fs: support special files in promises.readFile

[TimothyGu]

A size of 0 may indicate that the file is a Linux special file, which may still have content when read(). Instead of returning early, use the same approach as fs.readFile():

node/lib/internal/fs/read_file_context.js

Lines 74 to 88 in 6ced651

	if (this.size === 0) {
	buffer = this.buffer = Buffer.allocUnsafeSlow(kReadFileBufferLength);
	offset = 0;
	length = kReadFileBufferLength;
	} else {
	buffer = this.buffer;
	offset = this.pos;
	length = Math.min(kReadFileBufferLength, this.size - this.pos);
	}
	const req = new FSReqWrap();
	req.oncomplete = readFileAfterRead;
	req.context = this;
	read(this.fd, buffer, offset, length, -1, req);

Additionally, only the return code of 0 indicates end of file, and we should not use bytesRead !== chunkSize as a indication of EOF. Per POSIX:

The value returned may be less than nbyte if the number of bytes left in the file is less than nbyte, if the read() request was interrupted by a signal, or if the file is a pipe or FIFO or special file and has fewer than nbyte bytes immediately available for reading. For example, a read() from a file associated with a terminal may return one typed line of data.

Refs: http://pubs.opengroup.org/onlinepubs/9699919799/functions/read.html
Refs: https://groups.google.com/forum/#!topic/nodejs-dev/rxZ_RoH1Gn0
Fixes: #21331

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

[nodejs-github-bot]

@TimothyGu build started: https://ci.nodejs.org/blue/organizations/jenkins/node-test-pull-request-lite-pipeline/detail/node-test-pull-request-lite-pipeline/139/pipeline

[devsnek]

might wanna factor that number out into a constant

[TimothyGu]

Fixed.

[TimothyGu]

CI: https://ci.nodejs.org/job/node-test-pull-request/15584/

[joyeecheung]

Out of curiosity, is there context for this?

[TimothyGu]

Not really, it was there when I found it so I kept it that way. I just thought it was interesting to note.

[TimothyGu]

I guess we'd have to ask @jasnell about it.

[TimothyGu]

CI: https://ci.nodejs.org/job/node-test-pull-request/15726/ (linuxone passes after re-run)

[TimothyGu]

Landed in 5057dd4.