Motivation:
The `CNIOSHA1` module from SwiftNIO is an implementation detail. The fact that we were able to import and build the project with it is a bug/feature of Swift Package Manager.
Should the Swift ecosystem standardize on an implementation, that will replace the one we have chosen later.
Modifications:
Use the SHA1 native Swift implementation from CryptoSwift by reproducing the necessary code in our project.
Updates to the appropriate files to provide attribution to CryptoSwift have been included.
Result:
A reliable and pure Swift implementation of the SHA1 digest algorithm in the project for the sake of Lua script cache hashing.
### General usage
Most users should only need to use the `evalScript` method. We take the approach recommended by the [redis EVAL docs](https://redis.io/commands/eval):
>>>
The client library implementation can always optimistically send EVALSHA under the hood even when the client actually calls EVAL, in the hope the script was already seen by the server. If the NOSCRIPT error is returned EVAL will be used instead.
>>>
To facilitate this, we introduce a RedisScript struct, which calculates the sha1 hash on the client and stores it alongside the script source. I took inspiration for this approach from the [radix golang library](https://github.com/mediocregopher/radix). The struct makes using scripts a bit easier/nicer, and guides the developer toward the highest performance approach, but I'm curious whether you think it's an unnecessary abstraction.
One disadvantage of the optimistic approach using `EVALSHA` is that the `evalScript` may not be atomic if the script is not cached yet, which will be a problem if we are using it during an explicit pipeline or MULTI/EXEC operation. The redis docs recommend using `SCRIPT LOAD` before a multi/exec operation to avoid this. I think we could leave this up to the application developer.
### Should we implement the basic commands?
The approach above forces the application developer to use the evalScript method instead of exposing either `EVAL` or `EVALSHA` directly. This is to avoid the behavior of only using `EVAL` without caching, which might otherwise seem like the natural choice. If you think this is too opinionated, we can also add basic `eval` and `evalSha` methods.
### SHA1 calculation
I've copied the approach used by the SwiftNIO developers - RediStack already includes the `CNIOSHA1` module required.
There's a comment in [their private implementation](https://github.com/apple/swift-nio/blob/master/Sources/NIOWebSocket/SHA1.swift#L17) which indicates they're not entirely happy with this, but consider it the best approach for server-side Swift for now. I think we could follow their lead and migrate to a standard library method if/when it becomes available.
The `sha1` method is added as an extension property on String. Because it can theoretically fail, the `sha1` property is optional and thus the RedisScript initializer is failable, which is a bit unpleasant. I don't think the sha1 calculation should never actually fail given a String input. A force unwrap in the sha1 property would remove several code smells [like this](https://gitlab.com/liamdon/swift-redi-stack/blob/scripting/Sources/RediStack/Commands/ScriptingCommands.swift#L92), but I left it out in case you have a strong opposition to force unwraps - let me know what you think.
### Not implemented
I have not implemented `SCRIPT DEBUG` or `SCRIPT KILL`, because these seem more like sysadmin tasks that should not be in a client library. But they can easily be added if we want full coverage of the scripting commands.
Nathan Harris
Liam Don