Blocking Synchronous I/O on the Main Thread Causes UI Stutter

[Daksh7785]

Performance: Replace blocking filesystem operations in shell.execute with asynchronous APIs

Category: Performance
Priority: High

Affected Components

• packages/core/src/tools/shell.ts
• packages/cli (React Ink UI)

Summary

The shell tool performs synchronous filesystem operations (fs.mkdtempSync) on the main Node.js event loop immediately before spawning shell processes. Since the CLI UI is rendered using React Ink on the same event loop, these synchronous operations can briefly block rendering, resulting in input latency, dropped frames, and less responsive streaming output.

Current Behavior

The temporary directory used for shell execution is created synchronously:

tempDir = fs.mkdtempSync(path.join(os.tmpdir(), 'gemini-shell-'));

Although this operation is typically fast, it can become noticeably slower on busy systems, network-mounted filesystems (e.g. WSL/NFS), or under heavy concurrent tool execution.

Expected Behavior

Filesystem operations in the shell execution path should be non-blocking so the React Ink UI remains responsive during tool execution and streaming.

Steps to Reproduce

1. Run multiple shell tool invocations in quick succession.
2. While commands are executing, type into the CLI input.
3. Observe occasional input lag or UI stuttering.

Impact

• Reduced UI responsiveness during shell execution.
• Increased input latency while the agent is running commands.
• Brief interruptions in streamed terminal output.

Suggested Solution

Replace synchronous filesystem operations in the shell execution path with their asynchronous equivalents.

A minimal implementation would:

• Replace fs.mkdtempSync(...) with await fsPromises.mkdtemp(...).
• Audit the same execution path for other *Sync filesystem calls and replace them where appropriate.

Since execute() is already asynchronous, this change should integrate naturally without affecting behavior.

Testing

• Verify all existing shell execution tests continue to pass.
• Add or update performance tests to ensure concurrent shell executions do not introduce unnecessary event loop blocking.
• Manually verify the CLI remains responsive while multiple shell commands are executing.

Acceptance Criteria

• No synchronous filesystem operations remain in shell.execute.
• Existing functionality is preserved.
• CLI responsiveness is maintained during concurrent shell executions.
• All related tests pass.

[gemini-cli]

Effort Analysis:
The ShellToolInvocation.execute method in packages/core/src/tools/shell.ts currently uses fs.mkdtempSync (L501) and fs.existsSync/fs.statSync (L918-927) on the main Node.js event loop. Additionally, ShellExecutionService.background in packages/core/src/services/shellExecutionService.ts uses mkdirSync (L1467). These synchronous calls block the event loop, causing UI stuttering in the React Ink-based CLI. Resolving this requires refactoring these methods to use fsPromises, which involves changing ShellExecutionService.background from a synchronous to an asynchronous signature. This change has a cross-package impact, affecting the useExecutionLifecycle hook in packages/cli/src/ui/hooks/useExecutionLifecycle.ts (L210), and requires significant updates to the test suites in packages/core/src/tools/shell.test.ts and packages/core/src/services/shellExecutionService.test.ts. The architectural complexity of transitioning a core service to async and the resulting ripple effect on both the UI package and the testing infra map to effort/medium.