fix(dynamodb): paren-aware comma split in parse_update_clauses

[Bowbaq]

Heads up — this PR was written by an AI agent (Claude Code) under my direction. I don't write Rust day-to-day, so I can't speak to the idiom-level quality of the diff off the top of my head. The test coverage and reasoning should stand on their own; happy to revise anything that doesn't fit the project's style.

Summary

UpdateItem with a list_append expression silently no-oped:

SET #0 = list_append(#0, :0)

The item's target attribute was left unchanged even though the call returned no error, and the attribute already existed as an empty list (so if_not_exists was not the issue). Confirmed broken in v0.7.1 and v0.9.0 against real DynamoDB.

Root cause

parse_update_clauses split each action clause's content on every comma naively:

let assignments = content.split(',').map(...).collect();

The expression #0 = list_append(#0, :0) was torn apart at the comma inside the function call, producing two bogus tokens:

1. #0 = list_append(#0 — sent to apply_set_list_append as rest = "#0" (no closing ))
2. :0) — treated as a separate malformed assignment, silently ignored

In apply_set_list_append, rest.strip_suffix(')') found no ) and returned early — no write, no error.

Fix

Replace the naive split(',') with the existing split_on_top_level_keyword(content, ",") helper, which is already paren-depth-aware and is used by split_on_and / split_on_or for exactly the same reason. Commas inside balanced parentheses are now left intact.

Test plan

Five new unit tests (all failing before the fix):

• test_parse_update_clauses_list_append_single_assignment — parse level: list_append(a, b) stays as 1 assignment, not 2
• test_parse_update_clauses_list_append_mixed_with_plain_set — parse level: outer comma still splits two SET assignments correctly
• test_list_append_into_empty_list — exact repro from the bug report (empty existing list)
• test_list_append_into_nonempty_list — append to a non-empty existing list
• test_list_append_combined_with_plain_set — list_append and plain SET in the same expression both apply
• cargo test -p fakecloud-dynamodb — 98 pass (was 93, +5 new)
• cargo clippy -p fakecloud-dynamodb -- -D warnings — clean
• cargo fmt --check — clean
• Not run: real-AWS behavioral parity. I don't have credentials configured for this. The fix matches AWS's documented UpdateExpression SET syntax for list_append, and the bug was purely in the comma tokeniser.

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Summary by cubic

Fixes parenthesis-aware splitting of SET clauses in fakecloud-dynamodb UpdateExpression parsing so list_append(#a, :b) applies correctly instead of no-oping.

• Bug Fixes

  • Replace naive comma split in parse_update_clauses with split_on_top_level_keyword; add tests for list_append parsing and execution, including mixed SETs and empty/non-empty list appends.

• Refactors

  • Run cargo fmt (style-only).

^{Written for commit 89f8716. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 1 file

[vieiralucas]

Thanks for the thorough debugging and the test coverage — root cause analysis is spot on, and reusing the existing split_on_top_level_keyword helper is exactly the right call. Merging now.