Agent host: clearer worktree git timeout errors and 60s budget

[roblourens]

Problem

A user reported createSession failing on an SSH remote with this misleading error in the side-effects log:

[AgentSideEffects] sendMessage failed for session=copilotcli:/537534a9...:
code=undefined,
message=Preparing worktree (new branch 'agents/vsckb-implement-please-investigate-...')
Updating files:   0% (7/14834) ... Updating files:   1% (149/14834)

The remote (a Mac Air) happened to be under heavy disk-I/O contention from many concurrent npm install runs in other worktrees, so git worktree add only managed 149 of 14,834 files in 30s and got killed by the timeout in AgentHostGitService.addWorktree.

Two underlying bugs surfaced:

1. Error reporting was misleading. In _runGit, the timeout path did reject(new Error(stderr || error.message)). When git was killed before producing any diagnostic stderr, the surfaced error was just the progress meter, with code=undefined and no hint of what really happened.
2. 30s is tight for worktree ops on a large repo when the remote is under load. Normally sub-second, but no headroom for transient I/O storms.

Changes

• All three worktree operations (addWorktree, addExistingWorktree, removeWorktree --force) now use a 60s timeout instead of 30s. All three can do real filesystem work proportional to the worktree's content, so they share the same bumping consistently rather than leaving removeWorktree at 30s.rationale

• _runGit: replaced execFile's built-in timeout option with our own setTimeout that flips a didTimeOut flag before calling child.kill(). The flag lets us definitively distinguish a timeout from other kill paths (rather than "killed && signal, likely a timeout"). The timer is cleared via child.on('exit').

• New exported helper formatGitError: classifies the failure (timeout / signal / exit code / other) and produces messages like:

  • git worktree timed out after 60000ms: Updating files: 0% (149/14834)
  • git worktree exited with code 128: fatal: invalid reference: missing-branch
  • spawn git ENOENT

  The underlying ExecFileException is preserved as Error.cause so downstream consumers can still inspect it.

• summarizeStderrForError helper: git progress output is full of \r-separated repaints (e.g. Updating files: 0% (7/14834)\rUpdating files: 0% (149/14834)\r). Stripping naively still left a noisy multi-line blob in the error. The helper now keeps only the last non-empty line and caps at 200 chars, so the error message is a clean one-liner.

• Full stderr is still logged. Since the thrown error message is summarized, _runGit now logs the complete unmodified stderr at warn level via ILogService on any git failure, so no diagnostic context is lost.

• Unit tests for formatGitError (four classification branches) and summarizeStderrForError (empty / whitespace / multi-line progress / normal one-liner / truncation).

After this change, the same failure would be surfaced as a one-liner that immediately says "timed out after diagnosable from the side-effects log without log archaeology, with the full raw stderr available in the agent-host log if deeper inspection is needed.60000ms"

(Written by Copilot)

[Copilot]

Pull request overview

This PR improves agent-host git worktree operations by increasing the timeout budget and making git timeout failures diagnosable (rather than surfacing git progress output as the error), with unit tests covering the new error formatting helper.

Changes:

• Increased git worktree operation timeouts from 30s to 60s.
• Reworked _runGit to use a custom timeout timer so timeouts can be reliably distinguished from other kill paths.
• Added formatGitError helper (exported for tests) plus unit tests covering timeout/signal/exit-code/fallback formatting.

Show a summary per file

File	Description
src/vs/platform/agentHost/node/agentHostGitService.ts	Increases worktree timeouts; adds custom timeout handling; introduces formatGitError and uses Error.cause to preserve the original ExecFileException.
src/vs/platform/agentHost/test/node/agentHostGitService.test.ts	Adds unit tests for formatGitError classification branches.

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 3