Add support for request-time WebAssembly compilation

[danlapid]

Add the request_time_webassembly_compilation compatibility flag and keep the feature disabled by default.

Wrap WebAssembly.compile(), byte-based instantiate(), and Module() to snapshot exact input bytes before compilation.
Await retention for promise APIs and queue best-effort retention for the synchronous constructor.

Expose embedder upload hooks and per-request admission accounting while keeping eval() and Function() disabled. Preserve startup compilation and existing WebAssembly API behavior.

[ask-bonk]

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

This PR adds a request_time_webassembly_compilation experimental compat flag that enables request-time WebAssembly.compile(), byte-based instantiate(), and new Module() by wrapping them with a shim that snapshots input bytes before compilation and records them via embedder hooks.

Overall this is well-structured: the compat flag is correctly $experimental with no enable date, the JS shim captures trusted intrinsics to prevent tampering, the C++ callback properly bridges V8/KJ exceptions, and the shim layering with wasm-instantiate-shim.js composes correctly. Two findings:

1. [MEDIUM] Hardcoded limit in error message may drift from the production implementation.
2. [LOW] originalCompile is captured but never used in the JS shim.

---

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 a request_time_webassembly_compilation experimental compat flag that enables request-time WebAssembly.compile(), byte-based instantiate(), and new Module() by wrapping them with a JS shim that snapshots input bytes before compilation and records them via embedder hooks (uploadWasmForCodeGeneration / queueWasmUploadForCodeGeneration).

What looks good:

• Compat flag is correctly $experimental with no enable date -- no risk of ungated behavioral changes
• JS shim captures trusted intrinsics (Reflect.apply, Promise.prototype.then, etc.) before user code can tamper with them
• The shim layering with wasm-instantiate-shim.js composes correctly: codegen shim captures the already-wrapped Instance from the instantiate shim, so shutdown registration still works
• C++ callback properly bridges V8/KJ exceptions via kj::runCatchingExceptions + ThrowException
• Memory ownership is clean: source.copy() creates independent copies for the V8 snapshot ArrayBuffer and the upload bytes
• All IoChannelFactory subclasses (2 direct, 3 indirect) implement the new pure virtual methods
• Both positive and negative tests are included

Two findings posted as inline review comments:

1. [MEDIUM] Error message hardcodes "100 unique WebAssembly modules" but the actual limit is enforced by the production recordLoadedWasmModule() override -- suggested a generic message instead.
2. [LOW] originalCompile is captured but never used in the JS shim.

github run

[github-actions]

The generated output of @cloudflare/workers-types has been changed by this PR. If this is intentional, run just generate-types to update the snapshot. Alternatively, you can download the full generated types:

Full Type Diff

diff -r types/generated-snapshot/index.d.ts bazel-bin/types/definitions/index.d.ts
266c266,271
<   abstract class Module {
---
>   interface CompileOptions {
>     builtins?: string[];
>     importedStringConstants?: string;
>   }
>   class Module {
>     constructor(bytes: BufferSource, options?: CompileOptions);
283a289,301
>   interface WebAssemblyInstantiatedSource {
>     instance: Instance;
>     module: Module;
>   }
>   function compile(
>     bytes: BufferSource,
>     options?: CompileOptions,
>   ): Promise<Module>;
>   function instantiate(
>     bytes: BufferSource,
>     imports?: Imports,
>     options?: CompileOptions,
>   ): Promise<WebAssemblyInstantiatedSource>;
diff -r types/generated-snapshot/index.ts bazel-bin/types/definitions/index.ts
266c266,271
<   abstract class Module {
---
>   interface CompileOptions {
>     builtins?: string[];
>     importedStringConstants?: string;
>   }
>   class Module {
>     constructor(bytes: BufferSource, options?: CompileOptions);
283a289,301
>   interface WebAssemblyInstantiatedSource {
>     instance: Instance;
>     module: Module;
>   }
>   function compile(
>     bytes: BufferSource,
>     options?: CompileOptions,
>   ): Promise<Module>;
>   function instantiate(
>     bytes: BufferSource,
>     imports?: Imports,
>     options?: CompileOptions,
>   ): Promise<WebAssemblyInstantiatedSource>;