Enable AST Validation

This commit puts AST validation on the execution path. This means updates get
validated prior to being executed. There were quite a few tests that were not
working against Amazon DDB. These tests I considered broken and as such this
commit adapts them such that they pass against Amazon DDB.

test_update_item_on_map()
=> One of the SET actions would try to set a nested element by specifying the nesting on the path
   rather than by putting a map as a value for a non-existent key. This got changed.

test_item_size_is_under_400KB
=> Used the keyword "item" which DDB doesn't like. Change to cont in order to keep the same sizings.
=> Secondly the size error messages differs a bit depending whether it is part of the update or part
   of a put_item. For an update it should be:
   Item size to update has exceeded the maximum allowed size
   otherwise it is
   Item size has exceeded the maximum allowed size'

test_remove_top_level_attribute
=> Used a keyword item.  Use ExpressionAttributeNames

test_update_item_double_nested_remove
=> Used keywords name & first. Migrated to non-deprecated API and use ExpressionAttributeNames

test_update_item_set &
test_boto3_update_item_conditions_pass &
test_boto3_update_item_conditions_pass_because_expect_not_exists &
test_boto3_update_item_conditions_pass_because_expect_not_exists_by_compare_to_null &
test_boto3_update_item_conditions_pass_because_expect_exists_by_compare_to_not_null &
test_boto3_update_item_conditions_fail &
test_boto3_update_item_conditions_fail_because_expect_not_exists &
test_boto3_update_item_conditions_fail_because_expect_not_exists_by_compare_to_null
=> Were broken tests which had string literal instead of value placeholder

moto/dynamodb2/models/__init__.py

@@ -14,11 +14,16 @@ from moto.core import BaseBackend, BaseModel
 from moto.core.utils import unix_time
 from moto.core.exceptions import JsonRESTError
 from moto.dynamodb2.comparisons import get_filter_expression
-from moto.dynamodb2.comparisons import get_expected, get_comparison_func
-from moto.dynamodb2.exceptions import InvalidIndexNameError, ItemSizeTooLarge, InvalidUpdateExpression
+from moto.dynamodb2.comparisons import get_expected
+from moto.dynamodb2.exceptions import (
+    InvalidIndexNameError,
+    ItemSizeTooLarge,
+    ItemSizeToUpdateTooLarge,
+)
 from moto.dynamodb2.models.utilities import bytesize, attribute_is_list
 from moto.dynamodb2.models.dynamo_type import DynamoType
 from moto.dynamodb2.parsing.expressions import UpdateExpressionParser
+from moto.dynamodb2.parsing.validators import UpdateExpressionValidator
 
 
 class DynamoJsonEncoder(json.JSONEncoder):
@@ -151,7 +156,10 @@ class Item(BaseModel):
                     if "." in key and attr not in self.attrs:
                         raise ValueError  # Setting nested attr not allowed if first attr does not exist yet
                     elif attr not in self.attrs:
-                        self.attrs[attr] = dyn_value  # set new top-level attribute
+                        try:
+                            self.attrs[attr] = dyn_value  # set new top-level attribute
+                        except ItemSizeTooLarge:
+                            raise ItemSizeToUpdateTooLarge()
                     else:
                         self.attrs[attr].set(
                             ".".join(key.split(".")[1:]), dyn_value, list_index
@@ -1202,7 +1210,7 @@ class DynamoDBBackend(BaseBackend):
         # E.g. `a = b + c` -> `a=b+c`
         if update_expression:
             # Parse expression to get validation errors
-            UpdateExpressionParser.make(update_expression)
+            update_expression_ast = UpdateExpressionParser.make(update_expression)
             update_expression = re.sub(r"\s*([=\+-])\s*", "\\1", update_expression)
 
         if all([table.hash_key_attr in key, table.range_key_attr in key]):
@@ -1247,6 +1255,12 @@ class DynamoDBBackend(BaseBackend):
             item = table.get_item(hash_value, range_value)
 
         if update_expression:
+            UpdateExpressionValidator(
+                update_expression_ast,
+                expression_attribute_names=expression_attribute_names,
+                expression_attribute_values=expression_attribute_values,
+                item=item,
+            ).validate()
             item.update(
                 update_expression,
                 expression_attribute_names,