## Summary
Fixes#13316Fixes#18275
Running `bunx cowsay ""` (or any package with an empty string argument)
on Windows caused a panic. Additionally, `bunx concurrently "command
with spaces"` was splitting quoted arguments incorrectly.
**Repro #13316:**
```bash
bunx cowsay ""
# panic(main thread): reached unreachable code
```
**Repro #18275:**
```bash
bunx concurrently "bun --version" "bun --version"
# Only runs once, arguments split incorrectly
# Expected: ["bun --version", "bun --version"]
# Actual: ["bun", "--version", "bun", "--version"]
```
## Root Cause
The bunx fast path on Windows bypasses libuv and calls `CreateProcessW`
directly to save 5-12ms. The command line building logic had two issues:
1. **Empty strings**: Not quoted at all, resulting in invalid command
line
2. **Arguments with spaces**: Not quoted, causing them to be split into
multiple arguments
## Solution
Implement Windows command-line argument quoting using libuv's proven
algorithm:
- Port of libuv's `quote_cmd_arg` function (process backwards + reverse)
- Empty strings become `""`
- Strings with spaces/tabs/quotes are wrapped in quotes
- Backslashes before quotes are properly escaped per Windows rules
**Why not use libuv directly?**
- Normal `Bun.spawn()` uses `uv_spawn()` which handles quoting
internally
- bunx fast path bypasses libuv to save 5-12ms (calls `CreateProcessW`
directly)
- libuv's `quote_cmd_arg` is a static function (not exported)
- Solution: port the algorithm to Zig
## Test Plan
- [x] Added regression test for empty strings (#13316)
- [x] Added regression test for arguments with spaces (#18275)
- [x] Verified system bun (v1.3.3) fails both tests
- [x] Verified fix passes both tests
- [x] Implementation based on battle-tested libuv algorithm
🤖 Generated with [Claude Code](https://claude.com/claude-code)
---------
Co-authored-by: Claude <claude-bot@bun.sh>
Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>
