fix(core): Use atomic writes for session files to prevent data loss on crash

[jflam]

Problem

ChatRecordingService.writeConversation() currently uses fs.writeFileSync() directly:

fs.writeFileSync(this.conversationFile, newContent);

This is vulnerable to data loss if the process crashes or is killed mid-write - the session file could be left truncated or corrupted, losing the user's chat history.

Solution

Replace fs.writeFileSync() with write-file-atomic, which:

1. Writes to a temporary file first
2. Calls fsync() to ensure data is flushed to disk
3. Atomically renames the temp file to the target path

This guarantees that session files are always in a valid state - readers see either the previous complete version or the new complete version, never a partial write.

Changes

• Add write-file-atomic dependency to packages/core/package.json
• Add @types/write-file-atomic to devDependencies for TypeScript support
• Update ChatRecordingService.writeConversation() to use writeFileAtomic.sync()

Trade-offs

The performance overhead is minimal for session files (~10-50KB written once per message). write-file-atomic is widely used (40M+ weekly npm downloads) and is the standard solution for safe file writes in Node.js.

Testing

• Built and ran Gemini CLI locally
• Verified session files are created and updated correctly
• Confirmed JSON files remain valid after multiple chat interactions

[gemini-code-assist[bot]]

Summary of Changes

Hello @jflam, I'm Gemini Code Assist[^1]! 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 vulnerability where session files could become corrupted or truncated if the application crashed during a write operation. By integrating the write-file-atomic library, the system now performs atomic writes, guaranteeing that session files are always in a valid state. This enhancement prevents data loss and ensures a consistent user chat history, improving the overall robustness of the application.

Highlights

• Atomic Writes Implementation: Implemented atomic writes for session files to prevent data loss and corruption during unexpected application crashes or interruptions.
• New Dependency: write-file-atomic: Introduced the write-file-atomic package and its corresponding TypeScript type definitions (@types/write-file-atomic) as new dependencies to facilitate safe file operations.
• ChatRecordingService Update: Modified the ChatRecordingService.writeConversation() method to utilize writeFileAtomic.sync() instead of fs.writeFileSync(), ensuring data integrity and consistency for conversation history.

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