Do not read unnecessary settings in Conn::read_settings

src/conn/mod.rs

@@ -55,6 +55,8 @@ pub mod pool;
 pub mod routines;
 pub mod stmt_cache;
 
+const DEFAULT_WAIT_TIMEOUT: usize = 28800;
+
 /// Helper that asynchronously disconnects the givent connection on the default tokio executor.
 fn disconnect(mut conn: Conn) {
     let disconnected = conn.inner.disconnected;
@@ -962,42 +964,123 @@ impl Conn {
     /// * It reads and stores `wait_timeout` in the connection unless it's already in [`Opts`]
     ///
     async fn read_settings(&mut self) -> Result<()> {
-        let read_socket = self.inner.opts.prefer_socket() && self.inner.socket.is_none();
-        let read_max_allowed_packet = self.opts().max_allowed_packet().is_none();
-        let read_wait_timeout = self.opts().wait_timeout().is_none();
+        enum Action {
+            Load(Cfg),
+            Apply(CfgData),
+        }
 
-        let settings: Option<Row> = if read_socket || read_max_allowed_packet || read_wait_timeout {
-            self.query_internal("SELECT @@socket, @@max_allowed_packet, @@wait_timeout")
-                .await?
-        } else {
-            None
-        };
+        enum CfgData {
+            MaxAllowedPacket(usize),
+            WaitTimeout(usize),
+        }
 
-        // set socket inside the connection
-        if read_socket {
-            self.inner.socket = settings.as_ref().map(|s| s.get("@@socket")).unwrap_or(None);
+        impl CfgData {
+            fn apply(&self, conn: &mut Conn) {
+                match self {
+                    Self::MaxAllowedPacket(value) => {
+                        if let Some(stream) = conn.inner.stream.as_mut() {
+                            stream.set_max_allowed_packet(*value);
+                        }
+                    }
+                    Self::WaitTimeout(value) => {
+                        conn.inner.wait_timeout = Duration::from_secs(*value as u64);
+                    }
+                }
+            }
         }
 
-        // set max_allowed_packet
-        let max_allowed_packet = if read_max_allowed_packet {
-            settings
-                .as_ref()
-                .map(|s| s.get("@@max_allowed_packet"))
-                .unwrap()
-        } else {
-            self.opts().max_allowed_packet()
-        };
-        if let Some(stream) = self.inner.stream.as_mut() {
-            stream.set_max_allowed_packet(max_allowed_packet.unwrap_or(DEFAULT_MAX_ALLOWED_PACKET));
+        enum Cfg {
+            Socket,
+            MaxAllowedPacket,
+            WaitTimeout,
         }
 
-        // set read_wait_timeout
-        let wait_timeout = if read_wait_timeout {
-            settings.as_ref().map(|s| s.get("@@wait_timeout")).unwrap()
+        impl Cfg {
+            const fn name(&self) -> &'static str {
+                match self {
+                    Self::Socket => "@@socket",
+                    Self::MaxAllowedPacket => "@@max_allowed_packet",
+                    Self::WaitTimeout => "@@wait_timeout",
+                }
+            }
+
+            fn apply(&self, conn: &mut Conn, value: Option<crate::Value>) {
+                match self {
+                    Cfg::Socket => {
+                        conn.inner.socket = value.map(crate::from_value).flatten();
+                    }
+                    Cfg::MaxAllowedPacket => {
+                        if let Some(stream) = conn.inner.stream.as_mut() {
+                            stream.set_max_allowed_packet(
+                                value
+                                    .map(crate::from_value)
+                                    .flatten()
+                                    .unwrap_or(DEFAULT_MAX_ALLOWED_PACKET),
+                            );
+                        }
+                    }
+                    Cfg::WaitTimeout => {
+                        conn.inner.wait_timeout = Duration::from_secs(
+                            value
+                                .map(crate::from_value)
+                                .flatten()
+                                .unwrap_or(DEFAULT_WAIT_TIMEOUT) as u64,
+                        );
+                    }
+                }
+            }
+        }
+
+        let mut actions = vec![
+            if let Some(x) = self.opts().max_allowed_packet() {
+                Action::Apply(CfgData::MaxAllowedPacket(x))
+            } else {
+                Action::Load(Cfg::MaxAllowedPacket)
+            },
+            if let Some(x) = self.opts().wait_timeout() {
+                Action::Apply(CfgData::WaitTimeout(x))
+            } else {
+                Action::Load(Cfg::WaitTimeout)
+            },
+        ];
+
+        if self.inner.opts.prefer_socket() && self.inner.socket.is_none() {
+            actions.push(Action::Load(Cfg::Socket))
+        }
+
+        let loads = actions
+            .iter()
+            .filter_map(|x| match x {
+                Action::Load(x) => Some(x),
+                Action::Apply(_) => None,
+            })
+            .collect::<Vec<_>>();
+
+        let loaded = if !loads.is_empty() {
+            let query = loads
+                .iter()
+                .zip(std::iter::once(' ').chain(std::iter::repeat(',')))
+                .fold("SELECT".to_owned(), |mut acc, (cfg, prefix)| {
+                    acc.push(prefix);
+                    acc.push_str(cfg.name());
+                    acc
+                });
+
+            self.query_internal::<Row, String>(query)
+                .await?
+                .map(|row| row.unwrap())
+                .unwrap_or_else(|| vec![crate::Value::NULL; loads.len()])
         } else {
-            self.opts().wait_timeout()
+            vec![]
         };
-        self.inner.wait_timeout = Duration::from_secs(wait_timeout.unwrap_or(28800) as u64);
+        let mut loaded = loaded.into_iter();
+
+        for action in actions {
+            match action {
+                Action::Load(cfg) => cfg.apply(self, loaded.next()),
+                Action::Apply(cfg) => cfg.apply(self),
+            }
+        }
 
         Ok(())
     }