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`).

3 files changed, 118 insertions, 18 deletions

diff --git a/src/login/app.rs b/src/login/app.rs
index c1bfe6e..6da26e9 100644
--- a/src/login/app.rs
+++ b/src/login/app.rs
@@ -3,13 +3,12 @@ use sqlx::sqlite::SqlitePool;
 use super::repo::Provider as _;
 
 #[cfg(test)]
-use super::{validate, Login, Password};
-#[cfg(test)]
-use crate::{
-    clock::DateTime,
-    event::{repo::Provider as _, Broadcaster, Event},
-    name::Name,
+use super::{
+    create::{self, Create},
+    Login, Password,
 };
+#[cfg(test)]
+use crate::{clock::DateTime, event::Broadcaster, name::Name};
 
 pub struct Logins<'a> {
     db: &'a SqlitePool,
@@ -35,19 +34,14 @@ impl<'a> Logins<'a> {
         password: &Password,
         created_at: &DateTime,
     ) -> Result<Login, CreateError> {
-        if !validate::name(name) {
-            return Err(CreateError::InvalidName(name.clone()));
-        }
-
-        let password_hash = password.hash()?;
+        let create = Create::begin(name, password, created_at);
+        let validated = create.validate()?;
 
         let mut tx = self.db.begin().await?;
-        let created = tx.sequence().next(created_at).await?;
-        let login = tx.logins().create(name, &password_hash, &created).await?;
+        let stored = validated.store(&mut tx).await?;
         tx.commit().await?;
 
-        self.events
-            .broadcast(login.events().map(Event::from).collect::<Vec<_>>());
+        let login = stored.publish(self.events);
 
         Ok(login.as_created())
     }
@@ -67,7 +61,17 @@ pub enum CreateError {
     #[error("invalid login name: {0}")]
     InvalidName(Name),
     #[error(transparent)]
-    Database(#[from] sqlx::Error),
-    #[error(transparent)]
     PasswordHash(#[from] password_hash::Error),
+    #[error(transparent)]
+    Database(#[from] sqlx::Error),
+}
+
+#[cfg(test)]
+impl From<create::Error> for CreateError {
+    fn from(error: create::Error) -> Self {
+        match error {
+            create::Error::InvalidName(name) => Self::InvalidName(name),
+            create::Error::PasswordHash(error) => Self::PasswordHash(error),
+        }
+    }
 }
diff --git a/src/login/create.rs b/src/login/create.rs
new file mode 100644
index 0000000..693daaf
--- /dev/null
+++ b/src/login/create.rs
@@ -0,0 +1,95 @@
+use sqlx::{sqlite::Sqlite, Transaction};
+
+use super::{password::StoredHash, repo::Provider as _, validate, History, Password};
+use crate::{
+    clock::DateTime,
+    event::{repo::Provider as _, Broadcaster, Event},
+    name::Name,
+};
+
+pub struct Create<'a> {
+    name: &'a Name,
+    password: &'a Password,
+    created_at: &'a DateTime,
+}
+
+impl<'a> Create<'a> {
+    #[must_use = "dropping a login creation attempt is likely a mistake"]
+    pub fn begin(name: &'a Name, password: &'a Password, created_at: &'a DateTime) -> Self {
+        Self {
+            name,
+            password,
+            created_at,
+        }
+    }
+
+    #[must_use = "dropping a login creation attempt is likely a mistake"]
+    pub fn validate(self) -> Result<Validated<'a>, Error> {
+        let Self {
+            name,
+            password,
+            created_at,
+        } = self;
+
+        if !validate::name(name) {
+            return Err(Error::InvalidName(name.clone()));
+        }
+
+        let password_hash = password.hash()?;
+
+        Ok(Validated {
+            name,
+            password_hash,
+            created_at,
+        })
+    }
+}
+
+pub struct Validated<'a> {
+    name: &'a Name,
+    password_hash: StoredHash,
+    created_at: &'a DateTime,
+}
+
+impl<'a> Validated<'a> {
+    #[must_use = "dropping a login creation attempt is likely a mistake"]
+    pub async fn store<'c>(self, tx: &mut Transaction<'c, Sqlite>) -> Result<Stored, sqlx::Error> {
+        let Self {
+            name,
+            password_hash,
+            created_at,
+        } = self;
+
+        let created = tx.sequence().next(created_at).await?;
+        let login = tx.logins().create(name, &password_hash, &created).await?;
+
+        Ok(Stored { login })
+    }
+}
+
+pub struct Stored {
+    login: History,
+}
+
+impl Stored {
+    #[must_use = "dropping a login creation attempt is likely a mistake"]
+    pub fn publish(self, events: &Broadcaster) -> History {
+        let Self { login } = self;
+
+        events.broadcast(login.events().map(Event::from).collect::<Vec<_>>());
+
+        login
+    }
+
+    pub fn login(&self) -> &History {
+        &self.login
+    }
+}
+
+#[derive(Debug, thiserror::Error)]
+pub enum Error {
+    #[error("invalid login name: {0}")]
+    InvalidName(Name),
+    #[error(transparent)]
+    PasswordHash(#[from] password_hash::Error),
+}
diff --git a/src/login/mod.rs b/src/login/mod.rs
index 6d10e17..5a6d715 100644
--- a/src/login/mod.rs
+++ b/src/login/mod.rs
@@ -1,4 +1,5 @@
 pub mod app;
+pub mod create;
 pub mod event;
 mod history;
 mod id;
@@ -6,7 +7,7 @@ pub mod password;
 pub mod repo;
 mod routes;
 mod snapshot;
-pub mod validate;
+mod validate;
 
 pub use self::{
     event::Event, history::History, id::Id, password::Password, routes::router, snapshot::Login,