Memory leak in watch mode due to unique child compiler IDs

[helloitsjoe]

Current behaviour 💣

During watch mode, each new child compiler creates a copy of source strings, which are never removed from memory.

I think I've narrowed this down to this commit aa64b82 where HtmlWebpackPlugin started using a unique ID for each child compiler instead of the template path. This cache in MemoryCachePlugin uses the unique ID when storing an entry, so the cache entries grow with each compilation.

I don't see any mention of the change in the changelog or PR description (😄) so I'm not sure why the change was made, but id doesn't seem to be used anywhere else in HtmlWebpackPlugin. Let me know if there's an explanation I'm missing somewhere!

Potential fix 🛠️

The memory leak goes away when aa64b82 is reverted to use template, and unit tests still pass. Is it possible to revert this change, or if it's solving a problem, to implement it in a different way? I'm happy to help.

Screenshots

Duplicated strings in memory

The size of the leak is small in this example, but the reproduction repo below shows an example of a much larger leak.

MemoryCachePlugin cache

Note the HtmlWebpackPlugin_0-15 - this is the 15th iteration i.e. 15th child compiler.

Expected behaviour ☀️

Source strings are cleaned up (or able to be garbage collected) between recompilations in watch mode

Reproduction Example 👾

The main branch of this repo includes a custom loader that makes the leak obvious: https://github.com/helloitsjoe/webpack-cache-memory-leak

But the leak is also visible on this simplified branch with vanilla use of HtmlWebpackPlugin: https://github.com/helloitsjoe/webpack-cache-memory-leak/tree/simplified

Environment 🖥

Node.js v20.5.1
darwin 22.5.0
npm 9.8.0
webpack@5.89.0
html-webpack-plugin@5.5.4

[helloitsjoe]

I've created a pull request here: #1836