fix(middleawre/iscsi): do not allow duplicate tags for iSCCI Auth

Ticket:	#80085

src/middlewared/middlewared/plugins/iscsi.py

@@ -367,7 +367,7 @@ class Config:
 
     @accepts(Dict(
         'iscsi_auth_create',
-        Int('tag'),
+        Int('tag', required=True),
         Str('user', required=True),
         Str('secret', required=True),
         Str('peeruser'),
@@ -399,15 +399,13 @@ async def do_create(self, data):
         )
     )
     async def do_update(self, id, data):
-        verrors = ValidationErrors()
         old = await self._get_instance(id)
 
         new = old.copy()
         new.update(data)
 
-        await self.validate(
-            new, 'iscsi_auth_update', verrors
-        )
+        verrors = ValidationErrors()
+        await self.validate(new, 'iscsi_auth_update', verrors, old)
 
         if verrors:
             raise verrors
@@ -428,7 +426,15 @@ async def do_delete(self, id):
         )
 
     @private
-    async def validate(self, data, schema_name, verrors):
+    async def validate(self, data, schema_name, verrors, old=None):
+
+        filters = [('tag', '=', data['tag'])]
+        if old:
+            filters.append(('id', '!=', old['id']))
+
+        if await self.middleware.call('iscsi.auth.query', filters):
+            verrors.add(f'{schema_name}.tag', f'Tag {data["tag"]!r} is already in use.')
+
         secret = data.get('secret')
         peer_secret = data.get('peersecret')
         peer_user = data.get('peeruser', '')