Add an allocator for external strings

[dcarney-cf]

No description provided.

[ask-bonk]

I'm Bonk, and I've done a quick review of your PR.

This PR adds an ExternalStringAllocator abstraction for V8 external string resources, replacing direct new/delete with a configurable allocator threaded through IsolateBase.

Issues found (highest severity first):

1. [HIGH] Compile breakage — The primary Isolate constructor (with IsolateGroup) now requires an externalStringAllocator parameter, but existing callers in workerd-api.c++, setup-test.c++, modules-new-test.c++, and multiple-typewrappers-test.c++ were not updated.
2. [MEDIUM] Silent failure from allocate returning nullptr — createExtern returns an empty MaybeLocal when allocation fails, but the callers (newExternalOneByteString, newExternalTwoByteString) pass the result through check() which throws JsExceptionThrown without an actual JS exception pending.
3. [LOW] Allocator interface lacks alignment contract — allocate(size_t) doesn't specify alignment requirements. ExternString contains a vtable pointer and kj::ArrayPtr, requiring at least pointer alignment. Custom allocators could violate this.

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This review was generated by an AI assistant and may contain inaccuracies.

[ask-bonk]

I'm Bonk, and I've done a quick review of your PR.

This PR adds an ExternalStringAllocator abstraction for V8 external string resources, replacing direct new/delete with a configurable allocator threaded through IsolateBase.

I posted a batch review on PR #6307 with 3 inline comments covering:

1. [HIGH] Compile breakage — The primary Isolate constructor now requires externalStringAllocator but existing callers in workerd-api.c++, setup-test.c++, modules-new-test.c++, and multiple-typewrappers-test.c++ were not updated. Suggested defaulting the parameter to kj::heap<DefaultExternalStringAllocator>().

2. [MEDIUM] Silent failure path — When allocate() returns nullptr, createExtern returns an empty MaybeLocal, but callers pass it through check() which throws JsExceptionThrown without an actual JS exception pending.

3. [LOW] Missing alignment contract — The allocate(size_t) interface doesn't document or enforce alignment requirements needed by ExternString's vtable and pointer members.

github run

[github-actions]

The generated output of @cloudflare/workers-types matches the snapshot in types/generated-snapshot 🎉

[erikcorry]

Probably needs a test with an allocator that says no, to ensure the exception throwing works.