transpiler cache: require an absolute XDG_CACHE_HOME and HOME

[robobun]

Problem

• really_get_cache_dir (src/jsc/RuntimeTranspilerCache.rs:636) joins the value of XDG_CACHE_HOME or HOME onto the top level directory. A value that is not absolute resolves against the working directory, so the runtime transpiler cache is written inside the project.
• Verified on 1.4.3-canary.1 (f42e980): XDG_CACHE_HOME= bun ./big.ts creates ./bun/@t@/<hash>.pile, and XDG_CACHE_HOME=relxdg bun ./big.ts creates ./relxdg/bun/@t@/. An empty HOME writes ./.bun/install/cache/@t@.
• env_var::XDG_CACHE_HOME.get() returns Some("") for an empty value, so an empty value counts as set.

Fix

• Each of the three arms (XDG_CACHE_HOME, HOME on macOS, HOME) takes the value only when bun_paths::is_absolute accepts it, and falls through otherwise.
• The XDG base directory specification requires an absolute path and says to ignore a relative one. An empty value is not absolute, so it reads as unset. Nothing changes for an absolute value.
• A cache entry is executed as code, so the directory has to stay per-user. The working directory is shared with whoever can write the project.
• Verified: test/cli/run/transpiler-cache.test.ts, four new cases (an empty and a relative value for each of XDG_CACHE_HOME and HOME), all four fail on the released bun. The full file passes. Also ran bun-pm, npmrc, patch, bunx, bun-add and the global directory subset of bun-install-registry.

Background

• The runtime transpiler cache stores transpiled output for a file of 4 KB or more in a @t@ directory, keyed by a content hash. A later run with the same hash loads that output instead of parsing the file.
• really_get_cache_dir picks the directory once per thread, from the first candidate that answers: BUN_RUNTIME_TRANSPILER_CACHE_PATH, XDG_CACHE_HOME, HOME. It returns 0 to mean the cache is disabled.
• join_abs_string_buf_z(top, [value, ...]) is bun's path.resolve. An absolute part replaces the base. A relative part is appended to it, which is how the value ended up under the working directory.

Notes

Found while checking a report about environment derived paths. The same report named several other sites, which two open PRs already cover, so this PR is only the three arms above.

• Resolve relative and empty directory variables against the working directory instead of the filesystem root #39781 fixes the sites that pass an environment value as the join base. That is a different defect: join_abs_string_buf requires an absolute base, and with a relative one the POSIX branch prefixes / and normalizes from the second byte, so the first byte of the value is lost. On 1.4.3-canary.1 BUN_INSTALL=.bun bun pm bin -g resolves the global install directory to /bun/install/global, BUN_INSTALL=relbun gives /elbun/install/global, and XDG_CACHE_HOME=relxdg gives /elxdg/.bun/install/global. bun add -g creates that directory. That PR does not touch RuntimeTranspilerCache.rs.
• Transpiler cache: fall back to a private per-user temp directory when HOME is unset #40705 rewrites really_get_cache_dir to add a <tmpdir>/bun-<euid>/@t@ fallback when no per-user directory is available. It leaves the XDG_CACHE_HOME and HOME arms as they are, so it needs a rebase against this change, or this change folded in.

HOME is USERPROFILE on Windows (env_var::HOME), so both tests set both names. The macOS arm uses $HOME/Library/Caches/bun/@t@, so the test branches on isMacOS.

One unrelated flake during the runs: defines are part of the cache key > --drop invalidates cache timed out at its 5 s limit on a loaded machine, and passed on the next run.

---

no test proof · iteration 0 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/cli/run/transpiler-cache.test.ts

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• src/jsc/RuntimeTranspilerCache.rs
• test/cli/run/transpiler-cache.test.ts

Walkthrough

The cache directory resolver now ignores empty and relative XDG_CACHE_HOME and HOME values. Tests verify fallback to platform-appropriate absolute cache paths and prevent project-relative cache directories.

Changes

Cache path validation

Layer / File(s)	Summary
Runtime cache path validation src/jsc/RuntimeTranspilerCache.rs	XDG_CACHE_HOME and HOME are used only when they are absolute paths. Invalid values continue through fallback logic or disable caching.
Fallback path test coverage test/cli/run/transpiler-cache.test.ts	Parameterized tests cover empty and relative XDG_CACHE_HOME values on macOS and non-macOS platforms. The tests verify absolute HOME fallbacks and prevent project-relative cache directories.

Suggested reviewers: jarred-sumner, dylan-conway

Merge Risk: 🔵 Low · up to de53a

This change prevents invalid cache environment paths from creating project-local transpiler caches. The new HOME fallback behavior is not covered for invalid HOME values, leaving a bounded regression risk before merge.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly identifies the main change: requiring absolute XDG_CACHE_HOME and HOME values for the transpiler cache.
Description check	✅ Passed	The description explains the problem, fix, scope, background, and verification steps. It does not use the exact template headings, but it provides the required information and is complete.

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[robobun]

^{Updated 10:38 PM PT - Sep 6th, 2026}

❌ @robobun, your commit 56851c9 has 5 failures in Build #111955 (All Failures):

• test/js/node/dgram/node-dgram.test.js - 1 failing in the parallel batch on 🍎 any x64
• test/js/node/net/node-net-server.test.ts - 2 failing in the parallel batch on 🍎 any aarch64
• test/js/third_party/prompts/prompts.test.ts - 1 failing in the parallel batch on 🪟 2019 x64
• test/js/node/test/parallel/test-crypto-dh-leak.js - code 1 on 🐧 13 x64-asan
• step failed outside runner - exit 1 on 🍎 any aarch64 - test-bun

---

🧪   To try this PR locally:

bunx bun-pr 41763

That installs a local version of the PR into your bun-41763 executable, so you can run:

bun-41763 --bun

[robobun]

Status

How I reproduced it, on canary d316760 (1.4.3-canary.1), in an empty directory:

printf 'export const x: number = 1;\n%s\nconsole.log(x);\n' "$(for i in $(seq 200); do echo '// padding padding padding padding'; done)" > big.ts
XDG_CACHE_HOME= bun ./big.ts && XDG_CACHE_HOME= bun ./big.ts
find . -type d            # -> ./bun, ./bun/@t@   (the cache is in the project)
XDG_CACHE_HOME=relxdg bun ./big.ts && XDG_CACHE_HOME=relxdg bun ./big.ts
find . -type d            # -> ./relxdg/bun/@t@

The file has to be 4 KB or more, or the cache skips it.

With this change both runs write to $HOME/.bun/install/cache/@t@ instead, and nothing is created in the project. test/cli/run/transpiler-cache.test.ts covers an empty and a relative value for each of XDG_CACHE_HOME and HOME. All four new cases fail with the released bun and pass with the debug build.

CI: test/cli/run/transpiler-cache.test.ts is green on every lane that ran it in #111955: darwin x64 (24 pass, covers the isMacOS branch), Windows x64 and aarch64, alpine, debian. Both builds (#111853, #111955) are red for reasons outside this diff. One darwin aarch64 agent (biscuit) times out downloading the build artifacts before any test runs, and 50 of its last 50 jobs failed the same way, so one of the two darwin aarch64 shards never ran. The other red entries are unrelated tests in areas this diff does not touch: prompts.test.ts (Windows, timeout), node-net-server.test.ts (darwin aarch64), node-dgram.test.js and test-crypto-dh-leak.js (both also red on main), fetch-proxy-tls-intern-race.test.ts (darwin x64). This needs a maintainer to merge, or a rerun of the darwin aarch64 shard once that agent is out of rotation.

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Inline comments:
In `@test/cli/run/transpiler-cache.test.ts`:
- Around line 213-216: Add empty and relative HOME cases to the parameterized
transpiler-cache test alongside the existing XDG_CACHE_HOME cases, ensuring each
invalid HOME value exercises the RuntimeTranspilerCache HOME validation and
verifies that no project-relative cache directory is created.

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• src/jsc/RuntimeTranspilerCache.rs
• test/cli/run/transpiler-cache.test.ts

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[claude]

LGTM — the added HOME coverage in 88a90df rounds this out.

Checked that paths::is_absolute is the existing bun_paths helper already in scope, and that all three env-var arms (XDG, macOS HOME, generic HOME) are guarded symmetrically so no fall-through path can still reach join_abs_string_buf_z with a relative first part.
Verified the new HOME test's readdirSync(temp_dir)).toEqual(["a.js"]) can hold: beforeEach only assigns cache_dir without creating it, and BUN_RUNTIME_TRANSPILER_CACHE_PATH is unset, so nothing else writes into temp_dir.
Confirmed bunRun sets cwd to dirname(file) = temp_dir, so the negative assertions (bun/, relxdg/ absent) are checking the directory the unfixed build actually wrote into; both HOME and USERPROFILE are set so the Windows env_var::HOME alias is covered.

Extended reasoning...

Overview

The PR adds .filter(|d| paths::is_absolute(d)) to the three environment-variable reads in RuntimeTranspilerCache::really_get_cache_dir (XDG_CACHE_HOME, macOS HOME, generic HOME), so a non-absolute value falls through to the next candidate instead of being resolved against the process cwd by join_abs_string_buf_z. Two test.each blocks in test/cli/run/transpiler-cache.test.ts cover empty and relative values for each variable: the XDG_CACHE_HOME case asserts fall-through to the HOME-derived cache location (branching on isMacOS) and that nothing is created in the project directory; the HOME case asserts the cache is disabled entirely and the temp dir contains only the source file. Since the previous push, commit 88a90df added the second test.each covering the HOME arm.

Security risks

The transpiler cache stores bytecode that is later executed, so its location must stay per-user. This change tightens the directory selection so a stray empty or relative env value can no longer redirect the cache into a project or shared directory. It is a strict hardening — no behavior changes for absolute values, and the failure mode for bad input is now "fall through / disable" rather than "write next to the project". No new attack surface is introduced; the BUN_RUNTIME_TRANSPILER_CACHE_PATH override arm above is untouched and remains an explicit user opt-in.

Level of scrutiny

Low-to-moderate. The Rust change is three identical one-line filters using an already-imported bun_paths helper, applied symmetrically to every arm that previously joined an env value under top_level_dir. The #[cfg(target_os = "macos")] branch is touched but with the exact same edit as the non-cfg arm, so per-target compilation risk is negligible. The XDG Base Directory spec explicitly says relative values should be ignored, so the semantics are uncontroversial.

Other factors

Tests follow the file's existing conventions (bunRun, dummyFile, env spread with overrides), set both HOME and USERPROFILE for Windows, and assert the negative contract (no stray directories in cwd) as well as the positive fall-through. I traced bunRun in test/harness.ts to confirm it spawns with cwd = dirname(file) and spreads the caller's env last, so the undefined overrides on BUN_RUNTIME_TRANSPILER_CACHE_PATH and XDG_CACHE_HOME take effect. The readdirSync(temp_dir).toEqual(["a.js"]) assertion is safe because beforeEach does not pre-create .cache. The PR description notes interaction with two open PRs (#39781, #40705); those are separate call sites and a follow-up rebase note, not a defect in this change.