[fs] Add file.read and file.seek methods to the fs module (2/3)

[oleiade]

What?

This Pull Request builds (and is based) upon #3142 and adds a read and a seek method, the the File object exposed by the k6/experimental/fs module.

read

The File.read method allows to read a file into a Uint8Array. The amount of bytes read is equal to the size of the provided buffer.

The method resolves to either the number of bytes read during the operation or EOF (represented as null) if there is nothing more to read.

seek

The File.seek method allows to seek to a specified offset within a file under the given whence.

The offset is expressed in bytes. Whence allows you to seek from the start of the file (the offset needs to be strictly negative), the current position within the file (offset can be either positive or negative), or from the end of the file (offset needs to be strictly negative). A dedicated SeekMode type/enum is exposed to help with it.

The method resolves to the new offset in bytes after the seek operation.

In action

import { open } from "k6/experimental/fs";

export const options = {
	vus: 100,
	iterations: 1000,
};

// As k6 does not support asynchronous code in the init context, yet, we need to
// use a top-level async function to be able to use the `await` keyword.
let file;
(async function () {
	file = await open("bonjour.txt");
})();

export default async function () {
	// About information about the file
	const fileinfo = await file.stat();
	if (fileinfo.name != "bonjour.txt") {
		throw new Error("Unexpected file name");
	}

	// Define a buffer of the same size as the file
	// to read the file content into.
	const buffer = new Uint8Array(fileinfo.size);

	// Read the file's content into the buffer
	const bytesRead = await file.read(buffer);

	// Check that we read the expected number of bytes
	if (bytesRead != fileinfo.size) {
		throw new Error("Unexpected number of bytes read");
	}

	// Seek back to the beginning of the file
	await file.seek(0);
}

Checklist

• I have performed a self-review of my code.
• I have added tests for my changes.
• I have run linter locally (make ci-like-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 #3142
ref #2974
fixes #3145
fixes #3148

[codecov-commenter]

Codecov Report

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Additional details and impacted files

@@                Coverage Diff                 @@
##             experimental/fs    #3309   +/-   ##
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  Hits                       ?    14690           
  Misses                     ?     4460           
  Partials                   ?      942           

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[olegbespalov]

I did a round of reviewing, mostly looks good 👍 waiting for Mihail's comment to be addressed.

[olegbespalov]

Just for the record.

I'm OK with merging this and working on the fix in a separate PR. The tiny commit that could be done is to write the test case, which should catch the bug and t.Skip it for now.

My rationale is that merging this will put the focus on fixing the bug. And we still have plenty of time until the release; in the worst case, we could be transparent that a known issue exists. However, some of the users of this experiment already could try to use it and provide feedback.

[mstoykov]

LGTM!