aws configure set s3.* can raise UnboundLocalError when updating nested config blocks

[fallintoplace]

Describe the bug

aws configure set s3.signature_version s3v4 can crash when the target config already has a nested s3 = block and the next line is not another option line.

I reproduced this with all of the following existing config shapes:

• an empty nested block
• a comment immediately after s3 =
• an immediate next section header after s3 =

The crash comes from ConfigFileWriter._update_subattributes() reading current_indent even when OPTION_REGEX did not match the current line.

Regression Issue

This does not look like a recent regression. git blame shows _update_subattributes() was introduced with this behavior in 9c2fd8098a.

Expected Behavior

Updating a nested config value should append or update the nested key without crashing.

For example, starting from:

[default]
s3 =
# comment
    addressing_style = path

setting s3.signature_version should produce:

[default]
s3 =
# comment
    addressing_style = path
    signature_version = s3v4

Current Behavior

An uncaught traceback is raised:

Traceback (most recent call last):
  File "/Users/hoangvu/Code/OSS/aws-cli/awscli/customizations/configure/writer.py", line 115, in update_config
    self._update_section_contents(contents, section_name, new_values)
  File "/Users/hoangvu/Code/OSS/aws-cli/awscli/customizations/configure/writer.py", line 203, in _update_section_contents
    j = self._update_subattributes(
  File "/Users/hoangvu/Code/OSS/aws-cli/awscli/customizations/configure/writer.py", line 231, in _update_subattributes
    if starting_indent == current_indent or \
                          ^^^^^^^^^^^^^^
UnboundLocalError: cannot access local variable 'current_indent' where it is not associated with a value

Reproduction Steps

Minimal reproducer:

from awscli.customizations.configure.writer import ConfigFileWriter
import os
import tempfile

fd, path = tempfile.mkstemp()
os.close(fd)

with open(path, "w") as f:
    f.write("[default]\ns3 =\n# comment\n    addressing_style = path\n")

ConfigFileWriter().update_config(
    {"__section__": "default", "s3": {"signature_version": "s3v4"}},
    path,
)

This also reproduces with:

[default]
s3 =

and:

[default]
s3 =
[profile other]
region = us-east-1

Possible Solution

Handle non-option lines before comparing indentation, or initialize current_indent safely.

For example, _update_subattributes() could skip lines where OPTION_REGEX does not match, while still treating a new section header as the boundary for inserting new nested values.

It would also be helpful to add regression tests in tests/unit/customizations/configure/test_writer.py for:

• empty nested blocks
• comments after s3 =
• section headers immediately after s3 =

Additional Information/Context

I reproduced this against the current develop checkout:

• aws-cli/1.45.14
• develop @ b698a6a77
• macOS 26.5 arm64
• Python 3.13.2

The user-facing path appears to be aws configure set s3.signature_version ..., but the minimal reproducer above avoids unrelated environment/setup dependencies.

CLI version used

aws-cli/1.45.14

Environment details (OS name and version, etc.)

macOS 26.5 arm64, Python 3.13.2