Allow shard setting with comments (#293)

What
Allows shard selection by the client to come in via comments like /* shard_id: 1 */ select * from foo;

Why
We're using a setup in Ruby that makes it tough or impossible to inject commands on the connection to set the shard before it gets to the "real" SQL being run. Instead we have an updated PG adapter that allows injection of comments before each executed SQL statement. We need this support in pgcat in order to keep some complex shard picking logic in Ruby code while using pgcat for connection management.

Local Testing
Run postgres and pgcat with the default options. Run psql < tests/sharding/query_routing_setup.sql to setup the database for the tests and run ./tests/pgbench/external_shard_test.sh as often as needed to exercise the shard setting comment test.

src/pool.rs

@@ -8,6 +8,7 @@ use once_cell::sync::Lazy;
 use parking_lot::{Mutex, RwLock};
 use rand::seq::SliceRandom;
 use rand::thread_rng;
+use regex::Regex;
 use std::collections::{HashMap, HashSet};
 use std::sync::{
     atomic::{AtomicBool, Ordering},
@@ -104,6 +105,15 @@ pub struct PoolSettings {
 
     // Ban time
     pub ban_time: i64,
+
+    // Regex for searching for the sharding key in SQL statements
+    pub sharding_key_regex: Option<Regex>,
+
+    // Regex for searching for the shard id in SQL statements
+    pub shard_id_regex: Option<Regex>,
+
+    // Limit how much of each query is searched for a potential shard regex match
+    pub regex_search_limit: usize,
 }
 
 impl Default for PoolSettings {
@@ -121,6 +131,9 @@ impl Default for PoolSettings {
             healthcheck_delay: General::default_healthcheck_delay(),
             healthcheck_timeout: General::default_healthcheck_timeout(),
             ban_time: General::default_ban_time(),
+            sharding_key_regex: None,
+            shard_id_regex: None,
+            regex_search_limit: 1000,
         }
     }
 }
@@ -300,6 +313,15 @@ impl ConnectionPool {
                         healthcheck_delay: config.general.healthcheck_delay,
                         healthcheck_timeout: config.general.healthcheck_timeout,
                         ban_time: config.general.ban_time,
+                        sharding_key_regex: pool_config
+                            .sharding_key_regex
+                            .clone()
+                            .map(|regex| Regex::new(regex.as_str()).unwrap()),
+                        shard_id_regex: pool_config
+                            .shard_id_regex
+                            .clone()
+                            .map(|regex| Regex::new(regex.as_str()).unwrap()),
+                        regex_search_limit: pool_config.regex_search_limit.unwrap_or(1000),
                     },
                     validated: Arc::new(AtomicBool::new(false)),
                     paused: Arc::new(AtomicBool::new(false)),