Start background git by full path so it doesn't fork the whole app by zcy22606 · Pull Request #1030 · hardbeat920/monocode

zcy22606 · 2026-10-16T14:10:39Z

What changed

New harness::command_on_path(name, path): on Unix it looks name up on path with the existing which_in_path, builds the Command from the full path it finds (falling back to the bare name), and sets PATH as before. On Windows it keeps the bare name.
New harness::background_command_on_path(name, path) for the git helpers that 0.14.0 put behind background_work::configure_child. It keeps that policy (nice 10, plus IOPOL_THROTTLE on macOS, inherited by git-lfs, hooks and filters) but applies it with wrapper programs instead of a pre_exec hook:
- macOS: /usr/bin/nice -n 10 /usr/sbin/taskpolicy -d throttle /full/path/to/git …
- other Unix: nice -n 10 /full/path/to/git …
- If nice isn't on the PATH it starts git directly at normal priority (and on macOS skips taskpolicy if it's missing); it never falls back to pre_exec. Windows is unchanged.
The background git helpers use these instead of Command::new("git") + .env("PATH", …):
- fs::git_cmd / git_cmd_for_args_with_path (via a small git_cmd_on(path)), and the scratch-index run in git_selected_context_for: background_command_on_path, so they keep configure_child's priority
- worktrees::git, checkpoint::turn::git_command: command_on_path (they had no priority change before, so none now)
background_work::configure_child itself is unchanged; the harness spawns still use it (see "Not in this PR").

Why

Rust's std only uses posix_spawn on Unix when nothing forces it back to fork + exec (library/std/src/sys/process/unix/unix.rs):

if self.get_gid().is_some()
    || self.get_uid().is_some()
    || (self.env_saw_path() && !self.program_is_path())
    || !self.get_closures().is_empty()
    || self.get_groups().is_some()
    || self.get_chroot().is_some()
{
    return Ok(None); // fall back to fork
}

The git helpers hit two of these. Before 0.14.0 it was the bare git plus .env("PATH", gui_search_path()). Since 0.14.0 git_cmd also goes through configure_child, whose pre_exec closure makes get_closures() non-empty, so even a full path would still fork.

On macOS, fork runs libSystem_atfork_prepare, which takes the malloc locks for the whole process while the address space is copied. Until it finishes, any thread that allocates waits, the UI main thread included. The bigger the app's memory map, the longer that takes. These helpers run constantly while agents work: the file tree's list_dir asks git check-ignore per directory, and the Changes / worktree / checkpoint code refreshes after every tool call. In a 120-second sample of the app process (macOS 26, several agents running tools), the main thread spent 1,211 samples parked in _xzm_fork_lock_wait, with tokio workers sitting in:

monocode_lib::fs::list_dir
  std::process::Command::spawn
    fork
      libSystem_atfork_prepare
        _os_unfair_lock_lock_slow

To the user that's typing and hover freezing for 0.5–1 s at a time while an agent is busy, getting worse the longer the app has been open.

nice and taskpolicy give the same policy as the pre_exec hook, and the Command std sees is a full path with no closure, so it takes posix_spawn. Both exec in place, so the pid std returns is git's, and git-lfs / hooks inherit the policy. taskpolicy -d throttle is setiopolicy_np(IOPOL_TYPE_DISK, IOPOL_SCOPE_PROCESS, IOPOL_THROTTLE) (man taskpolicy), the same call configure_child makes.

Measurement

A standalone program (not in the repo) allocates and touches ~2 GB in 250k small malloc blocks plus 16k separate mappings. One thread allocates and frees in a loop (a ring of 4,096 live blocks, 16 B to 512 KB) and records how long each allocation takes. Another starts git --version 300 times. Apple M3 Max, macOS 27.0.1, the machine otherwise busy (load average ~7–10):

how git is started	std path	longest malloc wait	worst wait per spawn, p50 / p99	`spawn()` p50 / p99	spawn to exit p50 / p99
bare `git` + `PATH` (before 0.14.0)	fork	53 ms	45 / 53 ms	50 / 59 ms	58 / 69 ms
full path + `configure_child`'s `pre_exec` (0.14.0)	fork	97 ms	47 / 54 ms	52 / 62 ms	60 / 71 ms
full path, no hook	posix_spawn	0.07 ms	0 / 0 ms	0.21 / 0.33 ms	7.9 / 9.6 ms
`nice -n 10 taskpolicy -d throttle` + full path (this PR)	posix_spawn	< 0.05 ms	0 / 0 ms	0.21 / 0.36 ms	10.3 / 12.5 ms

Every forking spawn blocked the allocating thread for about as long as the fork itself took; none of the posix_spawn ones did. The two wrapper execs add about 2 ms per git run, off the UI thread. The real app is bigger than this test, which is why the waits there reach 0.5–1 s.

I also considered taskpolicy -b (darwin background), a single wrapper. It is much stronger than nice 10: on this busy machine git --version took 40 / 121 ms (p50 / p99) under it, git status on this repo took 2.2× as long, and an 8-process CPU loop took 41 s instead of 1.1 s. That would delay Changes refreshes, so I kept the policy configure_child already chose.

Linux

glibc's fork also takes the malloc arena locks around _Fork (__malloc_fork_lock_parent), so the same stall exists there, though copying page tables is cheaper than a macOS fork. I haven't measured it. nice -n 10 sets the same value configure_child's setpriority sets there, and on CFQ/BFQ the IO priority follows the nice value as before. One small difference everywhere: nice -n 10 adds 10, while configure_child uses max(current, 10). They only differ if the app itself is already niced.

Not in this PR

harness::isolate_child also calls configure_child, so harness spawns (starting a session, --version / --help probes, harness_exec, MCP listing, the login-shell printenv) fork too since 0.14.0. They happen about once per session rather than after every tool call. Wrapping them would turn spawn errors into exit codes: a missing binary would show up as nice exiting 127 instead of Failed to start …, and spawn_retrying_text_file_busy on Linux would no longer see ETXTBSY. So I left them for a separate change. Happy to do that as a follow-up if you want them off pre_exec too.

UI

No visual change.

Validation

background_work::tests::scheduling_policy_reaches_child_and_descendant now also runs the probe through background_command_on_path: the child sees nice ≥ 10 and IOPOL_THROTTLE, and a grandchild sees nice ≥ 10.
New background_command_wraps_the_full_path_in_nice: the program is an absolute nice, and the args are -n 10 /usr/sbin/taskpolicy -d throttle <full git path> on macOS (-n 10 <full git path> elsewhere).
which_in_path_takes_the_first_executable_hit also checks command_on_path: a hit gives the full path as the program, a miss keeps the bare name.
read_only_git_cmd_path_adds_fixed_dirs_to_inherited_path also asserts git_cmd()'s program is absolute.
With the fix reverted (git_cmd back to bare git, background_command_on_path returning command_on_path), the three runnable tests above fail.
cargo fmt --check, cargo clippy --workspace --all-targets -- -D warnings, cargo test: 667 passed, 0 failed, 1 ignored (macOS).

Checklist

I ran npm run check (check:rust; no web files changed)
This PR is small and focused
I did not mix unrelated changes

Summary by CodeRabbit

Bug Fixes
- Git commands now locate Git through the application’s configured search paths, including when Git is not available through the default system path.
- Repository-changing operations such as commits, pushes, pulls, fetches, and clones continue to use the login-shell search path, supporting installations where Git is available there but not through the default system path.

coderabbitai · 2026-10-16T14:11:15Z

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📝 Walkthrough

Git subprocess construction now uses helpers that resolve executables against supplied GUI search paths. Background commands can use platform-specific scheduling tools. Git call sites select cached or login-shell GUI paths.

Changes: Git command PATH resolution

Layer / File(s)	Summary
Resolve and wrap commands using a supplied PATH `src-tauri/src/harness.rs`, `src-tauri/src/background_work.rs`	`command_on_path` resolves executables on Unix and sets the child PATH. `background_command_on_path` can wrap commands with `nice` and, on macOS, `taskpolicy`. Tests cover command resolution, wrapper arguments, and successful policy probes.
Use selected GUI paths for Git commands `src-tauri/src/checkpoint/turn.rs`, `src-tauri/src/fs.rs`, `src-tauri/src/worktrees.rs`	Git call sites use the command helpers with cached or login-shell GUI paths. The turn command retains `GIT_LITERAL_PATHSPECS`, and the worktree helper retains its other environment settings. A test asserts that the Git executable path is absolute.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~12 minutes

Change: Bug fix

Sequence Diagram(s)

sequenceDiagram
  participant Caller
  participant background_command_on_path
  participant nice
  participant Target
  Caller->>background_command_on_path: supply target name and PATH
  background_command_on_path->>nice: run target with priority 10
  nice->>Target: launch resolved executable

Suggested reviewers: hardbeat920

Merge Risk: 🔵 Low · up to 23aa9

An unusual PATH configuration could cause Git commands to use a different executable than PATH search would select. This is a bounded risk to resolve or explicitly accept before merging.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 47.06% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 17 functions across 5 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Title check		The title clearly describes the main change: starting background Git by full path to avoid forking the application.
Description check		The description includes all required sections, explains what changed and why, states that there is no UI change, and includes the checklist and validation results.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

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Actionable comments posted: 1

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Inline comments:
Review comments at @src-tauri/src/harness.rs:
- Line 2795: Update command_on_path’s executable lookup so empty PATH components
are treated as the current directory when calling which_in_path, preserving Unix
search order; keep the child process’s PATH unchanged.

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zcy22606 · 2026-10-16T16:04:01Z

Before/after from daily use. Same app (my fork carries this change), same machine, same kind of load: several agent sessions running tools, 120-second sample <app pid> 120 1 of the app process each time.

	before	after
main-thread samples in `_xzm_fork_lock_wait`	1,211	0
`fork` / `libSystem_atfork_prepare` frames	present (`fs::list_dir` → `Command::spawn` → `fork`)	none
how git children start	`fork`	`posix_spawn`

The git helpers were busy in the "after" run too (git_branches_for, git_diff_stats_for, git_diff_index_with, worktrees::git, checkpoint diffs), all through posix_spawn. The typing and hover freezes while agents work are gone.

With PATH overridden and a bare program name, std falls back from posix_spawn to fork + exec on Unix. So does any pre_exec hook, which 0.14.0's background_work::configure_child added to git_cmd. On macOS fork takes the malloc locks for the whole process while the address space is copied, so every thread that allocates waits, the UI main thread included. The git helpers behind the file tree, Changes, worktrees and checkpoints all spawned that way and run after every agent tool call, freezing typing and hover for up to a second. command_on_path resolves the program on that PATH first, so std can use posix_spawn. Windows keeps the bare name. For the git helpers that configure_child covered, background_command_on_path keeps its policy (nice 10, IOPOL_THROTTLE on macOS, inherited by git-lfs and hooks) by starting git under `nice -n 10` and `/usr/sbin/taskpolicy -d throttle` instead of a pre_exec hook.

zcy22606 · 2026-10-18T07:51:56Z

Rebased on 0.14.0. One thing worth knowing: background_work::configure_child sets its priority in a pre_exec closure, and any pre_exec closure sends std back to fork (!self.get_closures().is_empty() in the same check as the bare-name-plus-PATH case). So once git_cmd went through it, git forked again even with a full path, and the malloc-lock stall came back.

I kept your policy and changed how it's applied: git_cmd now starts nice -n 10 /usr/sbin/taskpolicy -d throttle /full/path/to/git (just nice -n 10 off macOS). That's the same nice 10 + process-wide IOPOL_THROTTLE, git-lfs and hooks still inherit it, and std sees a full path with no closure, so it uses posix_spawn. Your probe test now runs through both paths.

Standalone test, ~2 GB process, one thread allocating in a loop while another starts git --version 300 times:

	std path	longest malloc wait	`spawn()` p50
bare `git` + `PATH`	fork	53 ms	50 ms
full path + `pre_exec`	fork	97 ms	52 ms
`nice` + `taskpolicy -d throttle` + full path	posix_spawn	< 0.05 ms	0.21 ms

I tried taskpolicy -b too, but darwin background is much stronger than nice 10 (git status took 2.2× as long on a busy machine), so I didn't use it. The harness spawns still use configure_child. They're once per session, and wrapping them changes spawn error handling, so I left them out (details in the description). I can follow up on them if you'd like.

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Actionable comments posted: 1

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src-tauri/src/harness.rs-2811-2811: 1

2811-2811: 🎯 Functional Correctness | 🟡 Minor | ⚡ Quick win

⚠️ Unverified finding
Verification ran but could not confirm this finding. It is shown for review, not as a verified issue.

Make a resolved executable path absolute before constructing the command.

If PATH contains a relative directory such as bin, which_in_path can find bin/git in the app’s working directory. If a caller then sets a different current_dir, the child can resolve bin/git in that directory instead. background_command_on_path also passes the relative path to nice. Make resolved paths absolute without changing the child’s PATH or the policy for empty PATH components. Rust documents relative program paths combined with current_dir as platform-specific and unstable. (doc.rust-lang.org)

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Inline comments:
Review comments at @src-tauri/src/fs.rs:
- Line 9403: Gate the absolute-path assertion in the test using git_cmd with a
Unix-only configuration, since Windows intentionally uses the bare “git”
command. Preserve the existing assertion on Unix.

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Comment thread src-tauri/src/harness.rs

zcy22606 force-pushed the fix/git-spawn-full-path branch from 161e4a9 to b411e34 Compare October 18, 2026 07:51

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Comment thread src-tauri/src/fs.rs

Check git_cmd's full path only on Unix, where std would otherwise fork

23aa967

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Start background git by full path so it doesn't fork the whole app - #1030

Start background git by full path so it doesn't fork the whole app#1030
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zcy22606:fix/git-spawn-full-path

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