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+//! PostgreSQL-backed persistence for Finito state machines
+//!
+//! This module implements ... TODO when I can write again.
+//!
+//! TODO: events & actions should have `SERIAL` keys
+
+#[macro_use]
+extern crate postgres;
+#[macro_use]
+extern crate postgres_derive;
+
+extern crate chrono;
+extern crate finito;
+extern crate r2d2_postgres;
+extern crate serde;
+extern crate serde_json;
+extern crate uuid;
+
+#[cfg(test)]
+mod tests;
+#[cfg(test)]
+extern crate finito_door;
+
+mod error;
+pub use error::{Error, ErrorKind, Result};
+
+use chrono::prelude::{DateTime, Utc};
+use error::ResultExt;
+use finito::{FSMBackend, FSM};
+use postgres::transaction::Transaction;
+use postgres::GenericConnection;
+use r2d2_postgres::{r2d2, PostgresConnectionManager};
+use serde::de::DeserializeOwned;
+use serde::Serialize;
+use serde_json::Value;
+use std::marker::PhantomData;
+use uuid::Uuid;
+
+type DBPool = r2d2::Pool<PostgresConnectionManager>;
+type DBConn = r2d2::PooledConnection<PostgresConnectionManager>;
+
+/// This struct represents rows in the database table in which events
+/// are persisted.
+#[derive(Debug, ToSql, FromSql)]
+struct EventT {
+    /// ID of the persisted event.
+    id: Uuid,
+
+    /// Timestamp at which the event was stored.
+    created: DateTime<Utc>,
+
+    /// Name of the type of FSM that this state belongs to.
+    fsm: String,
+
+    /// ID of the state machine belonging to this event.
+    fsm_id: Uuid,
+
+    /// Serialised content of the event.
+    event: Value,
+}
+
+/// This enum represents the possible statuses an action can be in.
+#[derive(Debug, PartialEq, ToSql, FromSql)]
+#[postgres(name = "actionstatus")]
+enum ActionStatus {
+    /// The action was requested but has not run yet.
+    Pending,
+
+    /// The action completed successfully.
+    Completed,
+
+    /// The action failed to run. Information about the error will
+    /// have been persisted in Postgres.
+    Failed,
+}
+
+/// This struct represents rows in the database table in which actions
+/// are persisted.
+#[derive(Debug, ToSql, FromSql)]
+struct ActionT {
+    /// ID of the persisted event.
+    id: Uuid,
+
+    /// Timestamp at which the event was stored.
+    created: DateTime<Utc>,
+
+    /// Name of the type of FSM that this state belongs to.
+    fsm: String,
+
+    /// ID of the state machine belonging to this event.
+    fsm_id: Uuid,
+
+    /// ID of the event that resulted in this action.
+    event_id: Uuid,
+
+    /// Serialised content of the action.
+    #[postgres(name = "content")] // renamed because 'action' is a keyword in PG
+    action: Value,
+
+    /// Current status of the action.
+    status: ActionStatus,
+
+    /// Detailed (i.e. Debug-trait formatted) error message, if an
+    /// error occured during action processing.
+    error: Option<String>,
+}
+
+// The following functions implement the public interface of
+// `finito-postgres`.
+
+/// TODO: Write docs for this type, brain does not want to do it right
+/// now.
+pub struct FinitoPostgres<S> {
+    state: S,
+
+    db_pool: DBPool,
+}
+
+impl<S> FinitoPostgres<S> {
+    pub fn new(state: S, db_pool: DBPool, _pool_size: usize) -> Self {
+        FinitoPostgres { state, db_pool }
+    }
+}
+
+impl<State: 'static> FSMBackend<State> for FinitoPostgres<State> {
+    type Key = Uuid;
+    type Error = Error;
+
+    fn insert_machine<S: FSM + Serialize>(&self, initial: S) -> Result<Uuid> {
+        let query = r#"
+          INSERT INTO machines (id, fsm, state)
+          VALUES ($1, $2, $3)
+        "#;
+
+        let id = Uuid::new_v4();
+        let fsm = S::FSM_NAME.to_string();
+        let state = serde_json::to_value(initial).context("failed to serialise FSM")?;
+
+        self.conn()?
+            .execute(query, &[&id, &fsm, &state])
+            .context("failed to insert FSM")?;
+
+        return Ok(id);
+    }
+
+    fn get_machine<S: FSM + DeserializeOwned>(&self, key: Uuid) -> Result<S> {
+        get_machine_internal(&*self.conn()?, key, false)
+    }
+
+    /// Advance a persisted state machine by applying an event, and
+    /// storing the event as well as all resulting actions.
+    ///
+    /// This function holds a database-lock on the state's row while
+    /// advancing the machine.
+    ///
+    /// **Note**: This function returns the new state of the machine
+    /// immediately after applying the event, however this does not
+    /// necessarily equate to the state of the machine after all related
+    /// processing is finished as running actions may result in additional
+    /// transitions.
+    fn advance<'a, S>(&'a self, key: Uuid, event: S::Event) -> Result<S>
+    where
+        S: FSM + Serialize + DeserializeOwned,
+        S::State: From<&'a State>,
+        S::Event: Serialize + DeserializeOwned,
+        S::Action: Serialize + DeserializeOwned,
+    {
+        let conn = self.conn()?;
+        let tx = conn.transaction().context("could not begin transaction")?;
+        let state = get_machine_internal(&tx, key, true)?;
+
+        // Advancing the FSM consumes the event, so it is persisted first:
+        let event_id = insert_event::<_, S>(&tx, key, &event)?;
+
+        // Core advancing logic is run:
+        let (new_state, actions) = finito::advance(state, event);
+
+        // Resulting actions are persisted (TODO: and interpreted)
+        let mut action_ids = vec![];
+        for action in actions {
+            let action_id = insert_action::<_, S>(&tx, key, event_id, &action)?;
+            action_ids.push(action_id);
+        }
+
+        // And finally the state is updated:
+        update_state(&tx, key, &new_state)?;
+        tx.commit().context("could not commit transaction")?;
+
+        self.run_actions::<S>(key, action_ids);
+
+        Ok(new_state)
+    }
+}
+
+impl<State: 'static> FinitoPostgres<State> {
+    /// Execute several actions at the same time, each in a separate
+    /// thread. Note that actions returning further events, causing
+    /// further transitions, returning further actions and so on will
+    /// potentially cause multiple threads to get created.
+    fn run_actions<'a, S>(&'a self, fsm_id: Uuid, action_ids: Vec<Uuid>)
+    where
+        S: FSM + Serialize + DeserializeOwned,
+        S::Event: Serialize + DeserializeOwned,
+        S::Action: Serialize + DeserializeOwned,
+        S::State: From<&'a State>,
+    {
+        let state: S::State = (&self.state).into();
+        let conn = self.conn().expect("TODO");
+
+        for action_id in action_ids {
+            let tx = conn.transaction().expect("TODO");
+
+            // TODO: Determine which concurrency setup we actually want.
+            if let Ok(events) = run_action(tx, action_id, &state, PhantomData::<S>) {
+                for event in events {
+                    self.advance::<S>(fsm_id, event).expect("TODO");
+                }
+            }
+        }
+    }
+
+    /// Retrieve a single connection from the database connection pool.
+    fn conn(&self) -> Result<DBConn> {
+        self.db_pool
+            .get()
+            .context("failed to retrieve connection from pool")
+    }
+}
+
+/// Insert a single state-machine into the database and return its
+/// newly allocated, random UUID.
+pub fn insert_machine<C, S>(conn: &C, initial: S) -> Result<Uuid>
+where
+    C: GenericConnection,
+    S: FSM + Serialize,
+{
+    let query = r#"
+      INSERT INTO machines (id, fsm, state)
+      VALUES ($1, $2, $3)
+    "#;
+
+    let id = Uuid::new_v4();
+    let fsm = S::FSM_NAME.to_string();
+    let state = serde_json::to_value(initial).context("failed to serialize FSM")?;
+
+    conn.execute(query, &[&id, &fsm, &state])?;
+
+    return Ok(id);
+}
+
+/// Insert a single event into the database and return its UUID.
+fn insert_event<C, S>(conn: &C, fsm_id: Uuid, event: &S::Event) -> Result<Uuid>
+where
+    C: GenericConnection,
+    S: FSM,
+    S::Event: Serialize,
+{
+    let query = r#"
+      INSERT INTO events (id, fsm, fsm_id, event)
+      VALUES ($1, $2, $3, $4)
+    "#;
+
+    let id = Uuid::new_v4();
+    let fsm = S::FSM_NAME.to_string();
+    let event_value = serde_json::to_value(event).context("failed to serialize event")?;
+
+    conn.execute(query, &[&id, &fsm, &fsm_id, &event_value])?;
+    return Ok(id);
+}
+
+/// Insert a single action into the database and return its UUID.
+fn insert_action<C, S>(conn: &C, fsm_id: Uuid, event_id: Uuid, action: &S::Action) -> Result<Uuid>
+where
+    C: GenericConnection,
+    S: FSM,
+    S::Action: Serialize,
+{
+    let query = r#"
+      INSERT INTO actions (id, fsm, fsm_id, event_id, content, status)
+      VALUES ($1, $2, $3, $4, $5, $6)
+    "#;
+
+    let id = Uuid::new_v4();
+    let fsm = S::FSM_NAME.to_string();
+    let action_value = serde_json::to_value(action).context("failed to serialize action")?;
+
+    conn.execute(query, &[
+        &id,
+        &fsm,
+        &fsm_id,
+        &event_id,
+        &action_value,
+        &ActionStatus::Pending,
+    ])?;
+
+    return Ok(id);
+}
+
+/// Update the state of a specified machine.
+fn update_state<C, S>(conn: &C, fsm_id: Uuid, state: &S) -> Result<()>
+where
+    C: GenericConnection,
+    S: FSM + Serialize,
+{
+    let query = r#"
+      UPDATE machines SET state = $1 WHERE id = $2
+    "#;
+
+    let state_value = serde_json::to_value(state).context("failed to serialize FSM")?;
+    let res_count = conn.execute(query, &[&state_value, &fsm_id])?;
+
+    if res_count != 1 {
+        Err(ErrorKind::FSMNotFound(fsm_id).into())
+    } else {
+        Ok(())
+    }
+}
+
+/// Conditionally alter SQL statement to append locking clause inside
+/// of a transaction.
+fn alter_for_update(alter: bool, query: &str) -> String {
+    match alter {
+        false => query.to_string(),
+        true => format!("{} FOR UPDATE", query),
+    }
+}
+
+/// Retrieve the current state of a state machine from the database,
+/// optionally locking the machine state for the duration of some
+/// enclosing transaction.
+fn get_machine_internal<C, S>(conn: &C, id: Uuid, for_update: bool) -> Result<S>
+where
+    C: GenericConnection,
+    S: FSM + DeserializeOwned,
+{
+    let query = alter_for_update(
+        for_update,
+        r#"
+      SELECT state FROM machines WHERE id = $1
+    "#,
+    );
+
+    let rows = conn
+        .query(&query, &[&id])
+        .context("failed to retrieve FSM")?;
+
+    if let Some(row) = rows.into_iter().next() {
+        Ok(serde_json::from_value(row.get(0)).context("failed to deserialize FSM")?)
+    } else {
+        Err(ErrorKind::FSMNotFound(id).into())
+    }
+}
+
+/// Retrieve an action from the database, optionally locking it for
+/// the duration of some enclosing transaction.
+fn get_action<C, S>(conn: &C, id: Uuid) -> Result<(ActionStatus, S::Action)>
+where
+    C: GenericConnection,
+    S: FSM,
+    S::Action: DeserializeOwned,
+{
+    let query = alter_for_update(
+        true,
+        r#"
+      SELECT status, content FROM actions
+      WHERE id = $1 AND fsm = $2
+    "#,
+    );
+
+    let rows = conn.query(&query, &[&id, &S::FSM_NAME])?;
+
+    if let Some(row) = rows.into_iter().next() {
+        let action =
+            serde_json::from_value(row.get(1)).context("failed to deserialize FSM action")?;
+        Ok((row.get(0), action))
+    } else {
+        Err(ErrorKind::ActionNotFound(id).into())
+    }
+}
+
+/// Update the status of an action after an attempt to run it.
+fn update_action_status<C, S>(
+    conn: &C,
+    id: Uuid,
+    status: ActionStatus,
+    error: Option<String>,
+    _fsm: PhantomData<S>,
+) -> Result<()>
+where
+    C: GenericConnection,
+    S: FSM,
+{
+    let query = r#"
+      UPDATE actions SET status = $1, error = $2
+      WHERE id = $3 AND fsm = $4
+    "#;
+
+    let result = conn.execute(&query, &[&status, &error, &id, &S::FSM_NAME])?;
+
+    if result != 1 {
+        Err(ErrorKind::ActionNotFound(id).into())
+    } else {
+        Ok(())
+    }
+}
+
+/// Execute a single action in case it is pending or retryable. Holds
+/// a lock on the action's database row while performing the action
+/// and writes back the status afterwards.
+///
+/// Should the execution of an action fail cleanly (i.e. without a
+/// panic), the error will be persisted. Should it fail by panicking
+/// (which developers should never do explicitly in action
+/// interpreters) its status will not be changed.
+fn run_action<S>(
+    tx: Transaction,
+    id: Uuid,
+    state: &S::State,
+    _fsm: PhantomData<S>,
+) -> Result<Vec<S::Event>>
+where
+    S: FSM,
+    S::Action: DeserializeOwned,
+{
+    let (status, action) = get_action::<Transaction, S>(&tx, id)?;
+
+    let result = match status {
+        ActionStatus::Pending => {
+            match S::act(action, state) {
+                // If the action succeeded, update its status to
+                // completed and return the created events.
+                Ok(events) => {
+                    update_action_status(&tx, id, ActionStatus::Completed, None, PhantomData::<S>)?;
+                    events
+                }
+
+                // If the action failed, persist the debug message and
+                // return nothing.
+                Err(err) => {
+                    let msg = Some(format!("{:?}", err));
+                    update_action_status(&tx, id, ActionStatus::Failed, msg, PhantomData::<S>)?;
+                    vec![]
+                }
+            }
+        }
+
+        _ => {
+            // TODO: Currently only pending actions are run because
+            // retryable actions are not yet implemented.
+            vec![]
+        }
+    };
+
+    tx.commit().context("failed to commit transaction")?;
+    Ok(result)
+}