add stuff and bits

src/database/abstraction/sqlite.rs

@@ -1,3 +1,11 @@
+use super::{DatabaseEngine, Tree};
+use crate::{database::Config, Result};
+use crossbeam::channel::{
+    bounded, unbounded, Receiver as ChannelReceiver, Sender as ChannelSender, TryRecvError,
+};
+use log::debug;
+use parking_lot::{Mutex, MutexGuard, RwLock};
+use rusqlite::{params, Connection, DatabaseName::Main, OptionalExtension};
 use std::{
     collections::BTreeMap,
     future::Future,
@@ -8,33 +16,12 @@ use std::{
     thread,
     time::{Duration, Instant},
 };
-
-use crate::{database::Config, Result};
-
-use super::{DatabaseEngine, Tree};
-
-use log::debug;
-
-use crossbeam::channel::{bounded, Sender as ChannelSender};
-use parking_lot::{Mutex, MutexGuard, RwLock};
-use rusqlite::{params, Connection, DatabaseName::Main, OptionalExtension};
-
 use tokio::sync::oneshot::Sender;
 
-// const SQL_CREATE_TABLE: &str =
-//     "CREATE TABLE IF NOT EXISTS {} {{ \"key\" BLOB PRIMARY KEY, \"value\" BLOB NOT NULL }}";
-// const SQL_SELECT: &str = "SELECT value FROM {} WHERE key = ?";
-// const SQL_INSERT: &str = "INSERT OR REPLACE INTO {} (key, value) VALUES (?, ?)";
-// const SQL_DELETE: &str = "DELETE FROM {} WHERE key = ?";
-// const SQL_SELECT_ITER: &str = "SELECT key, value FROM {}";
-// const SQL_SELECT_PREFIX: &str = "SELECT key, value FROM {} WHERE key LIKE ?||'%' ORDER BY key ASC";
-// const SQL_SELECT_ITER_FROM_FORWARDS: &str = "SELECT key, value FROM {} WHERE key >= ? ORDER BY ASC";
-// const SQL_SELECT_ITER_FROM_BACKWARDS: &str =
-//     "SELECT key, value FROM {} WHERE key <= ? ORDER BY DESC";
-
 struct Pool {
     writer: Mutex<Connection>,
     readers: Vec<Mutex<Connection>>,
+    spills: ConnectionRecycler,
     spill_tracker: Arc<()>,
     path: PathBuf,
 }
@@ -43,7 +30,7 @@ pub const MILLI: Duration = Duration::from_millis(1);
 
 enum HoldingConn<'a> {
     FromGuard(MutexGuard<'a, Connection>),
-    FromOwned(Connection, Arc<()>),
+    FromRecycled(RecycledConn, Arc<()>),
 }
 
 impl<'a> Deref for HoldingConn<'a> {
@@ -52,7 +39,57 @@ impl<'a> Deref for HoldingConn<'a> {
     fn deref(&self) -> &Self::Target {
         match self {
             HoldingConn::FromGuard(guard) => guard.deref(),
-            HoldingConn::FromOwned(conn, _) => conn,
+            HoldingConn::FromRecycled(conn, _) => conn.deref(),
+        }
+    }
+}
+
+struct ConnectionRecycler(ChannelSender<Connection>, ChannelReceiver<Connection>);
+
+impl ConnectionRecycler {
+    fn new() -> Self {
+        let (s, r) = unbounded();
+        Self(s, r)
+    }
+
+    fn recycle(&self, conn: Connection) -> RecycledConn {
+        let sender = self.0.clone();
+
+        RecycledConn(Some(conn), sender)
+    }
+
+    fn try_take(&self) -> Option<Connection> {
+        match self.1.try_recv() {
+            Ok(conn) => Some(conn),
+            Err(TryRecvError::Empty) => None,
+            // as this is pretty impossible, a panic is warranted if it ever occurs
+            Err(TryRecvError::Disconnected) => panic!("Receiving channel was disconnected. A a sender is owned by the current struct, this should never happen(!!!)")
+        }
+    }
+}
+
+struct RecycledConn(
+    Option<Connection>, // To allow moving out of the struct when `Drop` is called.
+    ChannelSender<Connection>,
+);
+
+impl Deref for RecycledConn {
+    type Target = Connection;
+
+    fn deref(&self) -> &Self::Target {
+        self.0
+            .as_ref()
+            .expect("RecycledConn does not have a connection in Option<>")
+    }
+}
+
+impl Drop for RecycledConn {
+    fn drop(&mut self) {
+        if let Some(conn) = self.0.take() {
+            log::debug!("Recycled connection");
+            if let Err(e) = self.1.send(conn) {
+                log::warn!("Recycling a connection led to the following error: {:?}", e)
+            }
         }
     }
 }
@@ -76,6 +113,7 @@ impl Pool {
         Ok(Self {
             writer,
             readers,
+            spills: ConnectionRecycler::new(),
             spill_tracker: Arc::new(()),
             path: path.as_ref().to_path_buf(),
         })
@@ -104,24 +142,38 @@ impl Pool {
     }
 
     fn read_lock(&self) -> HoldingConn<'_> {
+        // First try to get a connection from the permanent pool
         for r in &self.readers {
             if let Some(reader) = r.try_lock() {
                 return HoldingConn::FromGuard(reader);
             }
         }
 
-        let spill_arc = self.spill_tracker.clone();
+        // We didn't get a connection from the permanent pool, so we'll dumpster-dive for recycled connections.
+        // Either we have a connection or we dont, if we don't, we make a new one.
+        let conn = match self.spills.try_take() {
+            Some(conn) => conn,
+            None => Self::prepare_conn(&self.path, None).unwrap(),
+        };
+
+        // Clone the spill Arc to mark how many spilled connections actually exist.
+        let spill_arc = Arc::clone(&self.spill_tracker);
+
+        // Get a sense of how many connections exist now.
         let now_count = Arc::strong_count(&spill_arc) - 1 /* because one is held by the pool */;
 
-        log::warn!("read_lock: all readers locked, creating spillover reader...");
+        log::debug!("read_lock: all readers locked, creating spillover reader...");
 
-        if now_count > 1 {
-            log::warn!("read_lock: now {} spillover readers exist", now_count);
+        // If the spillover readers are more than the number of total readers, there might be a problem.
+        if now_count > self.readers.len() {
+            log::warn!(
+                "read_lock: possible high load; now {} spillover readers exist",
+                now_count
+            );
         }
 
-        let spilled = Self::prepare_conn(&self.path, None).unwrap();
-
-        HoldingConn::FromOwned(spilled, spill_arc)
+        // Return the recyclable connection.
+        HoldingConn::FromRecycled(self.spills.recycle(conn), spill_arc)
     }
 }
 
@@ -188,6 +240,26 @@ impl Engine {
             )
             .map_err(Into::into)
     }
+
+    // Reaps (at most) X amount of connections if `amount` is Some.
+    // If none, reaps all currently idle connections.
+    pub fn reap_spillover(&self, amount: Option<u32>) {
+        let mut reaped = 0;
+
+        if let Some(amount) = amount {
+            for _ in 0..amount {
+                if self.pool.spills.try_take().is_some() {
+                    reaped += 1;
+                }
+            }
+        } else {
+            while let Some(_) = self.pool.spills.try_take() {
+                reaped += 1;
+            }
+        }
+
+        log::debug!("Reaped {} connections", reaped);
+    }
 }
 
 pub struct SqliteTable {