refactor: pool fixes and breaking changes

* Fixed leak of `Arc<SharedPool>` in `DecrementSizeGuard::cancel()`
* Renamed `PoolOptions::connect_timeout` to `acquire_timeout` for clarity.
* Fixed `/* SQLx ping */` showing up in Postgres query logs
* Made `.close()` a regular function that returns a `Future`
* Deleted deprecated method `PoolConnection::release()`
* Document why connection might be dropped if `Pool::acquire()` is cancelled
* Added connection metadata to pool lifecycle callbacks
* Improved guarantees for `min_connections`
* Fixed `num_idle()` to not spin forever at high load
* Improved documentation across the `pool` module

sqlx-bench/benches/pg_pool.rs

@@ -26,7 +26,7 @@ fn do_bench_acquire(b: &mut Bencher, concurrent: u32, fair: bool) {
     let pool = sqlx_rt::block_on(
         PgPoolOptions::new()
             // we don't want timeouts because we want to see how the pool degrades
-            .connect_timeout(Duration::from_secs(3600))
+            .acquire_timeout(Duration::from_secs(3600))
             // force the pool to start full
             .min_connections(50)
             .max_connections(50)

sqlx-core/src/any/connection/mod.rs

@@ -136,6 +136,22 @@ impl Connection for AnyConnection {
         }
     }
 
+    fn close_hard(self) -> BoxFuture<'static, Result<(), Error>> {
+        match self.0 {
+            #[cfg(feature = "postgres")]
+            AnyConnectionKind::Postgres(conn) => conn.close_hard(),
+
+            #[cfg(feature = "mysql")]
+            AnyConnectionKind::MySql(conn) => conn.close_hard(),
+
+            #[cfg(feature = "sqlite")]
+            AnyConnectionKind::Sqlite(conn) => conn.close_hard(),
+
+            #[cfg(feature = "mssql")]
+            AnyConnectionKind::Mssql(conn) => conn.close_hard(),
+        }
+    }
+
     fn ping(&mut self) -> BoxFuture<'_, Result<(), Error>> {
         delegate_to_mut!(self.ping())
     }

sqlx-core/src/connection.rs

@@ -20,6 +20,12 @@ pub trait Connection: Send {
     /// will be faster at cleaning up resources.
     fn close(self) -> BoxFuture<'static, Result<(), Error>>;
 
+    /// Immediately close the connection without sending a graceful shutdown.
+    ///
+    /// This should still at least send a TCP `FIN` frame to let the server know we're dying.
+    #[doc(hidden)]
+    fn close_hard(self) -> BoxFuture<'static, Result<(), Error>>;
+
     /// Checks if a connection to the database is still valid.
     fn ping(&mut self) -> BoxFuture<'_, Result<(), Error>>;
 

sqlx-core/src/error.rs

@@ -165,6 +165,11 @@ pub trait DatabaseError: 'static + Send + Sync + StdError {
     #[doc(hidden)]
     fn into_error(self: Box<Self>) -> Box<dyn StdError + Send + Sync + 'static>;
 
+    #[doc(hidden)]
+    fn is_transient_in_connect_phase(&self) -> bool {
+        false
+    }
+
     /// Returns the name of the constraint that triggered the error, if applicable.
     /// If the error was caused by a conflict of a unique index, this will be the index name.
     ///

sqlx-core/src/mssql/connection/mod.rs

@@ -55,6 +55,10 @@ impl Connection for MssqlConnection {
         }
     }
 
+    fn close_hard(self) -> BoxFuture<'static, Result<(), Error>> {
+        self.close()
+    }
+
     fn ping(&mut self) -> BoxFuture<'_, Result<(), Error>> {
         // NOTE: we do not use `SELECT 1` as that *could* interact with any ongoing transactions
         self.execute("/* SQLx ping */").map_ok(|_| ()).boxed()

sqlx-core/src/mysql/connection/mod.rs

@@ -56,6 +56,13 @@ impl Connection for MySqlConnection {
         })
     }
 
+    fn close_hard(mut self) -> BoxFuture<'static, Result<(), Error>> {
+        Box::pin(async move {
+            self.stream.shutdown().await?;
+            Ok(())
+        })
+    }
+
     fn ping(&mut self) -> BoxFuture<'_, Result<(), Error>> {
         Box::pin(async move {
             self.stream.wait_until_ready().await?;