Bug Report: fs.promises.watch() emits change before file is saved

[fuad303]

Version

v22.14.0

Platform

Linux(Debian)

Subsystem

No response

What steps will reproduce the bug?

import fs from "fs/promises";
(async () => {
  const fileHandler = await fs.open("./command.txt", "r");
  fileHandler.on("change", async () => {
    const size = (await fileHandler.stat()).size;
    const buff = Buffer.alloc(size);
    const offset = 0;
    const length = buff.byteLength;
    const position = 0;
    const cont = await fileHandler.read(buff, offset, length, position);
    console.log(cont);
  });
  //watcher
  const watcher = await fs.watch("./command.txt");
  for await (const event of watcher) {
    if (event.eventType === "change") {
      fileHandler.emit("change");
    }
  }
})();

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

It happens every time I edit the file quickly. Even a single character change is enough to trigger the issue. There's no special condition — just editing command.txt (for example, in VS Code or nano) will do it.

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

[Object: null prototype] { bytesRead: 4, buffer: <Buffer 73 68 69 74> }
[Object: null prototype] { bytesRead: 4, buffer: <Buffer 73 68 69 74> }
[Object: null prototype] { bytesRead: 4, buffer: <Buffer 73 68 69 74> }
[Object: null prototype] { bytesRead: 4, buffer: <Buffer 73 68 69 74> }
[Object: null prototype] { bytesRead: 4, buffer: <Buffer 73 68 69 74> }
[Object: null prototype] { bytesRead: 4, buffer: <Buffer 73 68 69 74> }

What do you see instead?

Instead we see something like this:

[Object: null prototype] { bytesRead: 4, buffer: <Buffer 73 68 69 74> }
[Object: null prototype] { bytesRead: 4, buffer: <Buffer 73 68 69 74> }
[Object: null prototype] { bytesRead: 0, buffer: <Buffer 73 68 69 74> }
[Object: null prototype] { bytesRead: 4, buffer: <Buffer 73 68 69 74> }
[Object: null prototype] { bytesRead: 4, buffer: <Buffer 73 68 69 74> }
[Object: null prototype] { bytesRead: 4, buffer: <Buffer 73 68 69 74> }
[Object: null prototype] { bytesRead: 0, buffer: <Buffer > }
[Object: null prototype] { bytesRead: 4, buffer: <Buffer 73 68 69 74> }

Additional information

No response

[bnoordhuis]

Converted to a discussion because what you're seeing is not a bug, just a quirk or maybe a mismatched expectation. Your editor saves the file in multiple discrete steps and node¹ picks up and reports the first one.

¹ Not really node either; the event is reported by the operating system - i.e., the canonical source of truth - to node, which then passes it on to you.