bake: resolve app.root against the cwd and require it to be a string

[robobun]

Problem

• Bun.serve({ development: true, app: { framework, root: "." } }) (or any other relative root) aborts debug builds at startup:

  panic: assertion failed: paths::is_absolute(root)
    FrameworkRouter::init_empty   src/runtime/bake/FrameworkRouter.rs:171
    dev_server_body::init         src/runtime/bake/DevServer.rs:995
  

• An absolute root with a trailing separator (root: process.cwd() + "/") starts, then aborts debug builds on the first request:

  panic: assertion failed: self.root[self.root.len() - 1] != b'/'
    DevServer::relative_path      src/runtime/bake/DevServer.rs:5956
  

• Release builds accept the same configs and misbehave instead: FrameworkRouter::scan_inner builds each route pattern by slicing abs_root.len() - root.len() - 1 bytes off a path it computed relative to root (FrameworkRouter.rs:1575-1597). With root: "." no route matches and every request falls through to fetch(); with a root longer than the router directory's path the subtraction underflows and the process panics (range start index 18446744073709451655 out of range).
• root: 123 behaves like root: "123": the local get_optional_slice shim in bake_body.rs coerced any value with toString. The Zig it was ported from used getOptional(.., ZigString.Slice), which threw.
• Cause: UserOptions::from_js (src/runtime/bake/bake_body.rs:235 on main) stored the user's string verbatim, while the value it defaults to is an absolute directory. DevServer copies the field straight into dev.root (DevServer.rs:518) and both consumers above require an absolute path with no trailing separator.

Fix

• UserOptions::from_js now produces root through one helper, resolve_root, in both of its arms (app: "react" and the object form):
  • base directory: the resolver's top_level_dir. That is the directory Framework::resolve already resolves fileSystemRouterTypes[n].root and the entry points against, so app.root can no longer be derived from a different base than the paths it is compared with (before, it came from a separate getcwd). The default (root not given) is this directory, stripped of a trailing separator, which is the same string getcwd produced before.
  • a user supplied root is joined against it with join_abs_string_buf_checked, stripped of a trailing separator, and rejected with 'app.root' resolves to a path longer than <MAX_PATH_BYTES> bytes when it cannot fit in a path buffer.
• The local get_optional_slice shim is deleted in favor of JSValue::get_optional_slice, which throws ERR_INVALID_ARG_TYPE for non-strings, at its four call sites: root, plugins[n].name, serverComponents.serverRuntimeImportSource, serverComponents.serverRegisterClientReferenceExport. This restores the pre-port behavior and matches Bun.build's plugin parsing (JSBundler.rs:500). The remaining local shims (get_boolean_*, get_function) return None so that bake can raise its own messages; they are unrelated to this bug and unchanged.
• Why this fix should exist, given that today a root other than the cwd still fails at request time (the bundler keys modules relative to the cwd, DevServer relative to root):
  • bake: compute dev server module ids relative to the dev server root #39202 is fixing that mismatch and makes root a working option; it states that it needs an absolute root and does not touch bake_body.rs. This PR is the producer side it relies on: without it, root: "./app" still hits the assertions and the pattern arithmetic above.
  • Independently of bake: compute dev server module ids relative to the dev server root #39202, ".", "./" and dir + "/" are the natural spellings of the default, and today they are the difference between routes working and every request silently hitting fetch().
  • Relative directories resolving against the working directory is how Bun.build({ root }), Bun.FileSystemRouter({ dir }) and bake's own fileSystemRouterTypes[n].root already behave.
  • If instead the option is meant to be removed (it is not declared in bake.d.ts), this hunk goes with it; bake: compute dev server module ids relative to the dev server root #39202 is proceeding on the basis that it stays.
• Out of scope and tracked separately: the module id mismatch above (bake: compute dev server module ids relative to the dev server root #39202); a fileSystemRouterTypes[n].root longer than 4 KB panicking in Framework::resolve; every Err return from from_js after framework.plugins was parsed leaking the protected plugin cell, because only UserOptions::Drop releases it (raised in review, pre-existing on the other error paths). Consumer-side fixes for neighbouring symptoms (bake: strip trailing separator from dev server root after process.chdir() #36396 trailing separator after process.chdir() on the HTML route path, bake: make DevServer::relative_path work when the root is the filesystem root #38323 root of /, bake: support fileSystemRouterTypes roots outside the project root #33203 router roots outside the app root) do not overlap with this change.
• Tests: test/bake/app-options.test.ts (new file; jsc: let get_optional::<JSValue> replace the Terminal and bake get_optional_value shims #39150 adds a file of the same name for other app options, whichever lands second appends to it). Unset, "", ., ./, routes/.. and <cwd>/ are each served end to end, and root: 123, a 100k character root, root: null, and the three other migrated string options are checked for their exact outcome.
  • bun bd test test/bake/app-options.test.ts: 7 pass.
  • Same command with src/ stashed: everything except the unset spelling fails, the spellings with the two assertions above and the option test with is_absolute(root).
  • USE_SYSTEM_BUN=1 bun test test/bake/app-options.test.ts (release): 5 of 7 fail; ""/././/routes/.. get "fallback" instead of the route and the option test dies on the slice underflow panic (unset and <cwd>/ pass there: the default always worked, and the trailing separator only trips the debug assertion).
  • Also green on the debug build: test/bake/dev/plugins.test.ts, test/bake/dev/esm.test.ts, test/bake/dev/production.test.ts (bun build --app goes through the same from_js; the app: "react" shortcut was exercised by hand through bun build --app and gets past configuration loading).

Background

• app is the experimental Bake dev server config accepted by Bun.serve (behind BUN_FEATURE_FLAG_EXPERIMENTAL_BAKE, which the test harness sets) and by bun build --app. Both parse it through UserOptions::from_js; only the dev server reads root, bun build --app uses the cwd directly.
• top_level_dir is the working directory captured by the resolver's process-wide FileSystem singleton when the runtime starts; process.chdir() updates it (with a trailing separator, which is why the default is stripped). Everything the framework configures is resolved against it.
• DevServer.root is the project root: module ids sent to the HMR runtime are file paths with this prefix removed (DevServer::relative_path), and FrameworkRouter turns files under each fileSystemRouterTypes[n].root into URL patterns by the same prefix removal. That is why both hold the absolute, no trailing separator invariant this PR establishes at the point where the value enters.
• join_abs_string_buf_checked is bun's path.resolve over byte slices: it joins a base directory and parts, normalizes ./../separators, and returns None instead of overflowing when the result does not fit the caller's buffer. It keeps a trailing separator from its input, hence the explicit strip afterwards.
• JSValue::get_optional_slice returns None for a missing/null/undefined property, the UTF-8 bytes for a string, and throws The "<name>" property must be of type string, got <type> for anything else.

Probe of the release build before this change

cwd holds server.ts and routes/index.tsx; the script starts the server, fetches /, prints the body.

root: "/tmp/bake-root-repro"   -> {"status":200,"body":"rendered"}
root: "."                      -> {"status":200,"body":"fallback"}
root: "some/relative/dir"      -> {"status":200,"body":"fallback"}
root: "a".repeat(5000)         -> panic: range start index 18446744073709546642 out of range for slice of length 39

Debug build, same script: ".", "", 123, true abort in FrameworkRouter::init_empty; "/tmp/bake-root-repro/" starts and aborts in DevServer::relative_path on the fetch; "/tmp/bake-root-repro" serves the route.

Earlier shape of this PR

The first revision (84cffc3) resolved a user supplied root against a fresh getcwd() and left the default on getcwd() as before. Review pointed out that this derived the root from a different base than the one Framework::resolve uses for the paths it is compared with; c3f6ba5 switched both arms to top_level_dir through the shared helper, which also removed the getcwd error paths. Tests and behavior for the cases above are unchanged.

[coderabbitai]

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Walkthrough

Bake option parsing now resolves application roots against the resolver directory, normalizes paths, rejects oversized values, uses typed optional accessors, and adds coverage for valid and invalid options.

Changes

Bake option parsing

Layer / File(s)	Summary
Application root resolution src/runtime/bake/bake_body.rs, test/bake/app-options.test.ts	UserOptions::from_js resolves default and configured roots, normalizes trailing separators, arena-duplicates paths, and rejects oversized paths. Tests cover equivalent root spellings.
Typed optional accessors and validation src/runtime/bake/bake_body.rs, test/bake/app-options.test.ts	Optional plugin and server component values use JSValue.get_optional_slice. Tests cover null, non-string, oversized, and invalid nested options.

Possibly related PRs

• oven-sh/bun#39150: Updates the same optional-value helper usage and app option parsing tests.
• oven-sh/bun#39202: Addresses related Bun.serve({ app.root }) handling.
• oven-sh/bun#38902: Modifies UserOptions::from_js for related Bake option parsing.

Suggested reviewers: dylan-conway, jarred-sumner

🚥 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 summarizes the main changes to app.root resolution and string validation, although “cwd” is less precise than the resolver top-level directory.
Description check	✅ Passed	The description explains the problem, implementation, scope, and verification results in detail, despite not using the template headings exactly.

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[robobun]

^{Updated 5:48 PM PT - Aug 15th, 2026}

❌ @robobun, your commit 6b8450e has 1 failures in Build #98867 (All Failures):

• test/cli/run/require-cache.test.ts - code 1 on 🐧 13 x64-asan

---

🧪   To try this PR locally:

bunx bun-pr 39188

That installs a local version of the PR into your bun-39188 executable, so you can run:

bun-39188 --bun

[robobun]

Status: ready for a maintainer. Current revision is 6b8450e.

Reproduced on a debug build of main (bcab5ed) with Bun.serve({ development: true, app: { framework, root: "." } }): abort in FrameworkRouter::init_empty (assertion failed: paths::is_absolute(root)), and with root: process.cwd() + "/": abort in DevServer::relative_path on the first request. The release build serves fetch() instead of the routes for root: "." and panics on a very long root. test/bake/app-options.test.ts (7 cases) fails without the src/ change except for the unset root case, and passes with it.

CI for 6b8450e (build 98867): 178 of 179 jobs passed. The new test ran and passed on Linux (glibc, musl, ASAN), macOS 14 aarch64 (7 pass), Windows x64 and Windows aarch64. The one red job is an ASAN shard whose test/cli/run/require-cache.test.ts leak tests timed out; that file does not touch bake and the same failure was already filed for triage from build 98606. Nothing in this diff is implicated, so there will be no further retrigger pushes from my side.

Review: all threads resolved (three suggestions applied in 6b8450e, the rest withdrawn by the reviewer or declined with reasons). The plugin handle leak on from_js error paths is pre-existing and tracked separately. #39202 makes a non-cwd root work at request time and relies on the root being absolute, which this PR provides.

[claude]

LGTM — focused fix that normalizes app.root at the entry point using existing in-tree path helpers, with the shim deletion applied to all four call sites.

Checked: PoolGuard returns the path buffer to the pool on drop (including the too-long error path); cwd: ZBox and user_root: ZigStringSlice are both owned locals that drop after arena_dupe_z copies the bytes; without_trailing_slash preserves a lone / so a filesystem-root cwd doesn't become empty. The JSValue::get_optional_slice semantics match the deleted shim for the null/undefined case and tighten it for non-strings, which the new test asserts for every migrated caller. bunEnv already sets BUN_FEATURE_FLAG_EXPERIMENTAL_BAKE, so the spawned fixtures reach the parser.

Extended reasoning...

Overview

This PR touches one function in src/runtime/bake/bake_body.rs (UserOptions::from_js) to resolve the user-supplied app.root against the cwd via join_abs_string_buf_checked, strip a trailing separator, and reject overlong results with a catchable error instead of the debug-assert / release-underflow the PR description documents. It also deletes the local get_optional_slice shim (which coerced any value via toString) in favor of JSValue::get_optional_slice (which throws ERR_INVALID_ARG_TYPE for non-strings), and updates the four call sites — root, plugins[n].name, and the two serverComponents string options. A new test file exercises four spellings of the cwd end-to-end through the dev server plus six invalid-option cases.

Security risks

None. This is option parsing for an experimental dev-server feature behind BUN_FEATURE_FLAG_EXPERIMENTAL_BAKE. Path resolution uses the in-tree bun_paths helpers; the only new user-reachable behavior is that previously-panicking or silently-misbehaving inputs now throw a JS TypeError.

Level of scrutiny

Medium. The change is small and self-contained, uses existing helpers exclusively (no hand-rolled path logic), and lives in a flag-gated experimental feature. I traced resource ownership on the new paths: the pooled PathBuffer is an RAII PoolGuard (path_buffer_pool.rs:114) returned on both success and the too-long error return; getcwd_alloc returns an owned ZBox that drops at end-of-block; the ZigStringSlice from get_optional_slice drops after arena_dupe_z copies the resolved bytes into the arena. without_trailing_slash keeps at least one byte (while e > 1), so / is not stripped to empty.

Other factors

The tests are well-shaped: subprocess-spawned with bunEnv (which sets the feature flag per harness.ts:86), test.concurrent.each over the root spellings, all three pipes drained concurrently, and the invalid-options case asserts exact error messages for every migrated get_optional_slice caller. The PR description documents that the tests fail with src/ stashed and with USE_SYSTEM_BUN=1. The one inline nit (pre-existing SplitBundlerOptions.plugin leak on error paths, now marginally easier to hit) is real but structural, not introduced here, and correctly out of scope — the fix is a Drop impl on SplitBundlerOptions.

[claude]

LGTM — the invariant is established at the right layer and every call site now sees a normalized absolute root.

What was reviewed:

• join_abs_string_buf_checked + without_trailing_slash_windows_path produce an absolute, no-trailing-separator path; path_buffer_pool::get() is an RAII guard, and the pooled buffer / ZigStringSlice / ZBox cwd are all dropped after arena_dupe_z copies the bytes.
• Switching the four call sites to JSValue::get_optional_slice — null/undefined still map to None (test confirms root: null is accepted); non-strings now throw instead of toString-coercing, matching Bun.build's plugin parsing.
• New tests follow harness conventions (concurrent pipe drain, tempDir, exact error messages) and cover ././/routes/../<cwd>/ plus every rejection path.
• The pre-existing SplitBundlerOptions plugin-handle leak on from_js error paths was flagged earlier and is tracked separately (resolved thread).

Extended reasoning...

Overview

Two files: src/runtime/bake/bake_body.rs reshapes the root block in UserOptions::from_js to always fetch the cwd, then either dupe it (no root) or resolve the user's string against it with join_abs_string_buf_checked into a pooled path buffer, strip a trailing separator, and dupe the result into the arena. It also deletes the local get_optional_slice shim in favor of the typed JSValue::get_optional_slice at four sites (root, plugins[n].name, serverRuntimeImportSource, serverRegisterClientReferenceExport) and drops the trailing arena_dupe_z(&arena, root) since root is now already &'static ZStr. test/bake/app-options.test.ts is new and covers four spellings of the cwd end-to-end plus six rejection cases.

Security risks

None. This is startup-time option parsing for an experimental dev server behind BUN_FEATURE_FLAG_EXPERIMENTAL_BAKE. The change tightens validation (rejects non-strings, rejects paths that overflow a path buffer) rather than loosening it. No auth, crypto, network, or untrusted-data parsing is touched.

Level of scrutiny

Moderate-low. The fix is localized to one function, uses established in-tree helpers (join_abs_string_buf_checked, without_trailing_slash_windows_path, path_buffer_pool::get, JSValue::get_optional_slice), and the feature is experimental behind a flag. I verified each helper's semantics: get_optional_slice returns None for undefined/null and throws for non-strings; join_abs_string_buf_checked returns None on buffer overflow; path_buffer_pool::get() returns a PoolGuard that returns the buffer on Drop; getcwd_alloc returns an owned ZBox that drops after its bytes are copied.

Other factors

The one finding from the previous run — the two new early returns leaking the protected plugin handle when framework.plugins is set — is a pre-existing structural gap (every existing Err path between Framework::from_js and Ok already had it), was confirmed by the author, and is being fixed separately with a Drop on SplitBundlerOptions. That thread is resolved. The comment-cop lint was addressed in 0fd82fb (comment removed; the invariant it described is asserted at the consumers). CI on the substantive commit (84cffc3) passed all Linux and Windows lanes including the new test on Windows; the latest push only removes a comment. The tests demonstrably fail without the fix and pass with it, and the neighboring bake dev tests still pass.

[coderabbitai]

Actionable comments posted: 3

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Inline comments:
In `@src/runtime/bake/bake_body.rs`:
- Line 256: Update the invalid-arguments error for the “.root” path in the bake
validation to state the supported resolved-path limit and instruct users to set
a shorter app.root; then update the corresponding exact assertion in the
app-options test to match the expanded message.
- Line 254: Update resolve_root before the join involving user_root.slice() to
validate and reject embedded NUL bytes, before arena_dupe_z creates the ZStr.
Preserve existing length and normalization behavior, and add a regression test
confirming roots containing an embedded NUL are rejected.

In `@test/bake/app-options.test.ts`:
- Line 19: Add coverage in the route-serving tests around the existing
parameterized root cases for omitted or undefined app.root and an empty string,
and add a separate validation case for app.root set to 0. Keep these inputs
distinct so optional-property, empty-string, and zero-value handling are each
exercised.

🪄 Autofix

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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[claude]

I reviewed this PR again after 6b8450e and didn't find any bugs — my earlier nits (test.concurrent, plugin-leak scoping) are addressed and all threads are resolved. Because this is native runtime code that changes validation behavior at four get_optional_slice call sites and adds a path-resolution helper that a sibling PR (#39202) will build on, a maintainer look would still be worthwhile.

What was reviewed:

• resolve_root: pooled buffer is a RAII Guard, join_abs_string_buf_checked result is copied into the arena before the guard drops, and the ZigStringSlice from get_optional_slice is dropped after arena_dupe_z copies its bytes.
• top_level_dir base matches what Framework::resolve already uses for fileSystemRouterTypes[n].root in this same file.
• The four migrated call sites now throw ERR_INVALID_ARG_TYPE instead of coercing — matches JSValue::get_optional_slice semantics and the test asserts each message.
• New early-return leak of bundler_options.plugin on the root error paths is pre-existing (no Drop on SplitBundlerOptions), tracked separately per the thread.

Extended reasoning...

Overview

This PR fixes UserOptions::from_js in src/runtime/bake/bake_body.rs so that app.root is resolved to an absolute, no-trailing-separator path against the resolver's top_level_dir, and replaces the local get_optional_slice shim (which coerced via toString) with the canonical JSValue::get_optional_slice (which throws ERR_INVALID_ARG_TYPE for non-strings) at four call sites. A new test file (test/bake/app-options.test.ts) covers six spellings of the cwd end-to-end plus rejection of non-string / oversized values.

Security risks

None identified. app.root is developer-supplied configuration for the experimental Bake dev server (behind BUN_FEATURE_FLAG_EXPERIMENTAL_BAKE), not attacker-controlled input. The value is resolved with join_abs_string_buf_checked (bounds-checked, normalizes ..), copied into the arena, and used only for byte-prefix comparisons in DevServer::relative_path / FrameworkRouter — not passed to a C API. The embedded-NUL question was raised and reasonably declined in-thread: root never reaches as_cstr().

Level of scrutiny

Medium-high. Bake is experimental and gated, so blast radius is limited, but this is native Rust with the file-level 'static lifetime-erasure pattern, a new helper that other PRs will depend on, and a behavior change (coerce → throw) at four call sites. The repo's review bar for native code is strict; a maintainer should confirm the top_level_dir-as-base decision aligns with where #39202 is going.

Other factors

• All prior review threads (mine, CodeRabbit's, comment-cop's) are resolved; 6b8450e addressed the last round.
• Tests are thorough: six positive spellings, six negative cases, verified to fail on main / USE_SYSTEM_BUN=1 per the PR description.
• Memory: verified path_buffer_pool::get() returns a Guard (RAII), arena_dupe_z copies before the guard/slice drop, and the removed allocations.track(slice) for root is no longer needed since the bytes are now arena-owned.
• The pre-existing SplitBundlerOptions plugin-handle leak on from_js error paths is acknowledged and tracked separately; not introduced here.

[robobun]

Superseded by #39488, which consolidates the open Bake PRs from this period into one branch. It carries this PR's change and its test, reworked where needed (see the commit in #39488 that names this PR). Closing this one in its favor.