Bug #32223 SETting max_allowed_packet variable

      
Inconsistent behavior of session variable max_allowed_packet 
(and net_buffer_length); only assignment to the global variable 
has any effect, without this being obvious to the user.
      
The patch for Bug#22891 is backported to 5.0, making the two
session variables read-only. As this is a backport to GA 
software, the error used when trying to assign to the read-
only variable is ER_UNKNOWN_ERROR. The error message is the 
same as in 5.1+.

mysql-test/t/variables.test:
  Tests are changed to account for the new semantics, and assignment to the read-only variables is added to test 
  the emission of the correct error message.
sql/set_var.cc:
  Both max_allowed_packet and net_buffer_length are changed 
  to be of type sys_var_thd_ulong_session_readonly. ER_UNKNOWN_ERROR is used to indicate an attempt to assign 
  to an instance of a read-only variable.
sql/set_var.h:
  Class sys_var_thd_ulong_session_readonly is added.

2 files changed, 41 insertions, 4 deletions

diff --git a/sql/set_var.cc b/sql/set_var.cc
index 7a85c1c0fa3..aebebb3b465 100644
--- a/sql/set_var.cc
+++ b/sql/set_var.cc
@@ -228,8 +228,8 @@ sys_var_thd_bool	sys_sql_low_priority_updates("sql_low_priority_updates",
 						     &SV::low_priority_updates,
 						     fix_low_priority_updates);
 #endif
-sys_var_thd_ulong	sys_max_allowed_packet("max_allowed_packet",
-					       &SV::max_allowed_packet);
+sys_var_thd_ulong_session_readonly sys_max_allowed_packet("max_allowed_packet",
+                                               &SV::max_allowed_packet);
 sys_var_long_ptr	sys_max_binlog_cache_size("max_binlog_cache_size",
 						  &max_binlog_cache_size);
 sys_var_long_ptr	sys_max_binlog_size("max_binlog_size",
@@ -296,8 +296,8 @@ sys_var_thd_enum        sys_myisam_stats_method("myisam_stats_method",
                                                 &myisam_stats_method_typelib,
                                                 NULL);
 
-sys_var_thd_ulong	sys_net_buffer_length("net_buffer_length",
-					      &SV::net_buffer_length);
+sys_var_thd_ulong_session_readonly sys_net_buffer_length("net_buffer_length",
+                                              &SV::net_buffer_length);
 sys_var_thd_ulong	sys_net_read_timeout("net_read_timeout",
 					     &SV::net_read_timeout,
 					     0, fix_net_read_timeout);
@@ -2948,6 +2948,21 @@ byte *sys_var_max_user_conn::value_ptr(THD *thd, enum_var_type type,
 }
 
 
+bool sys_var_thd_ulong_session_readonly::check(THD *thd, set_var *var)
+{
+  if (var->type != OPT_GLOBAL)
+  {
+    /* Due to backporting, this is actually ER_VARIABLE_IS_READONLY in 5.1+ */
+    my_printf_error(ER_UNKNOWN_ERROR, 
+             "SESSION variable %s is read-only. Use SET GLOBAL %s "
+             "to assign the value", MYF(0), name, name);
+    return TRUE;
+  }
+
+  return sys_var_thd_ulong::check(thd, var);
+}
+
+
 bool sys_var_thd_lc_time_names::check(THD *thd, set_var *var)
 {
   MY_LOCALE *locale_match;
diff --git a/sql/set_var.h b/sql/set_var.h
index f43d3b75cee..5fb883e8b86 100644
--- a/sql/set_var.h
+++ b/sql/set_var.h
@@ -873,6 +873,28 @@ public:
   byte *value_ptr(THD *thd, enum_var_type type, LEX_STRING *base);
 };
 
+
+/**
+ * @brief This is a specialization of sys_var_thd_ulong that implements a 
+   read-only session variable. The class overrides check() and check_default() 
+   to achieve the read-only property for the session part of the variable.
+ */
+class sys_var_thd_ulong_session_readonly : public sys_var_thd_ulong
+{
+public:
+  sys_var_thd_ulong_session_readonly(const char *name_arg, ulong SV::*offset_arg, 
+				     sys_check_func c_func= NULL,
+                                     sys_after_update_func au_func= NULL): 
+    sys_var_thd_ulong(name_arg, offset_arg, c_func, au_func)
+  { }
+  bool check(THD *thd, set_var *var);
+  bool check_default(enum_var_type type)
+  {
+    return type != OPT_GLOBAL || !option_limits;
+  }
+};
+
+
 class sys_var_trust_routine_creators :public sys_var_bool_ptr
 {
   /* We need a derived class only to have a warn_deprecated() */