MDEV-7652 - More explanatory ERROR and WARNING messages when loading plugins

            with plugin-load-add that are already registered at mysql.plugin

- issue just one error message, without this extra warning
- don't abuse ER_UDF_EXISTS, instead add a proper error message for plugins
- report started initialization for each plugin source

mysql-test/r/blackhole_plugin.result

@@ -5,7 +5,7 @@ Warning	1266	Using storage engine MyISAM for table 't1'
 DROP TABLE t1;
 INSTALL PLUGIN blackhole SONAME 'ha_blackhole.so';
 INSTALL PLUGIN BLACKHOLE SONAME 'ha_blackhole.so';
-ERROR HY000: Function 'BLACKHOLE' already exists
+ERROR HY000: Plugin 'BLACKHOLE' already installed
 UNINSTALL PLUGIN blackhole;
 INSTALL PLUGIN blackhole SONAME 'ha_blackhole.so';
 CREATE TABLE t1(a int) ENGINE=BLACKHOLE;

mysql-test/r/plugin.result

@@ -5,7 +5,7 @@ Warning	1266	Using storage engine MyISAM for table 't1'
 DROP TABLE t1;
 INSTALL PLUGIN example SONAME 'ha_example';
 INSTALL PLUGIN EXAMPLE SONAME 'ha_example';
-ERROR HY000: Function 'EXAMPLE' already exists
+ERROR HY000: Plugin 'EXAMPLE' already installed
 UNINSTALL PLUGIN example;
 INSTALL SONAME 'ha_example';
 select * from information_schema.plugins where plugin_library like 'ha_example%';

mysql-test/suite/archive/archive_plugin.result

@@ -5,7 +5,7 @@ Warning	1266	Using storage engine MyISAM for table 't1'
 DROP TABLE t1;
 INSTALL PLUGIN archive SONAME 'ha_archive.so';
 INSTALL PLUGIN ARCHIVE SONAME 'ha_archive.so';
-ERROR HY000: Function 'ARCHIVE' already exists
+ERROR HY000: Plugin 'ARCHIVE' already installed
 UNINSTALL PLUGIN archive;
 INSTALL PLUGIN archive SONAME 'ha_archive.so';
 CREATE TABLE t1(a int) ENGINE=ARCHIVE;

mysql-test/suite/archive/archive_plugin.test

@@ -7,7 +7,7 @@ DROP TABLE t1;
 --replace_regex /\.dll/.so/
 eval INSTALL PLUGIN archive SONAME '$HA_ARCHIVE_SO';
 --replace_regex /\.dll/.so/
---error 1125
+--error ER_PLUGIN_INSTALLED
 eval INSTALL PLUGIN ARCHIVE SONAME '$HA_ARCHIVE_SO';
 
 UNINSTALL PLUGIN archive;

mysql-test/t/blackhole_plugin.test

@@ -7,7 +7,7 @@ DROP TABLE t1;
 --replace_regex /\.dll/.so/
 eval INSTALL PLUGIN blackhole SONAME '$HA_BLACKHOLE_SO';
 --replace_regex /\.dll/.so/
---error 1125
+--error ER_PLUGIN_INSTALLED
 eval INSTALL PLUGIN BLACKHOLE SONAME '$HA_BLACKHOLE_SO';
 
 UNINSTALL PLUGIN blackhole;

mysql-test/t/plugin.test

@@ -6,7 +6,7 @@ DROP TABLE t1;
 
 INSTALL PLUGIN example SONAME 'ha_example';
 --replace_regex /\.dll/.so/
---error 1125
+--error ER_PLUGIN_INSTALLED
 INSTALL PLUGIN EXAMPLE SONAME 'ha_example';
 
 UNINSTALL PLUGIN example;

sql/share/errmsg-utf8.txt

@@ -7111,3 +7111,6 @@ ER_SLAVE_SKIP_NOT_IN_GTID
 	eng "When using GTID, @@sql_slave_skip_counter can not be used. Instead, setting @@gtid_slave_pos explicitly can be used to skip to after a given GTID position."
 ER_TABLE_DEFINITION_TOO_BIG
         eng "The definition for table %`s is too big"
+ER_PLUGIN_INSTALLED
+        eng "Plugin '%-.192s' already installed"
+        rus "Плагин '%-.192s' уже установлен"

sql/sql_plugin.cc

@@ -1049,7 +1049,7 @@ static bool plugin_add(MEM_ROOT *tmp_root,
 
   if (name->str && plugin_find_internal(name, MYSQL_ANY_PLUGIN))
   {
-    report_error(report, ER_UDF_EXISTS, name->str);
+    report_error(report, ER_PLUGIN_INSTALLED, name->str);
     DBUG_RETURN(TRUE);
   }
   /* Clear the whole struct to catch future extensions. */
@@ -1546,6 +1546,9 @@ int plugin_init(int *argc, char **argv, int flags)
   /*
     First we register builtin plugins
   */
+  if (global_system_variables.log_warnings >= 9)
+    sql_print_information("Initializing built-in plugins");
+
   for (builtins= mysql_mandatory_plugins; *builtins || mandatory; builtins++)
   {
     if (!*builtins)
@@ -1627,11 +1630,17 @@ int plugin_init(int *argc, char **argv, int flags)
   {
     I_List_iterator<i_string> iter(opt_plugin_load_list);
     i_string *item;
+    if (global_system_variables.log_warnings >= 9)
+      sql_print_information("Initializing plugins specified on the command line");
     while (NULL != (item= iter++))
       plugin_load_list(&tmp_root, item->ptr);
 
     if (!(flags & PLUGIN_INIT_SKIP_PLUGIN_TABLE))
+    {
+      if (global_system_variables.log_warnings >= 9)
+        sql_print_information("Initializing installed plugins");
       plugin_load(&tmp_root);
+    }
   }
 
   /*
@@ -1772,9 +1781,7 @@ static void plugin_load(MEM_ROOT *tmp_root)
       the mutex here to satisfy the assert
     */
     mysql_mutex_lock(&LOCK_plugin);
-    if (plugin_add(tmp_root, &name, &dl, REPORT_TO_LOG))
-      sql_print_warning("Couldn't load plugin named '%s' with soname '%s'.",
-                        str_name.c_ptr(), str_dl.c_ptr());
+    plugin_add(tmp_root, &name, &dl, REPORT_TO_LOG);
     free_root(tmp_root, MYF(MY_MARK_BLOCKS_FREE));
     mysql_mutex_unlock(&LOCK_plugin);
   }