MDEV-23879 server hangs with threadpool, compression, and client pipelining

Amend check for unread client data in threadpool.

THD::NET will have unread data, in case client uses compression, and
wraps multiple commands into  a single compression packet

MariaDB C/C sends COM_STMT_RESET+COM_STMT_EXECUTE, and wraps it into
a single compressed packet, when compression is on, thus trying to use
compression and prepared statements against a threadpool-enabled server
will result into a hang, before this patch.

mysql-test/r/pool_of_threads.result

@@ -2188,3 +2188,8 @@ sleep(50)
 connection extracon;
 sleep(5.5)
 0
+connect  comp_con,localhost,root,,,,,COMPRESS;
+SELECT 1;
+1
+1
+disconnect comp_con;

mysql-test/t/pool_of_threads.test

@@ -92,3 +92,9 @@ connection con2;
 
 connection extracon;
 --reap
+
+# Check if compression works OK
+connect (comp_con,localhost,root,,,,,COMPRESS);
+SELECT 1;
+disconnect comp_con;
+

sql/threadpool_common.cc

@@ -324,6 +324,16 @@ static void handle_wait_timeout(THD *thd)
   thd->net.error= 2;
 }
 
+/** Check if some client data is cached in thd->net or thd->net.vio */
+static bool has_unread_data(THD* thd)
+{
+  NET *net= &thd->net;
+  if (net->compress && net->remain_in_buf)
+    return true;
+  Vio *vio= net->vio;
+  return vio->has_data(vio);
+}
+
 
 /**
  Process a single client request or a single batch.
@@ -358,7 +368,6 @@ static int threadpool_process_request(THD *thd)
   */
   for(;;)
   {
-    Vio *vio;
     thd->net.reading_or_writing= 0;
     if (mysql_audit_release_required(thd))
       mysql_audit_release(thd);
@@ -374,8 +383,7 @@ static int threadpool_process_request(THD *thd)
 
     set_thd_idle(thd);
 
-    vio= thd->net.vio;
-    if (!vio->has_data(vio))
+    if (!has_unread_data(thd))
     { 
       /* More info on this debug sync is in sql_parse.cc*/
       DEBUG_SYNC(thd, "before_do_command_net_read");