Create SettingsDiff structure and run extractions based on it

[ManyTheFish]

Related product team resources: PRD (internal only)

⚠️ this issue depends on #4478 to be implemented

Summary

This issue is a subset of the work implementing the settings diff-indexing enhancement.

The current approach of Settings::execute is to trigger a full reindexing if at least one setting-change impacts the database.

Instead, we should create a structure SettingsDiff computing the differences between the old and the new version of the Settings, then send the SettingsDiff everywhere in the indexing process, allowing each part to react depending on the changes.

Additionally, the Documents shouldn't be sent to the database writer if there is no document addition/modification/deletion.

Structure example

struct SettingsDiff {
  before: &Settings { ... }
  after:  &Settings { ... }
}

impl SettingsDiff {
  fn process_searchable_pipeline(&self) -> bool {
    before.searchable_attributes != after.searchable_attributes
    || before.stop_words != after.stop_words
    || before.non_separator_tokens != after.non_separator_tokens // ⚠️ This can be a big vector of strings
    || before.separator_tokens != after.separator_tokens // ⚠️ This can be a big vector of strings
    || before.dictionary != after.dictionary // ⚠️ This can be a big vector of strings
    || before.disable_on_words != after.disable_on_words
    || before.disable_on_attributes != after.disable_on_attributes
  }
}

TODO

• Ensure Meilisearch has tests triggering the Settings::execute methods
• Create the SettingsDiff structure
• Make the indexer react to the SettingsDiff
  • Process the searchable pipeline only if needed
  • Process the facet pipeline only if needed
  • Process the vector pipeline only if needed
  • Skip documents database writing if only the settings have been changed

[ManyTheFish]

fixed by #4504