feat: persistent cache between runs (webpack@5 only)

[alexander-akait]

This PR contains a:

• bugfix
• new feature
• code refactor
• test update
• typo fix
• metadata update

Motivation / Use-Case

fixes #504

Breaking Changes

No

Additional Info

only webpack@5

[codecov]

Codecov Report

Merging #541 into master will decrease coverage by 2.04%.
The diff coverage is 84.61%.

@@            Coverage Diff             @@
##           master     #541      +/-   ##
==========================================
- Coverage   97.72%   95.68%   -2.05%     
==========================================
  Files           3        3              
  Lines         220      255      +35     
  Branches       73       86      +13     
==========================================
+ Hits          215      244      +29     
- Misses          5       11       +6     

Impacted Files	Coverage Δ
src/index.js	95.85% <84.61%> (-2.21%)	⬇️

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[sokra]

You can pass undefined here, that saves a bit memory.

[sokra]

checkSnapshotValid on a fresh created snapshot will always be true. You need to call checkSnapshotValid on a snapshot that was created in the last run of webpack. The snapshot need to be stored and restored in and from the cache.

Here is some pseudo code:

// 1. check cache first, validate snapshot in cache entry
cacheEntry = await cache.getPromise(file.relativeFrom, null);
if (cacheEntry !== undefined) {
  const { snapshot, source } = cacheEntry;
  if (await checkSnapshotValid(snapshot)) {
    // return early when cache entry can be used
    return source;
  }
}
// 2. create a new snapshot
const snapshot = await createSnapshot(file.relativeFrom);
// 3. create new data
const source = await createNewSource(file.relativeFrom);
// 4. store both in cache
await cache.storePromise(file.relativeFrom, null, { source, snapshot });
// 5. return new data
return source;

You are in the good position that you can call 2. before 3. Usually that's not possible, because dependencies are not known before execution. In that case one would do this:

// 2a.
const startTime = Date.now();
// 2b. create new data
const source = await createNewSource(file.relativeFrom);
// 3. create a new snapshot
const snapshot = await createSnapshot(file.relativeFrom, startTime);

[sokra]

https://nodejs.org/api/util.html#util_util_promisify_original

const checkSnapshotValid = util.promisify((compilation, snapshot, callback) =>
  compilation.fileSystemInfo.checkSnapshotValid(snapshot, callback)
);
const createSnapshot = util.promisify((compilation, dependency, callback) =>
  compilation.fileSystemInfo.createSnapshot(Date.now(), [dependency], undefined, undefined, null, callback)
);

[sokra]

Passing an undefined startTime, would assume worse case.