DynamoDB: Attr-generated placeholders overwrite the caller's ExpressionAttributeValues

Describe the bug

ConditionExpressionBuilder restarts at #n0 / :v0 on every request (boto3/dynamodb/transform.py:172), and the generated placeholders are merged over the caller's with dict.update (boto3/dynamodb/transform.py:203-213):

if expr_attr_names_input in params:
    params[expr_attr_names_input].update(generated_names)

Any caller placeholder named #n<k> or :v<k> with k below the generated count is silently replaced. The request stays well-formed and is sent, writing data the caller never supplied. No exception is raised.

This is the root cause of #4032, which was closed with a workaround. The required naming isn't unusual; :v1 is enough.

Regression Issue

• Select this option if this issue appears to be a regression.

No. Present since the condition-expression transformation was introduced, and still on develop at ced31bb7.

Expected Behavior

Caller-supplied placeholders survive the merge, and generated ones take suffixes that are free. Failing that, an explicit error. A request must not write a value the caller did not supply.

Current Behavior

From #4032: val1 is set to "bar", a condition operand, instead of 321.

caller    ExpressionAttributeValues  {':v1': 321}
generated                            {':v0': 'foo', ':v1': 'bar'}   # is_in makes two
sent                                 {':v1': 'bar', ':v0': 'foo'}   # caller's :v1 gone
          UpdateExpression 'SET #f1=:v1'  ->  writes 'bar'

Names collide the same way via #n0, and reads are affected too: a query whose FilterExpression='#n0 > :v0' binds score / 10, combined with KeyConditionExpression=Key('id').eq('x'), is sent filtering id > 'x'.

The caller's own dictionaries are deep-copied first (boto3/dynamodb/transform.py:35-36), so nothing in the caller's process shows the substitution.

Reproduction Steps

No AWS account or network required:

import botocore.session
from boto3.dynamodb.conditions import Attr
from boto3.dynamodb.transform import TransformationInjector

model = botocore.session.get_session().get_service_model(
    'dynamodb').operation_model('UpdateItem')

params = {                                   # verbatim from #4032
    'TableName': 'test_table',
    'Key': {'test_key': 'foo-key'},
    'UpdateExpression': 'SET #f1=:v1',
    'ExpressionAttributeNames': {'#f1': 'val1'},
    'ExpressionAttributeValues': {':v1': 321},
    'ConditionExpression': Attr('filter_val').is_in(['foo', 'bar']),
}

TransformationInjector().inject_condition_expressions(params, model)
print(params['ExpressionAttributeValues'])
# {':v1': 'bar', ':v0': 'foo'}   -- the caller's :v1 = 321 is gone

The same arguments passed to table.update_item(...) against a real table reproduce it end to end, as reported in #4032.

Possible Solution

Reserve the caller's keys before the builder runs, and skip a taken suffix:

# boto3/dynamodb/conditions.py
def reset(self, reserved_names=(), reserved_values=()):
    self._name_count = 0
    self._value_count = 0
    self._reserved_names = set(reserved_names)
    self._reserved_values = set(reserved_values)

def _get_value_placeholder(self):
    value = f":{self._value_placeholder}{self._value_count}"
    while value in self._reserved_values:
        self._value_count += 1
        value = f":{self._value_placeholder}{self._value_count}"
    return value          # same shape for _get_name_placeholder

# boto3/dynamodb/transform.py:172
self._condition_builder.reset(
    params.get('ExpressionAttributeNames') or (),
    params.get('ExpressionAttributeValues') or (),
)

Rejecting the collision with an error is the alternative, but it breaks callers that work today by luck; skipping does not.

Additional Information/Context

• Affected code: boto3/dynamodb/transform.py:165-213 (:172 reset, :203-213 the two merges) and boto3/dynamodb/conditions.py:313-322 (placeholder generation).
• update_item has no Attr-based builder for UpdateExpression, so mixing a raw update expression with a generated condition is the normal shape for a conditional update — not an unusual usage.
• Related: #4032, closed with a workaround.
• Found with ESBMC bounded model checking over the merge; the counterexample collides on the first generated placeholder.

SDK version used

Reproduced on boto3 1.34.46 / botocore 1.34.46; code path unchanged on develop at ced31bb7.

Environment details (OS name and version, etc.)

Ubuntu 24.04.4 LTS, x86_64, CPython 3.12.3