fix: prevent null pointer dereference in rb_tree delete_fixup

[zhangzhuang15]

Pull Request Template

Description

Prevent null pointer dereference in rb_tree delete_fixup.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I ran bellow commands using the latest version of rust nightly.
• I ran cargo clippy --all -- -D warnings just before my last commit and fixed any issue that was found.
• I ran cargo fmt just before my last commit.
• I ran cargo test just before my last commit and all tests passed.
• I added my algorithm to the corresponding mod.rs file within its own folder, and in any parent folder(s).
• I added my algorithm to DIRECTORY.md with the correct link.
• I checked COUNTRIBUTING.md and my code follows its guidelines.

Please make sure that if there is a test that takes too long to run ( > 300ms), you #[ignore] that or
try to optimize your code or make the test easier to run. We have this rule because we have hundreds of
tests to run; If each one of them took 300ms, we would have to wait for a long time.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.58%. Comparing base (21f5615) to head (e3b4034).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #952      +/-   ##
==========================================
+ Coverage   95.39%   95.58%   +0.19%     
==========================================
  Files         332      332              
  Lines       21496    21516      +20     
==========================================
+ Hits        20506    20566      +60     
+ Misses        990      950      -40     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[Copilot]

Pull request overview

This PR fixes a potential null pointer dereference in the delete_fixup function of the red-black tree implementation. The fix adds a null check for the parent pointer at the beginning of the fixup loop, preventing undefined behavior when deleting the root node in certain edge cases. The fix follows the same pattern already used in insert_fixup (line 248) and is validated by a new regression test.

Key changes:

• Added null pointer guard in delete_fixup loop to handle edge case where parent is null
• Added regression test that triggers the specific deletion sequence that would have caused the null pointer dereference

Comments suppressed due to low confidence (1)

src/data_structures/rb_tree.rs:382

• The loop invariant comment at line 379 states "node is not the root (so parent is not null)", but the newly added null check on line 372 contradicts this invariant. The comment should be updated to reflect that parent can indeed be null, which is the edge case this fix addresses.

        /*
         * Loop invariants:
         * - node is black (or null on first iteration)
         * - node is not the root (so parent is not null)
         * - All leaf paths going through parent and node have a
         *   black node count that is 1 lower than other leaf paths.
         */

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[siriak]

Looks good, thanks!