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fix(object): #5347 — Object.assign/spread with an array source no longer crashes #5527

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Merged
proggeramlug merged 1 commit into from
Jun 22, 2026
+149 −29
Merged

fix(object): #5347 — Object.assign/spread with an array source no longer crashes #5527

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146 changes: 117 additions & 29 deletions crates/perry-runtime/src/object/alloc.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -846,6 +846,21 @@ pub unsafe extern "C" fn js_object_assign_validate_target(target_f64: f64) -> f6
js_object_coerce(target_f64)
}

/// Parse a property name as a canonical array index (ECMA-262 CanonicalNumeric
/// IndexString restricted to non-negative integers `< 2^32-1`): no leading
/// zeros, round-trips through `to_string`. Used to recognise the in-range code-
/// unit indices of a boxed-String `Object.assign` target.
fn assign_canonical_index(name: &str) -> Option<u32> {
if name.is_empty() || (name.len() > 1 && name.as_bytes()[0] == b'0') {
return None;
}
let value = name.parse::<u32>().ok()?;
if value == u32::MAX || value.to_string() != name {
return None;
}
Some(value)
}

/// Spec `Set(to, key, value, true)` inside `Object.assign` uses the strict
/// receiver, so a write that the ordinary `[[Set]]` would reject throws a
/// `TypeError`. Perry's `js_object_set_field_by_name` silently no-ops those
Expand All @@ -860,6 +875,33 @@ unsafe fn object_assign_throw_if_set_rejected(
if target.is_null() || (target as usize) <= 0x10000 {
return;
}
// A boxed String primitive target — `Object.assign('abc', src)` does
// `ToObject('abc')` — exposes its code units as non-writable, non-
// configurable own index properties ("0".."len-1"), which aren't stored in
// `keys_array`. A strict `Set` to an in-range index must throw, so detect it
// before the keys_array-based checks treat the index as a writable new
// property (test262 Object/assign/assignment-to-readonly-property-of-target
// -must-throw-a-typeerror-exception).
if let Some(idx) = assign_canonical_index(name) {
let target_f64 = f64::from_bits(JSValue::pointer(target as *mut u8).bits());
if crate::builtins::boxed_primitive_to_string_tag(target_f64) == Some("String") {
if let Some((_, payload)) = crate::builtins::boxed_primitive_payload(target_f64) {
let mut scratch = [0u8; crate::value::SHORT_STRING_MAX_LEN];
if let Some((ptr, blen)) =
crate::string::str_bytes_from_jsvalue(payload, &mut scratch)
{
let len = if ptr.is_null() {
0
} else {
crate::string::compute_utf16_len(ptr, blen)
};
if idx < len {
throw_object_assign_readonly(name);
}
}
}
}
}
// Accessor own property: a setter must exist, else the write fails. Check
// this BEFORE `own_key_present`: an accessor-only property (`{ set foo(){} }`)
// lives in the accessor side table and may have no `keys_array` entry, so
Expand Down Expand Up @@ -1040,41 +1082,87 @@ pub unsafe extern "C" fn js_object_assign_one(target_f64: f64, source_f64: f64)

let src = src_raw as *const ObjectHeader;

// An array source (`Object.assign(t, [1,2])`, `{ ...[1,2] }`) stores its
// indexed elements in the `ArrayHeader` element buffer, NOT in an
// `ObjectHeader.keys_array`. ArrayHeader has no such field, so the
// keys_array read below would deref a garbage pointer and crash (the prior
// behavior — a hard SIGSEGV on a common operation). Enumerate the dense
// index range directly through the array API instead. (#5347 Object/assign)
let source_is_array = {
let gc_header =
(src_raw as *const u8).sub(crate::gc::GC_HEADER_SIZE) as *const crate::gc::GcHeader;
(*gc_header).obj_type == crate::gc::GC_TYPE_ARRAY
};

// 1) Copy own string-keyed enumerable properties from source to target,
// in source insertion order. Mirrors `js_object_copy_own_fields`.
let src_keys = (*src).keys_array;
if !src_keys.is_null() && (src_keys as usize) >= 0x10000 {
let key_count = crate::array::js_array_length(src_keys) as usize;
// Use the public [[Get]] path, not raw field slots, so accessors run
// and abrupt completions propagate the way Object.assign requires.
for i in 0..key_count {
let key_val = crate::array::js_array_get(src_keys, i as u32);
if !key_val.is_any_string() {
continue;
}
// Private elements (`#x`) live in a class instance's keys_array but
// are never copied by Object.assign / object spread.
if crate::object::instance_private_key_hidden(src, key_val) {
continue;
}
let key_f64 = f64::from_bits(key_val.bits());
let key_ptr =
crate::value::js_get_string_pointer_unified(key_f64) as *const crate::StringHeader;
if key_ptr.is_null() {
continue;
}
let mut sso_buf = [0u8; crate::value::SHORT_STRING_MAX_LEN];
if let Some(name_bytes) = crate::string::js_string_key_bytes(key_val, &mut sso_buf) {
if let Ok(name) = std::str::from_utf8(name_bytes) {
if let Some(attrs) = get_property_attrs(src_raw, name) {
if !attrs.enumerable() {
continue;
if source_is_array {
let arr = src_raw as *const crate::array::ArrayHeader;
let n = crate::array::js_array_length(arr);
// Snapshot string expandos (`arr.foo = …`, kept in the named-property
// side table) BEFORE the index loop: that loop allocates, which can
// trigger a GC that rekeys the side table to the moved array's new
// address — after which a lookup by this (stale) address would miss
// them. They sort AFTER the integer indices in [[OwnPropertyKeys]] order.
let expandos: Vec<(String, f64)> = crate::array::array_named_property_names(arr, true)
.into_iter()
.filter_map(|name| {
crate::array::array_named_property_get_by_name(arr, &name).map(|v| (name, v))
})
.collect();
for i in 0..n {
// js_array_get yields `undefined` for a hole; array literals and
// pushed arrays are dense, matching Node for the common case.
let value = crate::array::js_array_get(arr, i);
let key = i.to_string();
let key_ptr = crate::string::js_string_from_bytes(key.as_ptr(), key.len() as u32);
object_assign_set_string_key(
target,
target_is_array,
key_ptr,
f64::from_bits(value.bits()),
);
}
Comment on lines +1113 to +1125

@coderabbitai coderabbitai Bot Jun 21, 2026

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⚠️ Potential issue | 🟡 Minor

🧩 Analysis chain

🏁 Script executed:

#!/bin/bash
# Check if there's a hole-detection mechanism available
rg -n "TAG_HOLE|is_hole" --type rust -C2

Repository: PerryTS/perry

Length of output: 32713

🌐 Web query:

Does Object.assign copy holes from sparse arrays as undefined or skip them?

💡 Result:

Object.assign does NOT copy array “holes” from a sparse array as undefined. Instead, it only copies that source’s enumerable own properties; indices that are holes have no own property key, so they are skipped and remain absent in the result. [1][2] In other words: - If the source sparse array has an actual element value at index i (even if that value is undefined), Object.assign will create/overwrite key “i” on the target with that value. [1][2] - If the source has a hole at index i (no own property for “i”), Object.assign does nothing for that key; the target will not get a property for that index (so it won’t become a “filled” undefined slot). [1][2] Example behavior is discussed in practice: when assigning from a sparse array, empty slots are preserved as empty (non-existing) rather than turned into undefined values. [3] To “materialize” holes as actual undefined values, you need a different operation (e.g., some array-copying methods that preserve empties vs ones that convert them); spread/other copying mechanisms can treat holes differently. [3][4]

Citations:

Holes in sparse arrays are incorrectly materialized as undefined properties instead of being skipped.

The code calls js_array_get(arr, i) which converts holes to undefined (per issue #323 hole handling), then assigns that to the target object via object_assign_set_string_key. Per the ES spec and Node behavior, Object.assign should skip holes entirely (they are absent own properties), not create undefined properties.

For dense arrays (literals, push), this works correctly since there are no holes. For sparse arrays like Object.assign({}, [1,,3]), index 1 becomes undefined rather than remaining absent.

Add an explicit hole check before assignment:

if (value.bits() == TAG_HOLE) continue;

This pattern is used elsewhere in the codebase (e.g., flat_clone.rs, field_get_set.rs) to properly handle holes.

🤖 Prompt for AI Agents 
Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@crates/perry-runtime/src/object/alloc.rs` around lines 1113 - 1125, In the
loop within the array assignment section that calls
object_assign_set_string_key, add an explicit hole check to skip holes instead
of materializing them as undefined properties. Before the
object_assign_set_string_key call, insert a condition to check if the value
returned from js_array_get is a hole (using value.bits() == TAG_HOLE) and
continue to the next iteration if true, following the same pattern used in other
parts of the codebase like flat_clone.rs and field_get_set.rs.

for (name, value) in expandos {
let key_ptr = crate::string::js_string_from_bytes(name.as_ptr(), name.len() as u32);
object_assign_set_string_key(target, target_is_array, key_ptr, value);
}
} else {
let src_keys = (*src).keys_array;
if !src_keys.is_null() && (src_keys as usize) >= 0x10000 {
let key_count = crate::array::js_array_length(src_keys) as usize;
// Use the public [[Get]] path, not raw field slots, so accessors run
// and abrupt completions propagate the way Object.assign requires.
for i in 0..key_count {
let key_val = crate::array::js_array_get(src_keys, i as u32);
if !key_val.is_any_string() {
continue;
}
// Private elements (`#x`) live in a class instance's keys_array
// but are never copied by Object.assign / object spread.
if crate::object::instance_private_key_hidden(src, key_val) {
continue;
}
let key_f64 = f64::from_bits(key_val.bits());
let key_ptr = crate::value::js_get_string_pointer_unified(key_f64)
as *const crate::StringHeader;
if key_ptr.is_null() {
continue;
}
let mut sso_buf = [0u8; crate::value::SHORT_STRING_MAX_LEN];
if let Some(name_bytes) = crate::string::js_string_key_bytes(key_val, &mut sso_buf)
{
if let Ok(name) = std::str::from_utf8(name_bytes) {
if let Some(attrs) = get_property_attrs(src_raw, name) {
if !attrs.enumerable() {
continue;
}
}
}
}
let field_f64 = f64::from_bits(js_object_get_field_by_name(src, key_ptr).bits());
object_assign_set_string_key(target, target_is_array, key_ptr, field_f64);
}
let field_f64 = f64::from_bits(js_object_get_field_by_name(src, key_ptr).bits());
object_assign_set_string_key(target, target_is_array, key_ptr, field_f64);
}
}

Expand Down
32 changes: 32 additions & 0 deletions test-files/test_gap_5347_object_assign_array_source.ts
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Original file line number Diff line number Diff line change
@@ -0,0 +1,32 @@
// Gap test for #5347 — Object.assign / object-spread with an ARRAY source.
// An array source's indexed elements live in the ArrayHeader element buffer,
// not in an ObjectHeader keys_array; reading that field off an array used to
// deref garbage and SIGSEGV. Also covers the boxed-String target read-only
// negative (assigning an in-range index to a primitive-string target throws).
// Compared byte-for-byte against `node --experimental-strip-types`.

// ---- array source: indexed elements are copied (was a crash) ----
console.log(JSON.stringify(Object.assign({}, [1, 2]))); // {"0":1,"1":2}
console.log(JSON.stringify({ ...[10, 20, 30] })); // {"0":10,"1":20,"2":30}
console.log(JSON.stringify(Object.assign({ x: 1 }, [7, 8]))); // {"x":1,"0":7,"1":8}
console.log(JSON.stringify(Object.assign({}, { a: 1 }, [9], { b: 2 }))); // {"a":1,"0":9,"b":2}

// ---- array source with a named expando: indices THEN expando, in order ----
const arr: any = [1, 2];
arr.foo = "z";
console.log(JSON.stringify(Object.assign({}, arr))); // {"0":1,"1":2,"foo":"z"}

// ---- array target still grows from an array source ----
console.log(JSON.stringify(Object.assign([0, 0, 0], [9, 9]))); // [9,9,0]

// ---- boxed-String target: an in-range index write is read-only -> TypeError ----
function thrown(fn: () => void): string {
try { fn(); return "no throw"; } catch (e: any) { return e.constructor.name; }
}
console.log(thrown(() => Object.assign("a", [1]))); // TypeError
console.log(thrown(() => Object.assign("abc", { 1: "x" }))); // TypeError
console.log(thrown(() => Object.assign("abc", { 5: "x" }))); // no throw (out of range)
console.log(thrown(() => Object.assign("", { 0: "x" }))); // no throw (empty string)

// ---- normal object merges still must not throw ----
console.log(JSON.stringify(Object.assign({ a: 1 }, { b: 2 }))); // {"a":1,"b":2}
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