The MIT license doesn't require that all files be prepended with this
licensing or copyright information. Realm confirmed that they're ok with this
change. This will enable some companies to contribute to SwiftLint and the
date & authorship information will remain accessible via git source control.
`DispatchQueue.sync` was required to avoid CoW on calling `subscript` from multiple copied instances of `Array`.
Since `UnsafeMutableBufferPointer` does not cause CoW on same situation, `DispatchQueue.sync` can be removed.
By applying concepts from Swift Talk 90: https://talk.objc.io/episodes/S01E90-concurrent-map
Notably:
* Removing `@escaping` from block parameter
* Avoiding Array.append & sorting
* Using a constant DispatchQueue label
to avoid having to lock calls to mutating functions that don't
actually perform mutating operations.
This fixes races seen "in the wild" causing crashes.
This has never worked for two reasons:
1. We've used dictionaries to represent cache descriptions, which
don't guarantee stable ordering of keys across invocations.
This is true both on Darwin and Linux, but in practice ordering
varies significantly more on Linux.
2. Storing a `TimeInterval` value in a `[String: Any]` dictionary
and retrieving it again will not be dynamically castable to
`Double` or `TimeInterval` but will be castable to `Int`.
also slightly speed up writing to the cache.
For example, on the Lyft codebase with 1,500 Swift files:
```bash
$ time swiftlint lint --quiet
swiftlint --quiet 3.53s user 0.27s system 388% cpu 0.979 total
$ rm -rf ~/Library/Caches/SwiftLint && time swiftlint lint --quiet
swiftlint --quiet 35.20s user 1.22s system 371% cpu 9.806 total
$ time swiftlint lint --quiet
swiftlint lint --quiet 0.90s user 0.13s system 218% cpu 0.472 total
$ rm -rf ~/Library/Caches/SwiftLint && time swiftlint lint --quiet
swiftlint lint --quiet 31.78s user 1.18s system 360% cpu 9.146 total
```
Marcelo Fabri
JP Simard
Daniel Metzing
Norio Nomura
Ash Furrow
Frederick Pietschmann
Cyril Lashkevich
Jamie Edge
Marcelo Fabri
Manabu Nakazawa
Noah McCann