Address potential races in fs.read and fs.seek operations

[oleiade]

What?

This Pull Request aims to address #3354, and avoid data races involved in using the currently proposed implementation of fs.read and fs.seek.

Why?

Namely, it modifies the behavior of the proposed fs.read and fs.seek methods to avoid data races due to:

1. accessing the fs.read buffer argument concurrently
2. updating the file offset concurrently when calling seek

In a nutshell, we addressed 1 by ensuring that the provided buffer was only modified from the JS runtime's main thread, by performing it as a side-effect of the promise resolution's. We addressed 2 by making sure the internal file structure's offset is an atomic int that cannot, as a result, be modified by two concurrent operations at the same time.

Checklist

• I have performed a self-review of my code.
• I have added tests for my changes.
• I have run linter locally (make lint) and all checks pass.
• I have run tests locally (make tests) and all tests pass.
• I have commented on my code, particularly in hard-to-understand areas.

Related PR(s)/Issue(s)

ref #3309
ref #2974
closes #3354

[codecov-commenter]

Codecov Report

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Additional details and impacted files

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##             experimental/fs-readseek    #3420   +/-   ##
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[codebien]

LGTM, added a comment regarding f.size usage polish, and to make explicit the assumption we are making regarding the concurrent use of Read and Seek.

[codebien]

LGTM, mostly waiting for the TODO comment as expressed on #3420 (comment)

[mstoykov]

I have left some none blocking comments

[oleiade]

Created issue #3433 to track the issue related to operations not being treated sequentially (cc @codebien ) 👍🏻