bgpd: fix crash in aspath remove/replace chain, add a test for it

bgpd/bgp_aspath.c

@@ -1151,7 +1151,7 @@ bool aspath_firstas_check(struct aspath *aspath, as_t asno)
 
 unsigned int aspath_get_first_as(struct aspath *aspath)
 {
-	if (aspath == NULL || aspath->segments == NULL)
+	if (aspath == NULL || aspath->segments == NULL || aspath->segments->length == 0)
 		return 0;
 
 	return aspath->segments->as[0];

tests/topotests/bgp_remove_private_as/test_bgp_remove_private_as.py

@@ -41,6 +41,7 @@
 from lib import topotest
 from lib.topogen import Topogen, TopoRouter, get_topogen
 from functools import partial
+from lib.topolog import logger
 
 pytestmark = [pytest.mark.bgpd]
 
@@ -389,6 +390,44 @@ def _validate_paths(remove_type):
                     real_path = adj_rib_in["receivedRoutes"][pfx]["path"]
                     return real_path == good_path
 
+    def __no_crash():
+        output = json.loads(
+            tgen.gears["r2"].vtysh_cmd(
+                "show ip bgp neighbor 203.0.113.0 advertised-routes detail json"
+            )
+        )
+        logger.info(output)
+        logger.info(output["totalPrefixCounter"])
+        if output["totalPrefixCounter"] != 0:
+            return True
+        return False
+
+    def _crash_scenario():
+        """add list of command making"""
+        tgen = get_topogen()
+        tgen.gears["r2"].vtysh_cmd(
+            """
+            configure terminal
+             route-map OLDCORE-INTERNET-IN-IPv4 permit 5
+              set as-path replace any 65398
+             exit
+             route-map OLDCORE-INTERNET-OUT-IPv4 permit 5
+              set as-path replace any 65399
+             exit
+            router bgp 65002
+             bgp deterministic-med
+             address-family ipv4 unicast
+              neighbor 203.0.113.0 remove-private-AS all
+              neighbor 203.0.113.0 soft-reconfiguration inbound
+              neighbor 203.0.113.0 route-map OLDCORE-INTERNET-IN-IPv4 in
+              neighbor 203.0.113.0 route-map OLDCORE-INTERNET-OUT-IPv4 out
+             exit-address-family
+            """
+        )
+        test_func = partial(__no_crash)
+        _, result = topotest.run_and_expect(test_func, True, count=60, wait=0.5)
+        assert result == True, "bgpd may has major failure"
+
     #######################
     # Begin Test
     #######################
@@ -408,6 +447,13 @@ def _validate_paths(remove_type):
         # the old flag after each iteration so we only test the flags we expect.
         _change_remove_type(rmv_type, "del")
 
+    # verify that nulled as_path don't make a crash.
+    _change_remove_type("remove-private-AS all", "add")
+    _crash_scenario()
+
+
+exit
+
 
 if __name__ == "__main__":
     args = ["-s"] + sys.argv[1:]