[wan9chi]

Motivation

A task that creates a symbolic link doesn't have it among its outputs unless an output glob matches it. Nitro's dependency tracer, for example, links .output/server/node_modules/tslib into .nf3. fspy records nothing for symlink, so a hit leaves the link out. When the task checks the path first, as nf3 does, the link is a plain input and a clean checkout misses.

Changes

fspy records the path of a new link as a write, before the call:

• Unix preload: symlink and symlinkat.
• Linux seccomp supervisor: the same calls for static binaries.
• Windows: NtFsControlFile with FSCTL_SET_REPARSE_POINT(_EX), which covers CreateSymbolicLinkW and junctions.

vt needs no change: a written symlink counts as a file, and the archive stores links as links (#885).

E2E cases: symlink_in_output (synthetic, vtt symlink) and nitro_auto_outputs_restore_links (the nitro_build_cache Nitro build with automatic tracking only). Both run only on Unix in the first commit, because Windows already behaves differently there, and on every platform after.

Part of a stack that lets automatic tracking cache tasks which read their own outputs and list the folders holding them, so they also hit on a clean checkout.

🤖 Generated with Claude Code

---

Note

Medium Risk
Changes core file-access tracing on all platforms (new syscalls/NT detour); mistakes could mis-classify I/O or miss links, affecting cache correctness for builds that rely on symlinks.

Overview
Automatically tracked outputs now include symlinks a task creates, so cache hits restore them (including Nitro-style dependency links under .output) without matching output globs, and a clean checkout can still hit when the task stat’d the link path first.

fspy records the new link path as a write before the syscall returns: Unix symlink / symlinkat (preload + seccomp), and on Windows NtFsControlFile when FSCTL_SET_REPARSE_POINT(_EX) runs (symbolic links and junctions). Link targets are not tracked. rebuildable documents that Windows junctions count as written symlinks, not empty mkdir folders.

Adds unit/integration tests and e2e fixtures (symlink_in_output, Nitro build-auto). vtt symlink works on Windows for tests; changelog entry for #918.

^{Reviewed by Cursor Bugbot for commit 93cb131. Bugbot is set up for automated code reviews on this repo. Configure here.}

[github-actions]

fspy benchmark

linux

dynamic/launch             change  +0.58%  [ -3.65% ..  +6.69%]  overhead   +60.37%
dynamic/access             change  +0.80%  [ -6.08% ..  +6.04%]  overhead   +19.83%
dynamic/access-relative    change  +1.17%  [ -3.90% ..  +6.27%]  overhead   +73.46%
dynamic/access-contended   change  +0.23%  [ -9.09% ..  +7.13%]  overhead   +51.32%
static/launch              change  +1.47%  [ -2.41% ..  +6.27%]  overhead  +200.38%
static/access              change  +0.26%  [ -1.67% ..  +1.98%]  overhead +1455.99%
static/access-relative     change  +0.75%  [ -0.15% ..  +2.64%]  overhead +1940.17%
static/access-contended    change  +0.45%  [ -1.84% ..  +1.55%]  overhead +2121.24%

macos

dynamic/launch             change  +1.10%  [ -3.16% ..  +4.91%]  overhead  +235.23%
dynamic/access             change  +0.71%  [ -4.18% ..  +4.01%]  overhead    +3.28%
dynamic/access-relative    change  -3.31%  [ -5.14% ..  -1.32%]  overhead  +246.94%
dynamic/access-contended   change  -0.18%  [ -5.49% ..  +3.70%]  overhead    +5.02%

windows

dynamic/launch             change  -0.06%  [ -5.14% ..  +5.17%]  overhead   +27.51%
dynamic/access             change  +0.56%  [ -0.56% ..  +1.50%]  overhead    +1.50%
dynamic/access-relative    change  +0.00%  [ -1.09% ..  +1.09%]  overhead    +0.91%
dynamic/access-contended   change  +0.18%  [ -0.53% ..  +1.07%]  overhead    +1.09%