Make query_classifier a wrapper.

The query_classifier library is now only a wrapper that loads an
actual query classifier implementation. Currently it is hardwired
to load qc_mysqlembedded, which implements the query classifier
API using MySQL embedded.

This will be changed, so that the library to load is specified
when qc_init() is called. That will then allow the query classifier
to be specified in the config file.

Currently there seems to be a conflict between the mysql_library_end()
call made in qc_mysqlembedded and the mysql_library_end() call made in
gateway.c. The reason is that they both finalize a shared library.
For the time being mysql_library_end() is not called in gateway.c.
This problem is likely to go away by switching from the client
library to the connector-c library.

query_classifier/CMakeLists.txt

@@ -3,7 +3,7 @@ add_library(query_classifier SHARED query_classifier.cc)
 target_link_libraries(query_classifier ${MYSQL_EMBEDDED_LIB} aio crypt crypto dl m ${PCRE_LINK_FLAGS} ssl stdc++ z)
 set_target_properties(query_classifier PROPERTIES VERSION "1.0.0")
 set_target_properties(query_classifier PROPERTIES LINK_FLAGS -Wl,--version-script=${CMAKE_CURRENT_SOURCE_DIR}/query_classifier.map)
-#set_target_properties(query_classifier PROPERTIES LINK_FLAGS -Wl,-z,defs)
+set_target_properties(query_classifier PROPERTIES LINK_FLAGS -Wl,-z,defs)
 install(TARGETS query_classifier COMPONENT lib DESTINATION ${MAXSCALE_LIBDIR})
 
 add_subdirectory(qc_mysqlembedded)

query_classifier/qc_mysqlembedded/qc_mysqlembedded.cc

@@ -1818,6 +1818,9 @@ void qc_thread_end()
  * EXPORTS
  */
 
+extern "C"
+{
+
 static char version_string[] = "V1.0.0";
 
 static QUERY_CLASSIFIER qc =
@@ -1862,4 +1865,4 @@ QUERY_CLASSIFIER* GetModuleObject()
     return &qc;
 }
 
-
+}

query_classifier/qc_mysqlembedded/qc_mysqlembedded.map

@@ -2,7 +2,7 @@
     global:
         info;
         version;
-        GetModuleInfo;
+        GetModuleObject;
         ModuleInit;
     local:
         *;