Use heic-convert npm package to process images

[glmaljkovich]

Spawn one child process per request and convert images sequentially, avoiding blocking the main thread.

Note that there are many possible ways to use child processes (worker.js instances) here:

• One worker per image
• A singleton worker shared across requests
• A worker pool.

In the end I ended up using one worker per request because I think it's flexible enough to scale with the server load but without unnecessarily straining the cpu like one worker per image would.

For a comparison of how those different approaches work (except for the worker pool) you can try the benchmark_test.js file on this branch

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closes #2

[glmaljkovich]

@boutell I could use some help testing this with apostrophe. I believe I have to:

• npm link the middleware to a copy of apostrophe that uses the middleware
• npm link that copy of apostrophe to a project made with the apostrophe cli

Correct me if I'm wrong.

[boutell]

This looks great to me! As for integration testing with apostrophe, the easiest way would be to npm link both this module and apostrophe in your project, and patch lib/modules/apostrophe-attachments/index.js to add this middleware as self.expressMiddleware in that module. I don't mind doing that test if you prefer.

The only "change" I'm requesting is that you update the documentation to address the change (no more need for tifig).

[boutell]

Nice!

Will this build on Windows? Any idea? Will it build if a compiler is not available on Linux?

If we're not sure about those things, or the answers aren't ideal, I can wrap it as an optional dependency in apostrophe, and we'll bump the major version of the middleware because the requirements will have changed. In practice most people will have a compiler available when installing npm packages.

[glmaljkovich]

Good questions. I was looking at the dependencies from heic-convert and the only use of code that isn't javascript only is this pre-compiled webassembly library: https://github.com/catdad-experiments/libheif-js/blob/master/scripts/install.js#L16

That being said I think it's reasonable to wrap it as an optional dependency as I haven't tested it on windows.

[boutell]

If it's prebuilt webassembly code windows should be fine. No need for optional dependency.