Handle the error properly when LESS source file is empty or having compilation error, and make the error message clear and clean.

[Hailong]

Description (*)

Sometimes developer may want to put a <css src=... reference in the layout xml file first to get it ready, while leave an empty LESS source file (or with some comments) there to work later on.

Now the setup:static-content:deploy command would fail and print a full callstack of exception, while the callstack doesn't really show the real problem. Moreover, the deployment is actually moving on, until it gets a PHP error that failed to open CSS file, which it should never try since its LESS source is empty already.

This fix is to address such issue, print a clear error message, and to avoid the final PHP open file error. It covers the following two conditions:

1. LESS file is empty.
2. LESS file has compilation errors.

Fixed Issues (if relevant)

1. Empty LESS source file causes setup:static-content:deploy command fail #22183: Empty LESS source file causes setup:static-content:deploy command fail

Manual testing scenarios (*)

1. Clone the 2.3-develop branch, and install a new site.
2. Add a LESS file to app/design/frontend/Magento/blank/web/css/test-empty.less, which has NO content or just some comments.
3. Add <css src="css/test-empty.css"/> to app/design/frontend/Magento/blank/Magento_Theme/layout/default_head_blocks.xml
4. Now run the setup:static-content:deploy command to compile.
5. We should be able to see the error message, and the compilation should complete with no other errors.

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[m2-assistant]

Hi @Hailong. Thank you for your contribution
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[josefbehr]

@Hailong great PR, thank you. Not sure why the travis build failed, error message had nothing to do with this change, I'll trigger the build again and approve.

[magento-engcom-team]

Hi @josefbehr, thank you for the review.
ENGCOM-5612 has been created to process this Pull Request
✳️ @josefbehr, could you please add one of the following labels to the Pull Request?

Label	Description
Auto-Tests: Covered	All changes in Pull Request is covered by auto-tests
Auto-Tests: Not Covered	Changes in Pull Request requires coverage by auto-tests
Auto-Tests: Not Required	Changes in Pull Request does not require coverage by auto-tests

[engcom-Alfa]

Hi @Hailong !

During testing we faced the issue

Problem: When LESS file is empty, compilation stops with error message

Steps to reproduce:

1. Clone the 2.3-develop branch;
2. Add a LESS file to app/design/frontend/Magento/blank/web/css/test-empty.less, which has NO content
3. Add <css src="css/test-empty.css"/> to app/design/frontend/Magento/blank/Magento_Theme/layout/default_head_blocks.xml
4. Now run the setup:static-content:deploy command to compile.

Actual Result:

@Hailong Is it correct message?

If we add some comments to test-empty.less then it will display correct message and continue compilation

@Hailong Could you take a look, please?

Thanks!

[Hailong]

Hi @engcom-Alfa , yes your first screenshot is the error message I want to remove with this pull request. While I think no matter there is comment in the empty file, we would get the first screenshot as I described in the issue report.

[engcom-Alfa]

Hi @Hailong , the error message from first screenshot occurs on branch with PR changes when test-empty.css file still empty, but if we add some comments to this file we got the result from second screenshot.

Could you take a look?

Thanks!

[Hailong]

Hi @engcom-Alfa , I have updated the code so that it prints the error message properly in both of these two cases.

[josefbehr]

@Hailong Thank you. It seems tests have to be adapted, too. Can you do this?

[Hailong]

Hi @josefbehr , I have adapted the tests accordingly.

[magento-engcom-team]

Hi @josefbehr, thank you for the review.
ENGCOM-5612 has been created to process this Pull Request

[engcom-Alfa]

✔️ QA Passed

[engcom-Foxtrot]

@magento run all tests

[m2-assistant]

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