Add self timeout and crash if exceeded. (sonic-net#1502)

Log callstack on timeout.

What I did
Add self timeout and crash on timeout.
Before crash log the error and call stack.

How I did it
Add a signal based alarm and the handler to print error & call stack.

How to verify it
Artificially introduce a sleep (> TIMEOUT, which is 2mins) in the script in any function that is in the call path.
Invoke the script. Watch it crash and note the error & stack logged in syslog.

scripts/route_check.py

@@ -43,6 +43,8 @@
 import sys
 import syslog
 import time
+import signal
+import traceback
 
 from swsscommon import swsscommon
 
@@ -53,6 +55,9 @@
 
 SUBSCRIBE_WAIT_SECS = 1
 
+# Max of 2 minutes
+TIMEOUT_SECONDS = 120
+
 UNIT_TESTING = 0
 
 os.environ['PYTHONUNBUFFERED']='True'
@@ -75,6 +80,14 @@ def __str__(self):
 report_level = syslog.LOG_ERR
 write_to_syslog = False
 
+def handler(signum, frame):
+    print_message(syslog.LOG_ERR,
+            "Aborting routeCheck.py upon timeout signal after {} seconds".
+            format(TIMEOUT_SECONDS))
+    print_message(syslog.LOG_ERR, str(traceback.extract_stack()))
+    raise Exception("timeout occurred")
+
+
 def set_level(lvl, log_to_syslog):
     """
     Sets the log level
@@ -429,7 +442,7 @@ def main():
     parser=argparse.ArgumentParser(description="Verify routes between APPL-DB & ASIC-DB are in sync")
     parser.add_argument('-m', "--mode", type=Level, choices=list(Level), default='ERR')
     parser.add_argument("-i", "--interval", type=int, default=0, help="Scan interval in seconds")
-    parser.add_argument("-s", "--log_to_syslog", action="store_true", default=False, help="Write message to syslog")
+    parser.add_argument("-s", "--log_to_syslog", action="store_true", default=True, help="Write message to syslog")
     args = parser.parse_args()
 
     set_level(args.mode, args.log_to_syslog)
@@ -444,8 +457,12 @@ def main():
         if UNIT_TESTING:
             interval = 1
 
+    signal.signal(signal.SIGALRM, handler)
+
     while True:
+        signal.alarm(TIMEOUT_SECONDS)
         ret, res= check_routes()
+        signal.alarm(0)
 
         if interval:
             time.sleep(interval)

tests/route_check_test.py

@@ -2,6 +2,7 @@
 import json
 import os
 import sys
+import time
 from unittest.mock import MagicMock, patch
 
 import pytest
@@ -276,6 +277,7 @@ def table_side_effect(db, tbl):
 
 class mock_selector:
     TIMEOUT = 1
+    EMULATE_HANG = False
 
     def __init__(self):
         self.select_state = 0
@@ -295,6 +297,9 @@ def select(self, timeout):
         state = self.select_state
         self.subs.update()
 
+        if mock_selector.EMULATE_HANG:
+            time.sleep(60)
+
         if self.select_state == 0:
             self.select_state = self.TIMEOUT
         else:
@@ -423,6 +428,21 @@ def test_server(self, mock_subs, mock_sel, mock_table, mock_conn):
                 assert res == expect_res
 
 
+        # Test timeout
+        route_check.TIMEOUT_SECONDS = 5
+        mock_selector.EMULATE_HANG = True
+        ex_raised = False
+
+        try:
+            ret, res = route_check.main()
+        except Exception as err:
+            ex_raised = True
+            expect = "timeout occurred"
+            ex_str = str(err)
+            assert ex_str == expect, "{} != {}".format(ex_str, expect)
+        assert ex_raised, "Exception expected"
+
+