feat(Stats): recursively get warnings or errors of a stats and its children

[harrisoff]

What kind of change does this PR introduce?

a feature

Did you add tests for your changes?

yes

Does this PR introduce a breaking change?

no

What needs to be documented once your changes are merged?

add getWarnings() and getErrors() methods to Stats object API

[jsf-clabot]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

[alexander-akait]

What is use case?

[harrisoff]

https://github.com/harrisoff/move-assets-webpack-plugin/blob/master/test/index.test.js#L210

I copied function webpack() from clean-webpack-plugin as part of my test codes.

function webpack(options: Configuration = {}) {
    const webpackActual = require('webpack');

    // https://webpack.js.org/concepts/mode/
    if (
        options.mode === undefined &&
        options.mode !== null &&
        webpackMajor !== null &&
        webpackMajor >= 4
    ) {
        // eslint-disable-next-line no-param-reassign
        options.mode = 'development';
    }

    if (options.mode === null) {
        // eslint-disable-next-line no-param-reassign
        delete options.mode;
    }

    const compiler = webpackActual(options);

    const runAsync = () =>
        new Promise((resolve, reject) => {
            compiler.run((error: Error, stats: Stats) => {
                if (error || stats.hasErrors()) {
                    reject(error);

                    return;
                }

                resolve(stats);
            });
        });

    return { ...compiler, run: runAsync };
}

And in my test codes, I used copy-webpack-plugin:

try {
  const options = {
    plugins: [
      new CopyPlugin({
        patterns: [
          {
            from: fePublicPath,
            to: feDistPath,
            globOptions: {
              ignore: [
                'index.html',
                {
                  glob: 'index.html',
                  matchBase: false,
                },
              ],
            },
          },
        ],
      }),
    ]，
  }
  const stats = await webpack(config).run()
} catch (error) {
  console.error(error)
}

globOptions.ignore should be an array of strings, but I set an object by mistake.

stats.hasErrors() is true so runAsync() throws an error, however outside the webpack() function I could only get a null.

stats.hasErrors() returns true not only when this stats encountered errors, but also its children. So I think maybe there should be a method to return them.

[webpack-bot]

For maintainers only:

• This needs to be documented (issue in webpack/webpack.js.org will be filed when merged)
• This needs to be backported to webpack 4 (issue will be created when merged)

[webpack-bot]

@ The most important CI builds failed. This way your PR can't be merged.

Please take a look at the CI results from azure (2 errors / 1 warnings) and appveyor (failure) and fix these issues.