[Bug] Text not appended to a file in a `mock-fs` filesystem when writing to a WritableStream created with the `a` flag

[Arc8ne]

New text does not get appended to a file in a mock-fs filesystem when the write() function of a writable stream created using the following code is called:

let fileWriteStream = fs.createWriteStream(
    "file",
    {
        flags: "a"
    }
);

The following snippet of code (originally part of a Jest test script) can be used to reproduce this bug:

const fs = require("fs");

const { test, expect } = require("@jest/globals");

const mock = require("mock-fs");

function cleanUp()
{
    mock.restore();
}

test(
    "Append to a file in a 'mock-fs' filesystem",
    async () =>
    {
        mock(
            {
                "test.log": "[Block 1]"
            }
        );

        let testLogFileWriteStream = fs.createWriteStream(
            "test.log",
            {
                flags: "a"
            }
        );

        testLogFileWriteStream.write("[Block 2]");

        let textInTestLogFile = await fs.promises.readFile(
            "test.log",
            {
                encoding: "utf-8"
            }
        );

        try
        {
            expect(textInTestLogFile).toEqual("[Block 1][Block 2]"); // Here, the 'test.log' file contains its initial text (i.e. '[Block 1]') but does not contain the text appended to it (i.e. '[Block 2]') causing this assertion to fail.
        }
        finally
        {
            cleanUp();
        }
    }
);

Is there a way to resolve this bug?

[tschaub]

In #412, I added a test demonstrating the use of fs.createWriteStream() with the append flag:

mock-fs/test/lib/fs.createWriteStream.spec.js

Lines 38 to 54 in d3c8959

	it('can be used with the append flag', function (done) {
	const filename = 'existing';
	const output = fs.createWriteStream(filename, {flags: 'a'});
	output.on('close', function () {
	fs.readFile(filename, function (err, data) {
	if (err) {
	return done(err);
	}
	assert.equal(String(data), 'contentmorecontent');
	done();
	});
	});
	output.on('error', done);
	output.write(Buffer.from('more'));
	output.end(Buffer.from('content'));
	});

One difference is that the added test calls end on the write stream. Another difference is that it doesn't use Jest. Does your test pass if you call end on the write stream?

[Arc8ne]

Thanks for letting me know about that. The test passes once I read the content of the file after the write stream's close event is fired.