🐛 Bug: Watch mode (-w) does not re-run after a run did not find any tests

[stigok]

Prerequisites

• Checked that your issue hasn't already been filed by cross-referencing issues with the faq label
• Checked next-gen ES issues and syntax problems by using the same environment and/or transpiler configuration without Mocha to ensure it isn't just a feature that actually isn't supported in the environment in question or a bug in your code.
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Description

Starting mocha with -w watch argument does not make mocha re-run on file save when no tests were executed in the previous run.

Steps to Reproduce

1. Create a file test-test.js with contents describe('hello');
2. Run mocha -w test-test.js
3. Edit file and see mocha not re-running the test suite

Expected behavior: mocha -w re-runs the test suite when file is edited again

Actual behavior: mocha -w process hangs on the stats from the last run, failing to re-run after file has been edited

Reproduces how often: 100%

Versions

• The output of mocha --version and node node_modules/.bin/mocha --version: 6.0.2 and 6.0.2
• The output of node --version: v11.10.1
• Your operating system
  • name and version: Arch Linux 5.0.3-arch1-1-ARCH
  • architecture (32 or 64-bit): 64-bit
• Your shell (e.g., bash, zsh, PowerShell, cmd): bash
• Your browser and version (if running browser tests):
• Any third-party Mocha-related modules (and their versions):
• Any code transpiler (e.g., TypeScript, CoffeeScript, Babel) being used (and its version):

[plroebuck]

Your reproduction steps are too imprecise to be usable.

1. File "test-test.js" with just those contents contains no tests. Not sure what you were expecting since there's nothing for Mocha to run to begin with...
3. Edit file how? What change(s) did you make?

[stigok]

I'll try to be more precise. When I say update here, open up your editor and change the file contents.

1. Create a file bug-test.js with contents describe('hello world', () => it('works'));
2. Run mocha -w bug-test.js
3. Update the contents of bug-test.js with: describe('hello world', () => {});
4. Update the contents of bug-test.js with: describe('hello world', () => it('does not trigger mocha'));

[plroebuck]

Automated MCVE (UN*X-only)

$ cd /var/tmp
$ mkdir testwatch
$ cd testwatch
$ mkdir test bin
$ cat < EOF > "package.json"
{
  "name": "testwatch",
  "version": "1.0.0",
  "description": "Issue 3851",
  "main": "index.js",
  "devDependencies": {
    "mocha": "latest"
  },
  "directories": {
    "bin": "./bin",
    "test": "./test"
  },
  "scripts": {
    "test": "mocha",
    "test:watch": "mocha --watch"
  },
  "keywords": [
    "mocha"
  ],
  "author": "P. Roebuck <plroebuck@users.noreply.github.com>",
  "license": "ISC"
}
EOF
$ npm install
$ cat < EOF > "bin/run_test.sh"
#! /bin/sh
  
PROJROOT="/var/tmp/testwatch"
SPECFILE="${PROJROOT}/test/bug-test.spec.js"
#RUNDIR="/var/run"                   # user acct has no permission to write
RUNDIR="/tmp"
PIDFILE="${RUNDIR}/testwatch.pid"

## Modify test specification
function init_test_script {
  echo "describe('hello world', () => it('works'));" > ${SPECFILE}
}

function use_unmodified_test_script {
  echo "**** unmodified" >&2; \
  cat ${SPECFILE} >&2
}

function _use_empty_test_script {
  echo "describe('hello world', () => {});" > ${SPECFILE}
}

function use_empty_test_script {
  echo "**** replace with empty describe" >&2; \
  _use_empty_test_script; \
  cat ${SPECFILE} >&2
}

function _use_fixed_test_script {
  echo "describe('hello world', () => it('does not trigger mocha'));" > ${SPECFILE}
}

function use_fixed_test_script {
  echo "**** replace with valid describe" >&2; \
  _use_fixed_test_script; \
  cat ${SPECFILE} >&2
}

## Utility
function create_pidfile {
  echo "$(ps -ef | grep _mocha | grep -v grep | awk '{print $2}')" > ${PIDFILE}
}

function kill_mocha {
  pid=$(cat ${PIDFILE})
  kill ${pid}
}

function pause {
  sleep 10
}

function terminate {
  echo "**** done" >&2; \
  kill_mocha
}

## Cleanup
function cleanup {
  err=$?
  init_test_script
  trap '' EXIT INT QUIT TERM
  tput init
  exit $err
}

function sig_cleanup {
    trap '' EXIT # some shells will call EXIT after the INT handler
    false # sets $?
    cleanup
}
trap cleanup EXIT
trap sig_cleanup INT QUIT TERM

##
## Main
##
init_test_script

use_unmodified_test_script
(env DEBUG=* npm run test:watch &) && \
  sleep 5; create_pidfile && \
  pause; use_empty_test_script && \
  pause; use_fixed_test_script && \
  pause; terminate

EOF
$ chmod 755 bin/run_test.sh

Run test

$ cd /var/tmp/testwatch
$ bin/run_test.sh

[plroebuck]

I can reproduce your results, but I can't tell you whether it's by design or not at this point (don't use --watch myself).

[juergba]

Mocha's watch option is far from working perfectly, there are a few open issues about that.

I guess this bug has something to do with that Mocha loads its test files with require(), which means the files are loaded only ones and then cached. And the cached files don't change when the original file in the directory is modified.

[juergba]

@stigok do you have some time left for me?
Could you patch "lib\cli\run-helpers.js" aprox. ln 234:

• old line: const watchFiles = utils.files(cwd, extension);
• new line: const watchFiles = files.length ? files : utils.files(cwd, extension);

and give me some feedback, please?

EDIT: it's not working, yet ...

[stigok]

No, it made no difference.

[JoshuaKGoldberg]

This doesn't reproduce for me on Mocha 10.2.0. Hooray 🥳! Maybe #3912 happened to fix this?

If anybody is still seeing this, please file a new issue with one of our new issue templates. They'll prompt for the info needed to help us triage the new issue. Cheers all! 🤎