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fix: zero-initialize primitive scratch buffers (EXP-089) #31258

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fix: zero-initialize primitive scratch buffers (EXP-089) #31258

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33 changes: 9 additions & 24 deletions src/bun_core/util.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -989,23 +989,13 @@ pub use bun_alloc::SEP;
pub struct PathBuffer(pub [u8; MAX_PATH_BYTES]);
impl PathBuffer {
pub const ZEROED: Self = Self([0; MAX_PATH_BYTES]);
/// Zig `= undefined`. The bytes are immediately overwritten by the syscall
/// that fills it, so the initial contents are never observed.
/// Zero-initialized scratch buffer.
///
/// On Windows `MAX_PATH_BYTES` is 98 302 (vs 4 096 Linux / 1 024 macOS), so
/// the previous `Self::ZEROED` body here was a ~100 KB `memset` at every
/// one of the ~400 call sites — turning hot loops (glob scan, module load,
/// stack-trace formatting) into multi-GB zero-fill workloads and timing out
/// the leak/stress tests. Match the Zig spec and leave the bytes uninit.
/// The name is kept for Zig-port call sites, but this safe constructor must
/// return a fully initialized `[u8; MAX_PATH_BYTES]`.
#[inline]
#[allow(invalid_value, clippy::uninit_assumed_init)]
pub fn uninit() -> Self {
// SAFETY: `PathBuffer` is `repr(transparent)` over `[u8; N]`; every bit
// pattern is a valid `u8`, and callers treat this as a write-only
// scratch buffer (length-tracked) exactly like Zig
// `var buf: bun.PathBuffer = undefined`. No byte is read before being
// written by the consuming syscall / encoder.
unsafe { core::mem::MaybeUninit::uninit().assume_init() }
Self::ZEROED
}
#[inline]
pub fn as_mut_slice(&mut self) -> &mut [u8] {
Expand Down Expand Up @@ -1041,18 +1031,13 @@ impl core::ops::DerefMut for PathBuffer {
pub struct WPathBuffer(pub [u16; PATH_MAX_WIDE]);
impl WPathBuffer {
pub const ZEROED: Self = Self([0; PATH_MAX_WIDE]);
/// Zig `= undefined`. See [`PathBuffer::uninit`] — `PATH_MAX_WIDE` is
/// 32 767 `u16`s (~64 KB), and these are allocated per Windows syscall
/// for UTF-8→UTF-16 path conversion, so zero-initialising dominated the
/// hot path on Windows.
/// Zero-initialized wide-path scratch buffer.
///
/// The name is kept for Zig-port call sites, but this safe constructor must
/// return a fully initialized `[u16; PATH_MAX_WIDE]`.
#[inline]
#[allow(invalid_value, clippy::uninit_assumed_init)]
pub fn uninit() -> Self {
// SAFETY: `repr(transparent)` over `[u16; N]`; every bit pattern is a
// valid `u16`. Callers treat this as a write-only scratch buffer and
// track the written length out-of-band — mirrors Zig
// `var wbuf: bun.WPathBuffer = undefined`.
unsafe { core::mem::MaybeUninit::uninit().assume_init() }
Self::ZEROED
}
/// Inherent `as_slice` so `wbuf.as_slice()` resolves here instead of the
/// unstable `<[u16]>::as_slice` (`str_as_str` feature) via `Deref`.
Expand Down
16 changes: 4 additions & 12 deletions src/install/lockfile/Tree.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -74,19 +74,10 @@ pub const MAX_DEPTH: usize = (MAX_PATH_BYTES / b"node_modules".len()) + 1;

pub type DepthBuf = [Id; MAX_DEPTH];

/// Zig `var depth_buf: Tree.DepthBuf = undefined;` — write-only scratch buffer
/// for [`relative_path_and_depth`]. Every slot is written before it is read
/// (index 0 unconditionally, indices `1..depth_buf_len` in the parent-walk
/// loop), so leaving the ~1.4 KB array uninitialised matches the spec and
/// avoids a `memset` per tree in the `--frozen-lockfile` no-change path.
/// Same shape/contract as [`bun_core::PathBuffer::uninit`].
/// Zero-initialized scratch buffer for [`relative_path_and_depth`].
#[inline]
#[allow(invalid_value, clippy::uninit_assumed_init)]
pub fn depth_buf_uninit() -> DepthBuf {
// SAFETY: `DepthBuf` is `[u32; N]`; every bit pattern is a valid `u32`.
// Callers treat this as a write-only scratch buffer — no element is read
// before being assigned by `relative_path_and_depth`.
unsafe { core::mem::MaybeUninit::uninit().assume_init() }
[0; MAX_DEPTH]
}

impl Tree {
Expand Down Expand Up @@ -236,7 +227,8 @@ impl<'a, const PATH_STYLE: IteratorPathStyle> Iterator<'a, PATH_STYLE> {
dependencies,
string_bytes,
path_buf: PathBuffer::uninit(),
// Zig: `depth_stack: DepthBuf = undefined` (Tree.zig:94)
// Zig used undefined storage here; Rust zero-initializes this buffer
// so the safe constructor returns a valid `DepthBuf`.
depth_stack: depth_buf_uninit(),
};
if PATH_STYLE == IteratorPathStyle::NodeModules {
Expand Down