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[Help-librejs] Webpack plugin proposal to generate Web Labels page

From: Antoine Lambert
Subject: [Help-librejs] Webpack plugin proposal to generate Web Labels page
Date: Mon, 4 Mar 2019 18:34:04 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.5.1

Dear all,

I am currently working on the web application enabling to browse the
Software Heritage archive (
I wanted to add LibreJS compliance for this web application but as I
am using webpack to generate the JavaScript assets, I had a doubt whether
it was feasible or not.

Using a tool like webpack is quite common nowadays as it really simplify
frontend web development, notably:

 - it enables to organize frontend code into logical modules

 - it can generate multiple JavaScript assets bundling multiple source files
   in them

 - it can consume any type of file format that get compiled to JavaScript
   (for instance Typescript or CoffeeScript)

 - it can transpile JavaScript code written in ES6 standard to ES5 one

So when using a tool like webpack, each JavaScript asset generated with it
consists in a file bundling multiple source files (which can be minified for
production use). Those bundles usually contain the content of  JavaScript source
files retrieved from the npm registry using a package manager like npm or yarn.
The licenses of those source files are usually compatible with those allowed
by LibreJS (MIT/Expat being the most used in the JavaScript ecosystem).

One of the strength of webpack is that it enables to write plugins to perform
a variety of tasks related to frontend development. Notably, it is possible to
add extra processing after a whole webpack compilation has been executed.
Such a plugin receives the webpack compilation statistics as input. Those 
are of interest here as it is quite easy to extract from them the list of all 
source files in each generated JavaScript assets.

When reading the current LibreJS documentation, Section 7.1.1 "Specifying 
license  files for a single JavaScript file" rings a bell to me. As it is quite
easy to find the license(s) associated to each JavaScript source file retrieved 
npm/yarn (by parsing the SPDX license expression in the associated package.json 
I thought writing a webpack plugin that automatically generates a Web Labels 
to be consumed by LibreJS could be an interesting solution here.

If I understand correctly, in order to be compliant with the LibreJS 
the Web Labels page must contain the following information. For each JavaScript 
generated by webpack, all bundled source files in it need to be referenced 
along with
their license(s) but also a link to their non-minified source code.

So I have implemented a webpack plugin processing the statistics available
after the whole webpack compilation that currently does the following:

  - For each bundled JavaScript source file, it will try to find its associated 
    compatible license by parsing the corresponding SPDX License Expression
    (, usually located in `package.json` files
  - It copies all original source files (non-minified) bundled into the 
generated JavaScript
    assets, into a directory located into the webpack output folder. Also if a 
license file
    can be found for a source file it will also be copied.
  - It generates either a Web Labels HTML page named `jslicenses.html` or a 
JSON file named
    `jslicenses.json`, into a directory located into the webpack output folder. 
The JSON file
    should then be used with an HTML template engine to generate the Web Labels 

You can find the detail of its implementation here [1] and the resulting 
generated Web Labels
table here [2]. The good news is that LibreJS successfully detected the 
licenses of bundled
source files and do not block the loading of webpack generated JavaScript 

However, I have the feeling that LibreJS specifications should better handle 
the "multiple
licenses for a single JavaScript file" case.

For instance, if a compatible license can not be found for a particular bundled 
source file,
current LibreJS will not block the loading of the associated JavaScript asset 
if there at
least one compatible license available in the Web Labels table. In other words, 
if any of
the bundled source files do not have any license information in the generated 
Web Labels,
corresponding webpack generated JavaScript asset loading should be blocked. 
Maybe the following
update to Web Labels specifications could work. In case of a JavaScript asset 
bundling multiple
source files, the content of the Licenses and Sources columns could be turned 
into tables.
Both tables should have the exact same number of rows in order to precisely 
match for each
bundled source file its corresponding license. If a license is missing, the 
asset loading
should then be blocked.

Another issue I stumbled across when implementing and testing the plugin is the 
licenses combination case. SPDX license expressions allow complex licenses 
using AND/OR keyword. For instance, the following is a valid SPDX license 
(LGPL-2.1 OR (BSD-3-Clause AND MIT)). It is not clear how LibreJS can handle 
this at
the moment. However, such cases are pretty rare.

Apart the issues mentionned above, the webpack plugin should work great on a 
majority of projects.
Currently, its implementation lives in a dedicated folder of the swh-web 
repository but I intend
to create a dedicated one for it and publish it on npm as other webpack users 
could benefit
from it.

What do you think of the proposed approach ? I will be happy to go further on 
the subject and
keep its implementation synchronized with any LibreJS specifications update.

Best regards,

Antoine Lambert


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