MDEV-19274 mariadb does not build on OSes that do not have HAVE_POOL_OF_THREADS

Do not try to compile threadpool_common.cc, if threadpool is not available
on current OS.

Also introduced undocumented/uncached CMake variable DISABLE_THREADPOOL,
to emulate builds on exotic OSes.

sql/CMakeLists.txt

@@ -126,7 +126,7 @@ SET (SQL_SOURCE
                opt_index_cond_pushdown.cc opt_subselect.cc
                opt_table_elimination.cc sql_expression_cache.cc
                gcalc_slicescan.cc gcalc_tools.cc
-               threadpool_common.cc ../sql-common/mysql_async.c
+               ../sql-common/mysql_async.c
                my_apc.cc mf_iocache_encr.cc item_jsonfunc.cc
                my_json_writer.cc
                rpl_gtid.cc rpl_parallel.cc
@@ -152,17 +152,21 @@ SET (SQL_SOURCE
                ${MYSYS_LIBWRAP_SOURCE}
 )
   
-IF (CMAKE_SYSTEM_NAME MATCHES "Linux" OR
-    CMAKE_SYSTEM_NAME MATCHES "Windows" OR
-    CMAKE_SYSTEM_NAME MATCHES "SunOS" OR
-    HAVE_KQUEUE)
+IF ((CMAKE_SYSTEM_NAME MATCHES "Linux" OR
+     CMAKE_SYSTEM_NAME MATCHES "SunOS" OR
+     WIN32 OR
+     HAVE_KQUEUE)
+    AND (NOT DISABLE_THREADPOOL))
  ADD_DEFINITIONS(-DHAVE_POOL_OF_THREADS)
  IF(WIN32)
    SET(SQL_SOURCE ${SQL_SOURCE} threadpool_win.cc)
-   SET(SQL_SOURCE ${SQL_SOURCE} handle_connections_win.cc)
  ENDIF()
  SET(SQL_SOURCE ${SQL_SOURCE} threadpool_generic.cc)
+ SET(SQL_SOURCE ${SQL_SOURCE} threadpool_common.cc)
+ENDIF()
 
+IF(WIN32)
+  SET(SQL_SOURCE ${SQL_SOURCE} handle_connections_win.cc)
 ENDIF()
 
 MYSQL_ADD_PLUGIN(partition ha_partition.cc STORAGE_ENGINE DEFAULT STATIC_ONLY

sql/handle_connections_win.cc

@@ -28,8 +28,13 @@
 /* From mysqld.cc */
 extern HANDLE hEventShutdown;
 extern MYSQL_SOCKET base_ip_sock, extra_ip_sock;
+#ifdef HAVE_POOL_OF_THREADS
 extern PTP_CALLBACK_ENVIRON get_threadpool_win_callback_environ();
 extern void tp_win_callback_prolog();
+#else
+#define get_threadpool_win_callback_environ() 0
+#define tp_win_callback_prolog() do{}while(0)
+#endif
 static SECURITY_ATTRIBUTES pipe_security;
 
 /**

sql/threadpool.h

+#ifndef THREADPOOL_H_INCLUDED
+#define THREADPOOL_H_INCLUDED
+
+#ifdef HAVE_POOL_OF_THREADS
 /* Copyright (C) 2012 Monty Program Ab
 
    This program is free software; you can redistribute it and/or modify
@@ -154,3 +158,6 @@ struct TP_pool_generic :TP_pool
   virtual int set_stall_limit(uint);
   virtual int get_idle_thread_count();
 };
+
+#endif /* HAVE_POOL_OF_THREADS */
+#endif /* THREADPOOL_H_INCLUDED */