WASM32 backend overrides global Worker

[mdekstrand]

I realize I'm probably operating in very unsupported territory here, but I was doing some experimenting with Sharp on WASM and wanted to report what I found in case the developers want to look at it in the future.

I am experimenting with using the WASM32 backend for Sharp in Deno, mostly to see if it works but also to reduce my use of native modules. Since Deno doesn't support fine-grained control of Node dependencies, I did this test by creating a modified Sharp package to remove all optional dependencies except the WASM32 backend, but the specific problem arises just from loading @img/sharp-wasm32/sharp.node.

Sharp correctly loads the WASM32 backend on Deno but the Enscripten-generated loader code assigns to global.Worker, overriding the Worker class with an incompatible one. The result is that code that uses web workers and loads @img/sharp-wasm32 will no longer work, as new Worker(...) creates an object that is not compatible with the Web Workers API.

Steps to reproduce:

1. Load @img/sharp-wasm32
2. Try to use Web Workers with new Worker (specifically, addEventListener is missing)

[lovell]

The WebAssembly build of sharp is compiled using ENVIRONMENT_IS_NODE as emscripten currently requires this to generate a Node.js compatible binary (with a global Worker defined, as you've seen).

sharp/src/binding.gyp

Line 194 in fc32e0b

'-sENVIRONMENT=node',

There's been some discussion around improving Deno support in emscripten at emscripten-core/emscripten#12203 and emscripten-core/emscripten#19359 so this is probably an upstream thing.