Fix newline insertion bug in replace tool

[werdnum]

Fix issue where Gemini would sometimes "pad" blocks of code by adding newlines between every line.

Summary

Restrict indentation capture to horizontal whitespace in calculateRegexReplacement and calculateFlexibleReplacement to prevent extra newlines from being inserted when the target block is preceded by blank lines.

Details

Related Issues

Fixes #18591

How to Validate

Give Gemini the following instructions:

1. Setup: Create a file named repro.py with exactly one leading blank line followed by a two-line function. Run this command:
   printf "\ndef foo():\n print("hello")\n" > repro.py
2. Trigger: Use the replace tool with an old_string that contains an intentional whitespace error (two spaces after def) to force a flexible/regex match. It must be a multi-line replacement to see the bug:
   • old_string: "def foo():\n print("hello")"
   • new_string: "def bar():\n print("hello")"
3. Verify: Read the file back.
   • Fixed behavior: The file should contain exactly one blank line at the top, followed by the two lines of the function.
   • Buggy behavior: An extra blank line will appear between def bar(): and print("hello") because the tool incorrectly includes the leading \n in its indentation capture.

Pre-Merge Checklist

• Updated relevant documentation and README (if needed)
• Added/updated tests (if needed)
• Noted breaking changes (if any)
• Validated on required platforms/methods:
  • MacOS
    • npm run
    • npx
    • Docker
    • Podman
    • Seatbelt
  • Windows
    • npm run
    • npx
    • Docker
  • Linux
    • npm run
    • npx
    • Docker

[gemini-code-assist]

Summary of Changes

Hello @werdnum, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a critical formatting bug within the replace tool that led to the unintended insertion of extra newlines in code blocks. By refining the indentation detection mechanism to strictly account for horizontal whitespace, the change ensures that code formatting remains consistent and free from erroneous blank lines, significantly improving the reliability of code modifications performed by the tool.

Highlights

• Bug Fix: Resolved an issue in the replace tool where it would incorrectly insert extra newlines, particularly when processing code blocks preceded by blank lines. This bug caused 'padding' of code blocks with unnecessary newlines.
• Indentation Logic Refinement: The core fix involves modifying the regular expressions used in calculateRegexReplacement and calculateFlexibleReplacement to capture only horizontal whitespace (spaces and tabs: [ \t]) for indentation. Previously, it captured all whitespace (\s), which incorrectly included newlines in the indentation calculation.
• Test Coverage: Added two new regression tests to edit.test.ts to ensure that the newline insertion bug does not reoccur and that the replacement logic correctly handles blocks preceded by blank lines or when the old_string itself starts with a blank line.

Changelog

• packages/core/src/tools/edit.test.ts
  • Added a new test case to verify that extra newlines are not inserted when replacing a code block preceded by a blank line.
  • Added another test case to confirm that flexible replacements correctly handle old_string values that start with a blank line without introducing additional newlines.
• packages/core/src/tools/edit.ts
  • Modified the regular expression in calculateFlexibleReplacement to capture only horizontal whitespace ([ \t]) for indentation, preventing newlines from being included in the indentation calculation.
  • Updated the regular expression in calculateRegexReplacement to similarly restrict indentation capture to horizontal whitespace ([ \t]), resolving the newline insertion bug for regex-based replacements.

Activity

• Updated relevant documentation and README (if needed)
• Added/updated tests (if needed)
• Noted breaking changes (if any)
• Validated on Linux (npm run)

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[gemini-code-assist]

Code Review

This pull request effectively resolves a bug that caused the replace tool to insert extra newlines by modifying the regular expressions in calculateFlexibleReplacement and calculateRegexReplacement to only capture horizontal whitespace for indentation. While the fix is correct and regression tests are well-written, a high-severity security vulnerability was identified in the calculateRegexReplacement function: it lacks proper escaping for the $ character in the replacement string, making it susceptible to replacement pattern injection. This vulnerability should be addressed to ensure the tool's robustness against malicious or unintended input.