BUG#54989 - With null_audit installed, server hangs on an

attempt to install a plugin twice

Server crashes when [UN]INSTALL PLUGIN fails (returns an
error) and general log is disabled and there are audit
plugins interested in MYSQL_AUDIT_GENERAL_CLASS. 

When audit event is triggered, audit subsystem acquires interested
plugins by walking through plugin list. Evidently plugin list
iterator protects plugin list by acquiring LOCK_plugin, see
plugin_foreach_with_mask().

On the other hand [UN]INSTALL PLUGIN is acquiring LOCK_plugin
rather for a long time.

When audit event is triggered during [UN]INSTALL PLUGIN, plugin
list iterator acquires the same lock (within the same thread)
second time.

Repeatable only with general_log disabled, because general_log
triggers MYSQL_AUDIT_GENERAL_LOG event, which acquires audit
plugins before [UN]INSTALL PLUGIN acquired LOCK_plugin.

With this fix we pre-acquire audit plugins for events that
may potentially occur during [UN]INSTALL PLUGIN.

This hack should be removed when LOCK_plugin is fixed so it
protects only what it supposed to protect.

No test case for this fix - we do not have facility to test
audit plugins yet.

sql/sql_audit.cc:
  Move "acquire audit plugin" logics to a separate
  function.
sql/sql_audit.h:
  Move "acquire audit plugin" logics to a separate
  function.
sql/sql_plugin.cc:
  Pre-acquire audit plugins for events that may potentially occur
  during [UN]INSTALL PLUGIN.

sql/sql_audit.cc

@@ -137,6 +137,30 @@ static my_bool acquire_plugins(THD *thd, plugin_ref plugin, void *arg)
 }
 
 
+/**
+  @brief Acquire audit plugins
+
+  @param[in]   thd              MySQL thread handle
+  @param[in]   event_class      Audit event class
+
+  @details Ensure that audit plugins interested in given event
+  class are locked by current thread.
+*/
+void mysql_audit_acquire_plugins(THD *thd, uint event_class)
+{
+  unsigned long event_class_mask[MYSQL_AUDIT_CLASS_MASK_SIZE];
+  DBUG_ENTER("mysql_audit_acquire_plugins");
+  set_audit_mask(event_class_mask, event_class);
+  if (thd && !check_audit_mask(mysql_global_audit_mask, event_class_mask) &&
+      check_audit_mask(thd->audit_class_mask, event_class_mask))
+  {
+    plugin_foreach(thd, acquire_plugins, MYSQL_AUDIT_PLUGIN, &event_class);
+    add_audit_mask(thd->audit_class_mask, event_class_mask);
+  }
+  DBUG_VOID_RETURN;
+}
+ 
+
 /**
   Notify the audit system of an event
   
@@ -151,21 +175,8 @@ void mysql_audit_notify(THD *thd, uint event_class, uint event_subtype, ...)
 {
   va_list ap;
   audit_handler_t *handlers= audit_handlers + event_class;
-  unsigned long event_class_mask[MYSQL_AUDIT_CLASS_MASK_SIZE];
-  
   DBUG_ASSERT(event_class < audit_handlers_count);
-
+  mysql_audit_acquire_plugins(thd, event_class);
-  set_audit_mask(event_class_mask, event_class);
-  /*
-    Check to see if we have acquired the audit plugins for the
-    required audit event classes.
-  */
-  if (thd && check_audit_mask(thd->audit_class_mask, event_class_mask))
-  {
-    plugin_foreach(thd, acquire_plugins, MYSQL_AUDIT_PLUGIN, &event_class);
-    add_audit_mask(thd->audit_class_mask, event_class_mask);
-  }
-
   va_start(ap, event_subtype);  
   (*handlers)(thd, event_subtype, ap);
   va_end(ap);
@@ -448,6 +459,11 @@ static void event_class_dispatch(THD *thd, const struct mysql_event *event)
 #else /* EMBEDDED_LIBRARY */
 
 
+void mysql_audit_acquire_plugins(THD *thd, uint event_class)
+{
+}
+
+
 void mysql_audit_initialize()
 {
 }

sql/sql_audit.h

@@ -29,6 +29,7 @@ extern void mysql_audit_finalize();
 
 extern void mysql_audit_init_thd(THD *thd);
 extern void mysql_audit_free_thd(THD *thd);
+extern void mysql_audit_acquire_plugins(THD *thd, uint event_class);
 
 
 extern void mysql_audit_notify(THD *thd, uint event_class,

sql/sql_plugin.cc

@@ -29,7 +29,7 @@
 #include "records.h"          // init_read_record, end_read_record
 #include <my_pthread.h>
 #include <my_getopt.h>
-#include <mysql/plugin_audit.h>
+#include "sql_audit.h"
 #include "lock.h"                               // MYSQL_LOCK_IGNORE_TIMEOUT
 #define REPORT_TO_LOG  1
 #define REPORT_TO_USER 2
@@ -1703,6 +1703,27 @@ bool mysql_install_plugin(THD *thd, const LEX_STRING *name, const LEX_STRING *dl
                              MYSQL_LOCK_IGNORE_TIMEOUT)))
     DBUG_RETURN(TRUE);
 
+  /*
+    Pre-acquire audit plugins for events that may potentially occur
+    during [UN]INSTALL PLUGIN.
+
+    When audit event is triggered, audit subsystem acquires interested
+    plugins by walking through plugin list. Evidently plugin list
+    iterator protects plugin list by acquiring LOCK_plugin, see
+    plugin_foreach_with_mask().
+
+    On the other hand [UN]INSTALL PLUGIN is acquiring LOCK_plugin
+    rather for a long time.
+
+    When audit event is triggered during [UN]INSTALL PLUGIN, plugin
+    list iterator acquires the same lock (within the same thread)
+    second time.
+
+    This hack should be removed when LOCK_plugin is fixed so it
+    protects only what it supposed to protect.
+  */
+  mysql_audit_acquire_plugins(thd, MYSQL_AUDIT_GENERAL_CLASS);
+
   mysql_mutex_lock(&LOCK_plugin);
   mysql_rwlock_wrlock(&LOCK_system_variables_hash);
 
@@ -1783,6 +1804,27 @@ bool mysql_uninstall_plugin(THD *thd, const LEX_STRING *name)
   if (! (table= open_ltable(thd, &tables, TL_WRITE, MYSQL_LOCK_IGNORE_TIMEOUT)))
     DBUG_RETURN(TRUE);
 
+  /*
+    Pre-acquire audit plugins for events that may potentially occur
+    during [UN]INSTALL PLUGIN.
+
+    When audit event is triggered, audit subsystem acquires interested
+    plugins by walking through plugin list. Evidently plugin list
+    iterator protects plugin list by acquiring LOCK_plugin, see
+    plugin_foreach_with_mask().
+
+    On the other hand [UN]INSTALL PLUGIN is acquiring LOCK_plugin
+    rather for a long time.
+
+    When audit event is triggered during [UN]INSTALL PLUGIN, plugin
+    list iterator acquires the same lock (within the same thread)
+    second time.
+
+    This hack should be removed when LOCK_plugin is fixed so it
+    protects only what it supposed to protect.
+  */
+  mysql_audit_acquire_plugins(thd, MYSQL_AUDIT_GENERAL_CLASS);
+
   mysql_mutex_lock(&LOCK_plugin);
   if (!(plugin= plugin_find_internal(name, MYSQL_ANY_PLUGIN)))
   {