.run(test_suite)
+    enable_faulthandler = False
+    try:
+        import faulthandler
+    except ImportError:
+        pass
+    else:
+        enable_faulthandler = not faulthandler.is_enabled()
+        if enable_faulthandler:
+            faulthandler.enable()
+
+    # Run the collected tests.
+    try:
+        if options.shard_num > -1:
+            sys.stderr.write("Tests in shard %d starting" % options.shard_num)
+        result = test_runner.run(test_suite)
+    except Exception as exc:
+        # Make sure we print exceptions also from shards.
+        if options.shard_num > -1:
+            sys.stderr.write("Tests in shard %d crashed: %s" % (options.shard_num, exc))
+        import traceback
+        traceback.print_exc()
+    finally:
+        if enable_faulthandler:
+            faulthandler.disable()
+        if options.shard_num > -1:
+            sys.stderr.write("Tests in shard %d terminated (%s)" % (
+                options.shard_num, sys.exc_info()[1] or "no crash"))
 
     if common_utility_dir and options.shard_num < 0 and options.cleanup_workdir:
         shutil.rmtree(common_utility_dir)