File blob is not invalidated if the file was initially empty then written to

[Jamesernator]

Version

v19.8.1

Platform

Linux 5.19.0-35-generic #36~22.04.1-Ubuntu SMP PREEMPT_DYNAMIC Fri Feb 17 15:17:25 UTC 2 x86_64 x86_64 x86_64 GNU/Linux

Subsystem

fs

What steps will reproduce the bug?

The following should produce an exception when reading the blob, but currently doesn't:

import fs from "node:fs";

// Create an empty file
fs.writeFileSync("./test.txt", "");

const blob = await fs.openAsBlob("./test.txt");

// write to the file, which should invalidate the blob
fs.writeFileSync("./test.txt", "foobar");

// This should throw an error, but returns the empty string instead
const result = await blob.text();

console.log(JSON.stringify(result)); 

How often does it reproduce? Is there a required condition?

This happens consistently.

What is the expected behavior? Why is that the expected behavior?

It should throw a DOMException, this works fine if the file was initially not empty, i.e. this throws an error as expected:

import fs from "node:fs";

// Create an empty file
fs.writeFileSync("./test.txt", "NOT_EMPTY");

const blob = await fs.openAsBlob("./test.txt");

// write to the file, which should invalidate the blob
fs.writeFileSync("./test.txt", "foobar");

// This should throw an error, but returns the empty string instead
const result = await blob.text();

console.log(JSON.stringify(result)); 

What do you see instead?

No exception is thrown.

Additional information

The Chrome behaviour for empty files being read after modification is the same as for non-empty files — i.e. an error is thrown in all cases.

(Curiously the Firefox behaviour allows reading the latest contents of the file regardless of modification status, however this seems buggy as the blob .size never actually changes).

[bnoordhuis]

It's probably because of this check (.text() calls .arrayBuffer()):

node/lib/internal/blob.js

Lines 269 to 271 in 0fd14e4

	if (this.size === 0) {
	return PromiseResolve(new ArrayBuffer(0));
	}

And it's arguably wrong because special files (like files under /proc on linux) are often "zero-sized" while actually containing readable data.

There's another instance of that pattern inside the stream() method:

node/lib/internal/blob.js

Lines 319 to 323 in 0fd14e4

	if (this.size === 0) {
	return new lazyReadableStream({
	start(c) { c.close(); },
	});
	}

Pull request welcome.