fix(wasi): resolve path_open paths against preopen directory, not cwd

[mkusaka]

What does this PR do?

Fixes path_open in the WASI implementation to resolve guest paths against the preopen directory's host path instead of the current working directory.

Bug

When a WASI guest opens a file using an absolute path (e.g., /tmp/foo.txt) with preopens: { "/": "/" }, Bun incorrectly resolves it as <cwd>/tmp/foo.txt instead of /tmp/foo.txt, causing ENOENT.

Root cause: path_open in src/js/node/wasi.ts discards the CHECK_FD() result and calls path.resolve(p), which uses cwd as the base. All other path syscalls (path_readlink, path_create_directory, path_unlink_file, etc.) correctly use path.resolve(stats.path, p).

Fix

Two-line change:

- CHECK_FD(dirfd, constants_1.WASI_RIGHT_PATH_OPEN);
+ const stats = CHECK_FD(dirfd, constants_1.WASI_RIGHT_PATH_OPEN);
  // ... (flag handling unchanged) ...
- const fullUnresolved = path.resolve(p);
+ const fullUnresolved = path.resolve(stats.path, p);

Test

Added a test that reads a temp file via a WASI binary to verify path_open resolves against preopens.

read-file.wasm is a minimal Rust WASI binary (wasm32-wasip1, 91KB) that reads a file path from args and prints contents to stdout. Built with:

// src/main.rs
use std::{env, fs, process};
fn main() {
    let args: Vec<String> = env::args().collect();
    if args.len() < 2 { eprintln!("usage: read-file <path>"); process::exit(1); }
    match fs::read_to_string(&args[1]) {
        Ok(c) => print!("{}", c),
        Err(e) => { eprintln!("error: {}: {}", args[1], e); process::exit(1); }
    }
}

cargo init --name read-file-wasi
cargo build --target wasm32-wasip1 --release

How did you verify your code works?

• Confirmed the test fails on unmodified Bun v1.3.10 (ENOENT with doubled path)
• Confirmed Node.js handles the same WASI binary correctly with identical preopens
• The fix matches the exact pattern used by all other path syscalls in the same file

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• test/js/bun/wasm/wasi.test.js

---

Walkthrough

Path resolution for WASI path_open now resolves relative to the opened directory (preopen) by using the directory's stored path when constructing the unresolved path. A test was added to verify path_open resolves against preopens rather than the current working directory.

Changes

Cohort / File(s)	Summary
WASI path resolution src/js/node/wasi.ts	Updated path_open handling to store the dirfd rights check result in stats and compute fullUnresolved with path.resolve(stats.path, p) so resolution is based on the opened directory.
WASI path resolution test test/js/bun/wasm/wasi.test.js	Added test "path_open should resolve paths against preopens, not cwd" which creates a temporary directory and file, runs read-file.wasm against the temp path, and asserts expected stdout/stderr/exitCode.

🚥 Pre-merge checks | ✅ 2

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and specifically describes the main change: fixing path_open to resolve paths against the preopen directory rather than cwd, which is the primary objective of this PR.
Description check	✅ Passed	The description comprehensively covers both required template sections: 'What does this PR do?' with detailed bug explanation and fix, and 'How did you verify your code works?' with verification steps.

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@test/js/bun/wasm/wasi.test.js`:
- Around line 4-6: Replace direct use of fs.mkdtempSync with the test harness
helper: import and call tempDir (from harness) instead of mkdtempSync to create
the temporary directory used in this test (references: mkdtempSync -> tempDir).
Remove the explicit tmpdir/mkdtempSync import and any manual cleanup (rmSync)
for that directory and rely on harness-managed cleanup; keep using writeFileSync
and join to create files inside the tempDir path. Update any other occurrences
(lines ~29-31) that create or remove temp dirs to use tempDir consistently.

---

ℹ️ Review info

Configuration used: Path: .coderabbit.yaml

Review profile: ASSERTIVE

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Disabled knowledge base sources:

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📥 Commits

Reviewing files that changed from the base of the PR and between 6c7e972 and b0783a0.

⛔ Files ignored due to path filters (1)

• test/js/bun/wasm/read-file.wasm is excluded by !**/*.wasm

📒 Files selected for processing (2)

• src/js/node/wasi.ts
• test/js/bun/wasm/wasi.test.js