--watch prints messages to stdout, but should print to stderr

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#56,478 4 comments 2 reactions 0 assignees View on GitHub

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Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
76/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Quiet
Tech stack
javascript
Domain
cli

Research direction

Start in lib/internal/main/watch_mode.js, especially the stdout use near the linked line, and run the reported node --watch index.mjs reproduction with stderr redirected. Done means watch-mode messages such as "Restarting 'index.mjs'" go to stderr so they are suppressed by 2>/dev/null and do not interfere with the program's stdout.

Written by the indexing model from the issue text.

Description

watch-mode
Version

v22.12.0

Platform
macOS (but reading the source code of Node.js the issue should be present on any platform)
Subsystem

node/lib/internal/main/watch_mode.js

What steps will reproduce the bug?
echo 'process.stdin.resume()' >index.mjs
node --watch index.mjs 2>/dev/null

In another terminal:

touch index.mjs
How often does it reproduce? Is there a required condition?

Always.

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

Nothing is printed, since I redirect stderr to /dev/null.

What do you see instead?

Restarting 'index.mjs' is printed (to stdout).

Additional information

How to use stdout and stderr is purely conventional, but the GNU C library says:

stdout: The standard output stream, which is used for normal output from the program.

stderr: The standard error stream, which is used for error messages and diagnostics issued by the program.

I’m making a program that reads input from stdin and writes to stdout. It is a long-lived program: Another program writes to stdin over time, and consumes output from stdout. I use --watch so I can iterate on the sub-program more easily. Unfortunately, --watch writes messages to stdout, causing the consuming program to receive output it does not expect.

I would argue that my program creates the “normal output” while the --watch messages are “diagnostics”.

Here’s one of the places where stdout is used instead of stderr: https://github.com/nodejs/node/blob/9400eae52ee10f25d63c06c52479c033e3149580/lib/internal/main/watch_mode.js#L139

Is there a reason for this? Should it be changed to stderr?

Dominant language
JavaScript
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