feat: This feature is to create an eslintcache that can be shared across developers or into ci machines. (#16493)

Author: cs6cs6

designs/2023-relative-cache/README.md

@@ -0,0 +1,77 @@
+ Repo: eslint/eslint
+- Start Date: 2023-02-14
+- RFC PR: <https://github.com/eslint/rfcs/pull/106>
+- Authors: Christian Schulz (@cschulz)i and cs6cs6
+
+# Relative cache location strategy
+
+## Summary
+
+To be able to share the created eslint cache between different systems f.e. local machine and CI, there is a need for a relative path stored in the cache.
+
+## Motivation
+
+The overall motivation for doing this is performance during the eslint process.
+
+Most people are running eslint on local developer machine and additional on the CI workers.
+In high changing repositories there are several pull requests at the same time changing different files requiring linting all the time.
+This is a time consuming process depending on the enabled rules like type checking rules.
+
+If the CI worker is able to cache its last run, it can just pick this cache up and use it.
+Also developers can reuse this cache or commit it as part of a repository for the CI.
+
+## Detailed Design
+
+I have a branch with the following design already. It is not fully tested, so I'm not 100% sure this will be the final implementation but I think it is a good start. 
+
+The LintResultCache takes a file path as parameter to find or store a file cache entry.
+
+One approach would be to truncate the given absolute path of the file by the path of the cache location.
+
+### Adding the command line parameter
+- conf/default-cli-options.js: add the property 'shearableCache' with a default of false. It should be put in the section with the other cache variables, below cacheStrategy. 
+- docs/src/use/command-line-interface.md: Add an explanation of the 'shareable-cache' variable with the other cache variables: "By default, an eslintcache contains full file paths and thus cannot readily be shared across developers or ci machines. "False" is that default. "True" changes the internal storage to store a relative path to the process execution directory, thus making the eslintcache shareable across developers and ci machines. . If you change this setting, you must regenerate your entire eslint cache."
+- eslint-helpers.js: add shareableCache variable (set to false) to the processOptions
+- lib/options.js: Add an option 'shareable-cache' of type Boolean with a nice description for people to read: By default, an eslintcache contains full file paths and thus cannot readily be shared across developers or ci machines. "False" is that default. "True" changes the internal storage to store a relative path to the process execution directory, thus making the eslintcache shareable across developers and ci machines. . If you change this setting, you must regenerate your entire eslint cache.
+
+
+### Changing cache file serialization
+- lib/eslint/flat-eslint.js: Define the shearableCache property at the top, with the other properties, as a boolean
+- lib/eslint/flat-eslint.js: Around line 873, where the flat-eslint.js file is saving the cache in the cachefile, if processOptions.shareableCache is set to true, pass a PARTIAL path to the lintResultCache.setCachedLintResults() function instead of the full path. 
+
+## Documentation
+
+Add the new CLI flag to the CLI documentation. The new cli flag will be called shareable-cache.
+
+## Drawbacks
+
+Adding a new CLI flag adds some complexity for the end user. They will need to understand why the feature exists. 
+
+Furthermore, defaulting to the existing behavior does help make upgrading less painful, but it is possible most people expect a shareable cache by default. This may cause confusion.
+
+## Backwards Compatibility Analysis
+
+The new flag, shareable-cache, passed in on the command line, will default to false. Unless people explicitly set it to true, then the old algorithms will
+be used. They will not need to regenerate their cache unless they set this flag. 
+
+## Alternatives
+
+A CLI tool which translates the existing cache to the current folder structure.
+
+## Open Questions
+
+No.
+
+## Help Needed
+
+I have a branch with code changes, but I am trying to figure out how to build an artifact to test with. Using the eslint.js artifact in the browser does not make sense
+with a cache change, and I would like to test it in my project. How can I build a package that I can install with NPM into my project to test?? I am able to run the tests
+on the eslintproject and they all pass. My fork is here: https://github.com/cs6cs6/eslint_patch/tree/fork/shareable-eslintcache
+
+## Frequently Asked Questions
+
+TBD
+
+## Related Discussions
+
+[Change Request: Eslintcache relative #16493](https://github.com/eslint/eslint/issues/16493)