ip: check interface existence before set/get mtu

Author: mcgov

lisa/tools/ip.py

@@ -259,7 +259,12 @@ def get_mtu(self, nic_name: str) -> int:
         if self.node.shell.exists(self.node.get_pure_path(mtu_file)):
             return int(cat.read(mtu_file, force_run=True))
         else:
-            return int(self.get_detail(nic_name, "mtu"))
+            mtu = self.get_detail(nic_name, "mtu")
+            if mtu:
+                return int(mtu)
+            else:
+                self.node.log.debug(f"Could not find mtu information for interface {nic_name}")
+                return 0
 
     def set_mtu(self, nic_name: str, mtu: int, assert_success: bool = True) -> None:
         # Check if mtu is integer
@@ -268,14 +273,14 @@ def set_mtu(self, nic_name: str, mtu: int, assert_success: bool = True) -> None:
         except ValueError:
             raise LisaException(f"MTU value is not an integer: {mtu}")
         # check if the device exists
-        exists = self.run(f"link show {nic_name}", force_run=True).exit_code
+        exists = self.run(f"link show {nic_name}", force_run=True).exit_code == 0
         if not exists:
             if assert_success:
                 raise LisaException(f"MTU set failed, could not find interface {nic_name}.")
             else:
                 self.node.log.debug(
-                    "set_mtu: skipping since device does not exist "
-                    "and assert_success was False."
+                    f"set_mtu: device {nic_name} not found,"
+                    " skipping assertion since assert_success was False. "
                 )
                 return
 
@@ -286,9 +291,12 @@ def set_mtu(self, nic_name: str, mtu: int, assert_success: bool = True) -> None:
             if assert_success:
                 raise LisaException(f"set mtu failed, wanted {mtu} and got {new_mtu}")
             else:
-                self.node.log.debug(
-                    "set_mtu: skipping result assertion since assert_success was False. "
-                )
+                 # warn if assertion is turned off. 
+                 # Weird enough situation to justify log.warning
+                 self.node.log.warning(
+                        f"set_mtu: expected new mtu {mtu}, got {new_mtu} instead. "
+                    )
+                
 
     def nic_exists(self, nic_name: str) -> bool:
         result = self.run(f"link show {nic_name}", force_run=True, sudo=True)