Change event is not emitted on Windows when continuously appending to an initially existing file

[bersbersbers]

Originally posted in microsoft/vscode#202446

Steps to Reproduce:

In the VS Code Terminal

1. echo 1 > output.log (and wait >1s)
2. code output.log
3. (for /l %f in () do @echo 1) >> output.log

File does not auto-reload until editor is re-focused.

This is similar to microsoft/vscode#93277, and I observe similar behavior in paulmillr/chokidar#1297.

I rarely use node, so I did not test reproduction using this code. However, bpasero wrote in microsoft/vscode#202446 (comment):

I am able to reproduce with the file watching library already, so please report this over at https://github.com/parcel-bundler/watcher

This is what I am doing here.

[bersbersbers]

I have reproduction steps, finally.

watcher.bat

@echo off
pushd "%~dp0"
set PATH=%PATH%;C:\Program Files\Microsoft Visual Studio\2022\Professional\MSBuild\Microsoft\VisualStudio\NodeJs

echo. > watcher.log

call npm i @parcel/watcher
start cmd /k node watcher.js

(for /l %%f in () do @echo %time%) >> watcher.log

@REM (for /l %%f in () do @echo %time% && timeout /t 1) >> watcher.log
@REM for /l %%f in () do (@echo %time% >> watcher.log)
@REM for /l %%f in () do ((@echo %time% >> watcher.log) && @echo %time%)
@REM for /l %%f in () do ((@echo %time% >> watcher.log) && @echo %time% && timeout /t 0)
@REM for /l %%f in () do ((@echo %time% >> watcher.log) && @echo %time% && timeout /t 1)

watcher.js

const watcher = require('@parcel/watcher');
watcher.subscribe(".", (err, events) => {
  console.log(events);
});

Interestingly, all those variants with @REM do not reproduce this problem, so there must be something very particular about piping the whole output at high rates.

[bersbersbers]

And even more interestingly, events start coming in as soon as I read the file in another process. E.g., it can be enough to run watcher.bat in one terminal, see that no events are seen at all, only to then see them come in after I run more watcher.log in another terminal. Very strange.

[bersbersbers]

It seems this module is using ReadDirectoryChangesW on Windows, which is known to miss events in high-activity scenarios. However, here it seems the module is missing all events, which makes me wonder if this is a different issue.

[bersbersbers]

And even more interestingly, events start coming in as soon as I read the file in another process.

Current workaround: a background process doing something like

monitor-logs.bat

start /min "Monitor log files..." cmd /k for /l %%i in () do @(for %%f in (C:\log\*.txt) do @dir /b %%f ^> nul)