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Shrink JS-allocated strings down to the correct size before passing them to Rust #3808

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Jan 25, 2024
Merged

Shrink JS-allocated strings down to the correct size before passing them to Rust #3808

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38c1098
Shrink JS-allocated strings down to the correct size before passing t…
Liamolucko Jan 22, 2024
3f59fe9
Update reference tests
Liamolucko Jan 22, 2024
170aba2
Add changelog entry
Liamolucko Jan 23, 2024
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5 changes: 5 additions & 0 deletions CHANGELOG.md
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Original file line number Diff line number Diff line change
Expand Up @@ -25,6 +25,11 @@
* Deprecate `wasm_bindgen_test_configure!`s `run_in_worker` in favor of `run_in_dedicated_worker`.
[#3804](https://github.com/rustwasm/wasm-bindgen/pull/3804)

### Fixed

* Fixed UB when freeing strings received from JS if not using the default allocator.
[#3808](https://github.com/rustwasm/wasm-bindgen/pull/3808)

## [0.2.90](https://github.com/rustwasm/wasm-bindgen/compare/0.2.89...0.2.90)

Released 2024-01-06
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6 changes: 1 addition & 5 deletions crates/cli-support/src/js/mod.rs
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Expand Up @@ -1296,11 +1296,6 @@ impl<'a> Context<'a> {
mem = mem,
);

// TODO:
// When converting a JS string to UTF-8, the maximum size is `arg.length * 3`,
// so we just allocate that. This wastes memory, so we should investigate
// looping over the string to calculate the precise size, or perhaps using
// `shrink_to_fit` on the Rust side.
self.global(&format!(
"function {name}(arg, malloc, realloc) {{
{debug}
Expand All @@ -1314,6 +1309,7 @@ impl<'a> Context<'a> {
const ret = encodeString(arg, view);
{debug_end}
offset += ret.written;
ptr = realloc(ptr, len, offset, 1) >>> 0;
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Could add a check for whether len != offset before reallocing again, to avoid spurious allocs.

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I considered that, but I didn't think it'd be worth it since in the vast majority of cases the string won't take up the maximum amount of space anyway.

I'd be fine to add it if you want but I don't think the difference would be significant either way.

}}

WASM_VECTOR_LEN = offset;
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1 change: 1 addition & 0 deletions crates/cli/tests/reference/string-arg.js
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Expand Up @@ -75,6 +75,7 @@ function passStringToWasm0(arg, malloc, realloc) {
const ret = encodeString(arg, view);

offset += ret.written;
ptr = realloc(ptr, len, offset, 1) >>> 0;
}

WASM_VECTOR_LEN = offset;
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