Improve send message checks

[Kharchevskyi]

This PR extracts message validation and sending logic to ComposeMessageService.
Adds improvements to compose message validation

close #399

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Tests:

• Tests added

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To be filled by reviewers

I have reviewed that this PR... (tick whichever items you personally focused on during this review):

• addresses the issue it closes (if any)
• code is readable and understandable
• is accompanied with tests, or tests are not needed
• is free of vulnerabilities
• is documented clearly and usefully, or doesn't need documentation

[Kharchevskyi]

LGTM

[tomholub]

There's only one error scenario and one success scenario when pulling public keys, I think an if or a guard would be better suited then a switch.

[tomholub]

Isn't there some mechanism like

let recipientPublicKeys = await getPubKeys(for: recipients)

let replyToMimeMsg = input.replyToMime
   .flatMap { String(data: $0, encoding: .utf8) }

let msg = SendableMsg(
    text: text,
    to: recipients.map(\.email),
    cc: [],
    bcc: [],
    from: email,
    subject: subject,
    replyToMimeMsg: replyToMimeMsg,
    atts: atts,
    pubKeys: allRecipientPubs + [myPubKey]
)
return .success(msg)

That would re-propagate the error from getPubKeys automatically?

[Kharchevskyi]

getPubKeys returns Result<[String], MessageValidationError>, it can be success or failure cases with some associated values in it.
I can't use guard or if because I will miss one of 2 associated values

guard case .success(let allRecipientPubs) = getPubKeys(for: recipients) else {
           return ...(here I don't have associated error if getPubKeys fails)
}

It's possible to use do catch here, but I wouldn't recommend this solution, because we need to cast to some particular error in catch

do {
            try getPubKeys(for: recipients).get()
        } catch let error {
            (here it's Error type, not MessageValidationError)
            print(error)
        }

best way is to use Result<SendableMsg, ComposeMessageError>.Publisher and then in view controller we can chain validateMessage with encryptAndSend(message: sendableMessage) functions.
Please check latest commit.

[tomholub]

What does .mapError with .eraseToAnyPublisher do?

[Kharchevskyi]

func sendMail(mime: Data) -> Future<Void, Error> will return just Error for us. But we are expecting to have ComposeMessageError, so mapError in case of error will transform this to ComposeMessageError.gatewayError with associated error. either from Imap or Gmail api.

eraseToAnyPublisher - exposes an instance of AnyPublisher to the downstream subscriber, rather than this publisher’s actual type.
So without this it will have a return type as Publishers.MapError<Future<Void, Error>, ComposeMessageError>.
Please don't ask why)) Swift - 🤷
Type erasure is not the best in swift nowadays.
https://developer.apple.com/documentation/combine/just/erasetoanypublisher()

[tomholub]

It's possible for this to fail for various reasons. For example, try to run this with an invalid public key. It will produce an error, and this should be handled gratiously.

[Kharchevskyi]

Just "replayed" previous app behaviour.
Should I fix it in scope of this PR?

[tomholub]

Got it, let's make another PR then. There, we'll check exactly what happens when we give it some bad input.

[tomholub]

It's similarly weird to use map if we are talking about a single Future executing one time with a single failure or success. Map implies working with array of potentially many items, which seems a mismatch for what we're doing (even if this is otherwise common in iOS development).

I'll merge this but file a separate issue.