Gracefully handle fatal webpack errors.

[bradleyayers]

Issue:

When a fatal webpack error occurs (e.g. wrong configuration) storybook's error handler incorrectly assumes that the stats object is provided by webpack. See https://webpack.js.org/api/node/#error-handling for details on error handling.

The effect is the callback throwing an exception at https://github.com/storybooks/storybook/blob/master/app/react/src/server/build.js#L86, with a message to the effect of 'undefined has no property hasErrors'.

This is a problem because some webpack loaders will run timers or open sockets that prevent the node process from exiting, and when the callback throws an error before reaching the process.exit(1); code which would kill it.

There have been cases where in continuous integration environments the build will run indefinitely (which is a problem when you pay per-second).

What I did

Added a simple check to ensure stats is passed, before using it.

How to test

Create a webpack configuration that causes a fatal error, and run storybook.

[codecov]

Codecov Report

Merging #1918 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1918   +/-   ##
=======================================
  Coverage   21.31%   21.31%           
=======================================
  Files         257      257           
  Lines        5737     5737           
  Branches      691      687    -4     
=======================================
  Hits         1223     1223           
+ Misses       3989     3982    -7     
- Partials      525      532    +7

Impacted Files	Coverage Δ
app/react/src/server/build.js	0% <ø> (ø)	⬆️
...ponents/left_panel/stories_tree/tree_decorators.js	33.33% <0%> (ø)	⬆️
lib/ui/src/modules/api/actions/api.js	50.47% <0%> (ø)	⬆️
addons/knobs/src/react/WrapStory.js	12% <0%> (ø)	⬆️
lib/ui/src/modules/ui/components/menu_item.js	19.14% <0%> (ø)	⬆️
app/react-native/src/bin/storybook-build.js	0% <0%> (ø)	⬆️
lib/ui/src/modules/ui/libs/filters.js	47.36% <0%> (ø)	⬆️
lib/ui/src/modules/ui/configs/handle_keyevents.js	33.33% <0%> (ø)	⬆️
lib/ui/src/modules/ui/containers/down_panel.js	23.52% <0%> (ø)	⬆️
addons/info/src/components/markdown/code.js	24.13% <0%> (ø)	⬆️
... and 14 more

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[ndelangen]

👌

[ndelangen]

Thank you @bradleyayers !!!

[Hypnosphi]

@ndelangen do we neet the same for app/vue?

[ndelangen]

Very good point! Yes we do.