Create a dedicated workflow type for creating logins.

Nasty design corner. Logins need to be created in three places:

1. In tests, using app.logins().create(…);
2. On initial setup, using app.setup().initial(…); and
3. When accepting invites, using app.invites().accept(…).

These three places do the same thing with respect to logins, but also do a varying mix of other things. Testing is the simplest and _only_ creates a login. Initial setup and invite acceptance both issue a token for the newly-created login. Accepting an invite also invalidates the invite. Previously, those three functions have been copy-pasted variations on a theme. Now that we have validation, the copy-paste approach is no longer tenable; it will become increasingly hard to ensure that the three functions (plus any future functions) remain in synch.

To accommodate the variations while consolidating login creation, I've added a typestate-based state machine, which is driven by method calls:

* A creation attempt begins with `let create = Create::begin()`. This always succeeds; it packages up arguments used in later steps, but does nothing else.
* A creation attempt can be validated using `let validated = create.validate()?`. This may fail. Input validation and password hashing are carried out at this stage, making it potentially expensive.
* A validated attempt can be stored in the DB, using `let stored = validated.store(&mut tx).await?`. This may fail. The login will be written to the DB; the caller is responsible for transaction demarcation, to allow other things to take place in the same transaction.
* A fully-stored attempt can be used to publish events, using `let login = stored.publish(self.events)`. This always succeeds, and unwraps the state machine to its final product (a `login::History`).

1 files changed, 20 insertions, 13 deletions

diff --git a/src/invite/app.rs b/src/invite/app.rs
index 182eb67..d4e877a 100644
--- a/src/invite/app.rs
+++ b/src/invite/app.rs
@@ -5,8 +5,11 @@ use super::{repo::Provider as _, Id, Invite, Summary};
 use crate::{
     clock::DateTime,
     db::{Duplicate as _, NotFound as _},
-    event::{repo::Provider as _, Broadcaster, Event},
-    login::{repo::Provider as _, validate, Login, Password},
+    event::Broadcaster,
+    login::{
+        create::{self, Create},
+        Login, Password,
+    },
     name::Name,
     token::{repo::Provider as _, Secret},
 };
@@ -44,9 +47,7 @@ impl<'a> Invites<'a> {
         password: &Password,
         accepted_at: &DateTime,
     ) -> Result<(Login, Secret), AcceptError> {
-        if !validate::name(name) {
-            return Err(AcceptError::InvalidName(name.clone()));
-        }
+        let create = Create::begin(name, password, accepted_at);
 
         let mut tx = self.db.begin().await?;
         let invite = tx
@@ -59,23 +60,20 @@ impl<'a> Invites<'a> {
         // the invite. Final validation is in the next tx.
         tx.commit().await?;
 
-        let password_hash = password.hash()?;
+        let validated = create.validate()?;
 
         let mut tx = self.db.begin().await?;
         // If the invite has been deleted or accepted in the interim, this step will
         // catch it.
         tx.invites().accept(&invite).await?;
-        let created = tx.sequence().next(accepted_at).await?;
-        let login = tx
-            .logins()
-            .create(name, &password_hash, &created)
+        let stored = validated
+            .store(&mut tx)
             .await
             .duplicate(|| AcceptError::DuplicateLogin(name.clone()))?;
-        let secret = tx.tokens().issue(&login, accepted_at).await?;
+        let secret = tx.tokens().issue(stored.login(), accepted_at).await?;
         tx.commit().await?;
 
-        self.events
-            .broadcast(login.events().map(Event::from).collect::<Vec<_>>());
+        let login = stored.publish(self.events);
 
         Ok((login.as_created(), secret))
     }
@@ -105,3 +103,12 @@ pub enum AcceptError {
     #[error(transparent)]
     PasswordHash(#[from] password_hash::Error),
 }
+
+impl From<create::Error> for AcceptError {
+    fn from(error: create::Error) -> Self {
+        match error {
+            create::Error::InvalidName(name) => Self::InvalidName(name),
+            create::Error::PasswordHash(error) => Self::PasswordHash(error),
+        }
+    }
+}