connection: quietly prepare unprepared queries in batch before sending

Before sending an unprepared statement in batch, quietly prepare it on one connection to obtain the information about the
bind markers. If the list of values to be bound is empty, we
just send it as an unprepared query.

scylla/src/transport/connection.rs

@@ -26,7 +26,7 @@ pub(crate) use ssl_config::SslConfig;
 
 use crate::authentication::AuthenticatorProvider;
 use scylla_cql::frame::response::authenticate::Authenticate;
-use std::collections::{BTreeSet, HashMap};
+use std::collections::{BTreeSet, HashMap, HashSet};
 use std::convert::TryFrom;
 use std::io::ErrorKind;
 use std::net::{IpAddr, SocketAddr};
@@ -52,7 +52,7 @@ use crate::frame::{
     request::{self, batch, execute, query, register, SerializableRequest},
     response::{event::Event, result, NonErrorResponse, Response, ResponseOpcode},
     server_event_type::EventType,
-    value::{BatchValues, ValueList},
+    value::{BatchValues, BatchValuesIterator, ValueList},
     FrameParams, SerializedRequest,
 };
 use crate::query::Query;
@@ -772,11 +772,62 @@ impl Connection {
 
     pub(crate) async fn batch_with_consistency(
         &self,
-        batch: &Batch,
+        init_batch: &Batch,
         values: impl BatchValues,
         consistency: Consistency,
         serial_consistency: Option<SerialConsistency>,
     ) -> Result<QueryResult, QueryError> {
+        let batch = {
+            let mut to_prepare = HashSet::<String>::new();
+
+            {
+                let mut value_iter = values.batch_values_iter();
+                for stmt in &init_batch.statements {
+                    if let BatchStatement::Query(query) = stmt {
+                        let value = value_iter.next_serialized().transpose()?;
+                        if let Some(v) = value {
+                            if v.len() > 0 {
+                                to_prepare.insert(query.contents.clone());
+                            }
+                        }
+                    } else {
+                        value_iter.skip_next();
+                    }
+                }
+            }
+
+            let mut prepared_queries = HashMap::<String, PreparedStatement>::new();
+
+            for query in &to_prepare {
+                let prepared = self.prepare(&Query::new(query)).await?;
+                prepared_queries.insert(query.clone(), prepared);
+            }
+
+            let mut batch: Cow<Batch>;
+
+            if to_prepare.is_empty() {
+                batch = Cow::Borrowed(init_batch);
+            } else {
+                batch = Cow::Owned(Default::default());
+                batch.to_mut().config = init_batch.config.clone();
+                for stmt in &init_batch.statements {
+                    match stmt {
+                        BatchStatement::Query(query) => {
+                            match prepared_queries.get(&query.contents) {
+                                Some(prepared) => batch.to_mut().append_statement(prepared.clone()),
+                                None => batch.to_mut().append_statement(query.clone())
+                            }
+                        }
+                        BatchStatement::PreparedStatement(prepared) => {
+                            batch.to_mut().append_statement(prepared.clone());
+                        }
+                    }
+                }
+            }
+
+            batch
+        };
+
         let batch_frame = batch::Batch {
             statements: Cow::Borrowed(&batch.statements),
             values,