mentat/src/conn.rs

// Copyright 2016 Mozilla
//
// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
// this file except in compliance with the License. You may obtain a copy of the
// License at http://www.apache.org/licenses/LICENSE-2.0
// Unless required by applicable law or agreed to in writing, software distributed
// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
// CONDITIONS OF ANY KIND, either express or implied. See the License for the
// specific language governing permissions and limitations under the License.

#![allow(dead_code)]

use std::collections::HashMap;
use std::sync::{Arc, Mutex};

use rusqlite;

use edn;
use errors::*;
use mentat_core::{
    Schema,
    TypedValue,
};
use mentat_db::db;
use mentat_db::{
    transact,
    PartitionMap,
    TxReport,
};
use mentat_tx_parser;
use query::{
    q_once,
    QueryResults,
};


/// Connection metadata required to query from, or apply transactions to, a Mentat store.
///
/// Owned data for the volatile parts (generation and partition map), and `Arc` for the infrequently
/// changing parts (schema) that we want to share across threads.
///
/// See https://github.com/mozilla/mentat/wiki/Thoughts:-modeling-db-conn-in-Rust.
pub struct Metadata {
    pub generation: u64,
    pub partition_map: PartitionMap,
    pub schema: Arc<Schema>,
}

impl Metadata {
    // Intentionally not public.
    fn new(generation: u64, partition_map: PartitionMap, schema: Arc<Schema>) -> Metadata {
        Metadata {
            generation: generation,
            partition_map: partition_map,
            schema: schema,
        }
    }
}

/// A mutable, safe reference to the current Mentat store.
struct Conn {
    /// `Mutex` since all reads and writes need to be exclusive.  Internally, owned data for the
    /// volatile parts (generation and partition map), and `Arc` for the infrequently changing parts
    /// (schema) that we want to share across threads.  A consuming thread may use a shared
    /// reference after the `Conn`'s `Metadata` has moved on.
    ///
    /// The motivating case is multiple query threads taking references to the current schema to
    /// perform long-running queries while a single writer thread moves the metadata -- partition
    /// map and schema -- forward.
    metadata: Mutex<Metadata>,

    // TODO: maintain set of change listeners or handles to transaction report queues. #298.

    // TODO: maintain cache of query plans that could be shared across threads and invalidated when
    // the schema changes. #315.
}

impl Conn {
    // Intentionally not public.
    fn new(partition_map: PartitionMap, schema: Schema) -> Conn {
        Conn {
            metadata: Mutex::new(Metadata::new(0, partition_map, Arc::new(schema)))
        }
    }

    pub fn connect(sqlite: &mut rusqlite::Connection) -> Result<Conn> {
        let db = db::ensure_current_version(sqlite)
            .chain_err(|| "Unable to initialize Mentat store")?;
        Ok(Conn::new(db.partition_map, db.schema))
    }

    /// Yield the current `Schema` instance.
    pub fn current_schema(&self) -> Arc<Schema> {
        // We always unwrap the mutex lock: if it's poisoned, this will propogate panics to all
        // accessing threads.  This is perhaps not reasonable; we expect the mutex to be held for
        // very short intervals, but a panic during a critical update section is possible, since the
        // lock encapsulates committing a SQL transaction.
        //
        // That being said, in the future we will provide an interface to take the mutex, providing
        // maximum flexibility for Mentat consumers.
        //
        // This approach might need to change when we support interrupting query threads (#297), and
        // will definitely need to change if we support interrupting transactor threads.
        //
        // Improving this is tracked by https://github.com/mozilla/mentat/issues/356.
        self.metadata.lock().unwrap().schema.clone()
    }

    /// Query the Mentat store, using the given connection and the current metadata.
    pub fn q_once<T, U>(&self,
                     sqlite: &rusqlite::Connection,
                     query: &str,
                     inputs: T,
                     limit: U) -> Result<QueryResults>
        where T: Into<Option<HashMap<String, TypedValue>>>,
              U: Into<Option<u64>>
        {

        q_once(sqlite,
               &*self.current_schema(),
               query,
               inputs,
               limit)
    }

    /// Transact entities against the Mentat store, using the given connection and the current
    /// metadata.
    pub fn transact(&mut self,
                    sqlite: &mut rusqlite::Connection,
                    transaction: &str) -> Result<TxReport> {

        let assertion_vector = edn::parse::value(transaction)?;
        let entities = mentat_tx_parser::Tx::parse(assertion_vector)?;

        let tx = sqlite.transaction()?;

        let (current_generation, current_partition_map, current_schema) =
        {
            // The mutex is taken during this block.
            let ref current: Metadata = *self.metadata.lock().unwrap();
            (current.generation,
             // Expensive, but the partition map is updated after every committed transaction.
             current.partition_map.clone(),
             // Cheap.
             current.schema.clone())
        };

        // The transaction is processed while the mutex is not held.
        let (report, next_partition_map, next_schema) = transact(&tx, current_partition_map, &*current_schema, &*current_schema, entities)?;

        {
            // The mutex is taken during this block.
            let mut metadata = self.metadata.lock().unwrap();

            if current_generation != metadata.generation {
                // Somebody else wrote!
                // Retrying is tracked by https://github.com/mozilla/mentat/issues/357.
                bail!("Lost the transact() race!");
            }

            // Commit the SQLite transaction while we hold the mutex.
            tx.commit()?;

            metadata.generation += 1;
            metadata.partition_map = next_partition_map;
            if let Some(next_schema) = next_schema {
                metadata.schema = Arc::new(next_schema);
            }
        }

        Ok(report)
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    extern crate mentat_parser_utils;

    #[test]
    fn test_transact_errors() {
        let mut sqlite = db::new_connection("").unwrap();
        let mut conn = Conn::connect(&mut sqlite).unwrap();

        // Good: empty transaction.
        let report = conn.transact(&mut sqlite, "[]").unwrap();
        assert_eq!(report.tx_id, 0x10000000 + 1);

        // Bad EDN: missing closing ']'.
        let report = conn.transact(&mut sqlite, "[[:db/add \"t\" :db/ident :a/keyword]");
        match report.unwrap_err() {
            Error(ErrorKind::EdnParseError(_), _) => { },
            x => panic!("expected EDN parse error, got {:?}", x),
        }

        // Good EDN.
        let report = conn.transact(&mut sqlite, "[[:db/add \"t\" :db/ident :a/keyword]]").unwrap();
        assert_eq!(report.tx_id, 0x10000000 + 2);

        // Bad transaction data: missing leading :db/add.
        let report = conn.transact(&mut sqlite, "[[\"t\" :db/ident :b/keyword]]");
        match report.unwrap_err() {
            Error(ErrorKind::TxParseError(::mentat_tx_parser::errors::ErrorKind::ParseError(_)), _) => { },
            x => panic!("expected EDN parse error, got {:?}", x),
        }

        // Good transaction data.
        let report = conn.transact(&mut sqlite, "[[:db/add \"u\" :db/ident :b/keyword]]").unwrap();
        assert_eq!(report.tx_id, 0x10000000 + 3);

        // Bad transaction based on state of store: conflicting upsert.
        let report = conn.transact(&mut sqlite, "[[:db/add \"u\" :db/ident :a/keyword]
                                                  [:db/add \"u\" :db/ident :b/keyword]]");
        match report.unwrap_err() {
            Error(ErrorKind::DbError(::mentat_db::errors::ErrorKind::NotYetImplemented(_)), _) => { },
            x => panic!("expected EDN parse error, got {:?}", x),
        }
    }
}