Fixes try_execute_command message parsing bug (#560)

* Fixes try_execute_command message parsing bug

* Fix initial segment logic

* Add test

src/query_router.rs

@@ -19,9 +19,9 @@ use crate::plugins::{Intercept, Plugin, PluginOutput, QueryLogger, TableAccess};
 use crate::pool::PoolSettings;
 use crate::sharding::Sharder;
 
-use std::cmp;
 use std::collections::BTreeSet;
 use std::io::Cursor;
+use std::{cmp, mem};
 
 /// Regexes used to parse custom commands.
 const CUSTOM_SQL_REGEXES: [&str; 7] = [
@@ -141,6 +141,7 @@ impl QueryRouter {
         let mut message_cursor = Cursor::new(message_buffer);
 
         let code = message_cursor.get_u8() as char;
+        let len = message_cursor.get_i32() as usize;
 
         // Check for any sharding regex matches in any queries
         match code as char {
@@ -150,9 +151,13 @@ impl QueryRouter {
                     || self.pool_settings.sharding_key_regex.is_some()
                 {
                     // Check only the first block of bytes configured by the pool settings
-                    let len = message_cursor.get_i32() as usize;
                     let seg = cmp::min(len - 5, self.pool_settings.regex_search_limit);
-                    let initial_segment = String::from_utf8_lossy(&message_buffer[0..seg]);
+
+                    let query_start_index = mem::size_of::<u8>() + mem::size_of::<i32>();
+
+                    let initial_segment = String::from_utf8_lossy(
+                        &message_buffer[query_start_index..query_start_index + seg],
+                    );
 
                     // Check for a shard_id included in the query
                     if let Some(shard_id_regex) = &self.pool_settings.shard_id_regex {
@@ -192,7 +197,6 @@ impl QueryRouter {
             return None;
         }
 
-        let _len = message_cursor.get_i32() as usize;
         let query = message_cursor.read_string().unwrap();
 
         let regex_set = match CUSTOM_SQL_REGEX_SET.get() {
@@ -1291,6 +1295,11 @@ mod test {
         // Shard should start out unset
         assert_eq!(qr.active_shard, None);
 
+        // Don't panic when short query eg. ; is sent
+        let q0 = simple_query(";");
+        assert!(qr.try_execute_command(&q0) == None);
+        assert_eq!(qr.active_shard, None);
+
         // Make sure setting it works
         let q1 = simple_query("/* shard_id: 1 */ select 1 from foo;");
         assert!(qr.try_execute_command(&q1) == None);