MDEV-21258: (p1) Can't uninstall plugin if the library file doesn't exist

Part 1: removing plugin from the mysql.plugin even if the plugin is  not loaded

mysql-test/r/plugin.result

@@ -341,3 +341,16 @@ RENAME TABLE t1 TO t2;
 ERROR 42S02: Table 'test.t1' doesn't exist
 DROP TABLE t1;
 # End of 10.1 test
+#
+# MDEV-21258: Can't uninstall plugin if the library file doesn't exist
+#
+insert into  mysql.plugin values ("unexisting_plugin", "soname");
+select * from mysql.plugin WHERE name='unexisting_plugin';
+name	dl
+unexisting_plugin	soname
+UNINSTALL PLUGIN unexisting_plugin;
+select * from mysql.plugin WHERE name='unexisting_plugin';
+name	dl
+UNINSTALL PLUGIN unexisting_plugin;
+ERROR 42000: PLUGIN unexisting_plugin does not exist
+# End of 10.2 tests

mysql-test/suite/plugins/r/audit_null_debug.result

@@ -11,5 +11,6 @@ uninstall plugin audit_null;
 ERROR HY000: Index for table './mysql/plugin.MYI' is corrupt; try to repair it
 SET debug_dbug=@old_dbug;
 uninstall plugin audit_null;
+uninstall plugin audit_null;
 ERROR 42000: PLUGIN audit_null does not exist
 delete from mysql.plugin where name='audit_null';

mysql-test/suite/plugins/t/audit_null_debug.test

@@ -26,7 +26,8 @@ SET debug_dbug='+d,myisam_pretend_crashed_table_on_usage';
 uninstall plugin audit_null;
 SET debug_dbug=@old_dbug;
 
---error 1305
+uninstall plugin audit_null;
+--error ER_SP_DOES_NOT_EXIST
 uninstall plugin audit_null;
 
 delete from mysql.plugin where name='audit_null';

mysql-test/t/plugin.test

@@ -276,3 +276,23 @@ RENAME TABLE t1 TO t2;
 DROP TABLE t1;
 
 --echo # End of 10.1 test
+
+--echo #
+--echo # MDEV-21258: Can't uninstall plugin if the library file doesn't exist
+--echo #
+
+insert into  mysql.plugin values ("unexisting_plugin", "soname");
+
+# check that we have the plugin installed
+select * from mysql.plugin WHERE name='unexisting_plugin';
+
+# make attempt to uninstall the plugin
+UNINSTALL PLUGIN unexisting_plugin;
+
+# check that we have the plugin uninstalled
+select * from mysql.plugin WHERE name='unexisting_plugin';
+
+--error ER_SP_DOES_NOT_EXIST
+UNINSTALL PLUGIN unexisting_plugin;
+
+--echo # End of 10.2 tests

sql/sql_plugin.cc

@@ -2216,26 +2216,30 @@ static bool do_uninstall(THD *thd, TABLE *table, const LEX_STRING *name)
   if (!(plugin= plugin_find_internal(name, MYSQL_ANY_PLUGIN)) ||
       plugin->state & (PLUGIN_IS_UNINITIALIZED | PLUGIN_IS_DYING))
   {
-    my_error(ER_SP_DOES_NOT_EXIST, MYF(0), "PLUGIN", name->str);
-    return 1;
-  }
-  if (!plugin->plugin_dl)
-  {
-    my_error(ER_PLUGIN_DELETE_BUILTIN, MYF(0));
-    return 1;
+    // maybe plugin is in mysql.plugin present so postpond the error
+    plugin= NULL;
   }
-  if (plugin->load_option == PLUGIN_FORCE_PLUS_PERMANENT)
+
+  if (plugin)
   {
-    my_error(ER_PLUGIN_IS_PERMANENT, MYF(0), name->str);
-    return 1;
-  }
+    if (!plugin->plugin_dl)
+    {
+      my_error(ER_PLUGIN_DELETE_BUILTIN, MYF(0));
+      return 1;
+    }
+    if (plugin->load_option == PLUGIN_FORCE_PLUS_PERMANENT)
+    {
+      my_error(ER_PLUGIN_IS_PERMANENT, MYF(0), name->str);
+      return 1;
+    }
 
-  plugin->state= PLUGIN_IS_DELETED;
-  if (plugin->ref_count)
-    push_warning(thd, Sql_condition::WARN_LEVEL_WARN,
-                 WARN_PLUGIN_BUSY, ER_THD(thd, WARN_PLUGIN_BUSY));
-  else
-    reap_needed= true;
+    plugin->state= PLUGIN_IS_DELETED;
+    if (plugin->ref_count)
+      push_warning(thd, Sql_condition::WARN_LEVEL_WARN,
+          WARN_PLUGIN_BUSY, ER_THD(thd, WARN_PLUGIN_BUSY));
+    else
+      reap_needed= true;
+  }
 
   uchar user_key[MAX_KEY_LENGTH];
   table->use_all_columns();
@@ -2260,6 +2264,11 @@ static bool do_uninstall(THD *thd, TABLE *table, const LEX_STRING *name)
       return 1;
     }
   }
+  else if (!plugin)
+  {
+    my_error(ER_SP_DOES_NOT_EXIST, MYF(0), "PLUGIN", name->str);
+    return 1;
+  }
   return 0;
 }