Automatic key import / export + Expected monad

Summary:
There are many things going on here (perhaps a bit much for a single patch):

- When an attachment is of mime type "application/pgp-keys", a button is added to import the key to GPG
- When sending a mail and crypto is enabled (encryption, signing or both), the public key of the first private key found is sent as an un-encrypted attachment (T6994)
- The `mailcrypto.{h,cpp}` was, for the most part, rewritten
- Introduction of the expected monad, inspired by what was proposed for C++ [here](https://isocpp.org/files/papers/n4015.pdf), but not at all a strict implementation of this specification. We may want to add some more features of this standard later.

The rationale for some of the choices:

- I found mailcrypto a bit hard to edit to add new features, and a great part was commented code to prepare for the support the SMIME crypto format, which would (in my current knowledge) not be used for sending emails.
- One thing I found that may be missing in the code base was a standardized way of handling errors in C++ code. Since exceptions are disabled I think that the functional way is the way to go. After some research I found the Expected monad / tagged union / sum type, which seemed to suit the problem particularly well.

In the long run, I hope we would move the entire code base to use `Expected` to indicate if a function might fail.

Of course every choice made here is to be considered as a proposition for doing things / RFC, critics wholeheartedly accepted.

Reviewers: cmollekopf

Tags: #kube

Maniphest Tasks: T6994, T8147, T6995

Differential Revision: https://phabricator.kde.org/D11158

1 files changed, 17 insertions, 12 deletions

diff --git a/framework/src/domain/mime/mailcrypto.h b/framework/src/domain/mime/mailcrypto.h
index 0a6c2f4c..832f68ec 100644
--- a/framework/src/domain/mime/mailcrypto.h
+++ b/framework/src/domain/mime/mailcrypto.h
@@ -19,19 +19,24 @@
 
 #pragma once
 
-#include <QByteArray>
+#include "framework/src/errors.h"
+
 #include <KMime/Message>
 #include <gpgme++/key.h>
+
+#include <QByteArray>
+
 #include <functional>
+#include <memory>
+
+namespace MailCrypto {
+
+Expected<GpgME::Error, std::unique_ptr<KMime::Content>>
+processCrypto(std::unique_ptr<KMime::Content> content, const std::vector<GpgME::Key> &signingKeys,
+    const std::vector<GpgME::Key> &encryptionKeys, const GpgME::Key &attachedKey);
+
+std::vector<GpgME::Key> findKeys(const QStringList &filter, bool findPrivate = false, bool remote = false);
+
+void importKeys(const std::vector<GpgME::Key> &keys);
 
-namespace MailCrypto
-{
-    enum Protocol {
-        OPENPGP,
-        SMIME
-    };
-    KMime::Content *processCrypto(KMime::Content *content, const std::vector<GpgME::Key> &signingKeys, const std::vector<GpgME::Key> &encryptionKeys, MailCrypto::Protocol protocol);
-    KMime::Content *sign(KMime::Content *content, const std::vector<GpgME::Key> &signers);
-    std::vector<GpgME::Key> findKeys(const QStringList &filter, bool findPrivate = false, bool remote = false, Protocol protocol = OPENPGP);
-    void importKeys(const std::vector<GpgME::Key> &keys);
-};
+}; // namespace MailCrypto