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.

mysql-test/r/func_compress.result

@@ -68,7 +68,7 @@ Warnings:
 Error	1259	ZLIB: Input data corrupted
 Error	1256	Uncompressed data size too large; the maximum size is 1048576 (probably, length of uncompressed data was corrupted)
 drop table t1;
-set @@max_allowed_packet=1048576*100;
+set @@global.max_allowed_packet=1048576*100;
 select compress(repeat('aaaaaaaaaa', IF(XXX, 10, 10000000))) is null;
 compress(repeat('aaaaaaaaaa', IF(XXX, 10, 10000000))) is null
 0

mysql-test/r/packet.result

@@ -3,32 +3,22 @@ set @net_buffer_length=@@global.net_buffer_length;
 set global max_allowed_packet=100;
 Warnings:
 Warning	1292	Truncated incorrect max_allowed_packet value: '100'
-set max_allowed_packet=100;
-Warnings:
-Warning	1292	Truncated incorrect max_allowed_packet value: '100'
 set global net_buffer_length=100;
 Warnings:
 Warning	1292	Truncated incorrect net_buffer_length value: '100'
-set net_buffer_length=100;
-Warnings:
-Warning	1292	Truncated incorrect net_buffer_length value: '100'
 SELECT length("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa") as len;
 len
 1024
 select repeat('a',2000);
 repeat('a',2000)
-NULL
-Warnings:
-Warning	1301	Result of repeat() was larger than max_allowed_packet (1024) - truncated
+aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
 select @@net_buffer_length, @@max_allowed_packet;
 @@net_buffer_length	@@max_allowed_packet
 1024	1024
 SELECT length("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa") as len;
 ERROR 08S01: Got a packet bigger than 'max_allowed_packet' bytes
 set global max_allowed_packet=default;
-set max_allowed_packet=default;
 set global net_buffer_length=default;
-set net_buffer_length=default;
 SELECT length("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa") as len;
 len
 100

mysql-test/r/variables.result

@@ -187,7 +187,7 @@ Variable_name	Value
 myisam_max_sort_file_size	FILE_SIZE
 set global net_retry_count=10, session net_retry_count=10;
 set global net_buffer_length=1024, net_write_timeout=200, net_read_timeout=300;
-set session net_buffer_length=2048, net_write_timeout=500, net_read_timeout=600;
+set session net_write_timeout=500, net_read_timeout=600;
 show global variables like 'net_%';
 Variable_name	Value
 net_buffer_length	1024
@@ -196,35 +196,39 @@ net_retry_count	10
 net_write_timeout	200
 show session variables like 'net_%';
 Variable_name	Value
-net_buffer_length	2048
+net_buffer_length	16384
 net_read_timeout	600
 net_retry_count	10
 net_write_timeout	500
-set session net_buffer_length=8000, global net_read_timeout=900, net_write_timeout=1000;
+set global net_buffer_length=8000, net_read_timeout=900, net_write_timeout=1000;
 show global variables like 'net_%';
 Variable_name	Value
-net_buffer_length	1024
+net_buffer_length	7168
 net_read_timeout	900
 net_retry_count	10
 net_write_timeout	1000
 show session variables like 'net_%';
 Variable_name	Value
-net_buffer_length	7168
+net_buffer_length	16384
 net_read_timeout	600
 net_retry_count	10
 net_write_timeout	500
-set net_buffer_length=1;
+set global net_buffer_length=1;
 Warnings:
 Warning	1292	Truncated incorrect net_buffer_length value: '1'
-show variables like 'net_buffer_length';
+show global variables like 'net_buffer_length';
 Variable_name	Value
 net_buffer_length	1024
-set net_buffer_length=2000000000;
+set global net_buffer_length=2000000000;
 Warnings:
 Warning	1292	Truncated incorrect net_buffer_length value: '2000000000'
-show variables like 'net_buffer_length';
+show global variables like 'net_buffer_length';
 Variable_name	Value
 net_buffer_length	1048576
+set session net_buffer_length=1024;
+ERROR HY000: SESSION variable net_buffer_length is read-only. Use SET GLOBAL net_buffer_length to assign the value
+set session max_allowed_packet=1024;
+ERROR HY000: SESSION variable max_allowed_packet is read-only. Use SET GLOBAL max_allowed_packet to assign the value
 set character set cp1251_koi8;
 show variables like "character_set_client";
 Variable_name	Value
@@ -393,7 +397,7 @@ set last_insert_id=1;
 set global local_infile=1;
 set long_query_time=100;
 set low_priority_updates=1;
-set max_allowed_packet=100;
+set global max_allowed_packet=100;
 Warnings:
 Warning	1292	Truncated incorrect max_allowed_packet value: '100'
 set global max_binlog_cache_size=100;
@@ -417,7 +421,7 @@ select @@max_user_connections;
 100
 set global max_write_lock_count=100;
 set myisam_sort_buffer_size=100;
-set net_buffer_length=100;
+set global net_buffer_length=100;
 Warnings:
 Warning	1292	Truncated incorrect net_buffer_length value: '100'
 set net_read_timeout=100;

mysql-test/t/func_compress.test

@@ -43,8 +43,10 @@ drop table t1;
 # note that when LOW_MEMORY is set the "test" below is meaningless
 #
 
-set @@max_allowed_packet=1048576*100;
+set @@global.max_allowed_packet=1048576*100;
 --replace_result "''" XXX "'1'" XXX
+# reconnect to make the new max packet size take effect
+--connect (newconn, localhost, root,,)
 eval select compress(repeat('aaaaaaaaaa', IF('$LOW_MEMORY', 10, 10000000))) is null;
 
 #

mysql-test/t/packet.test

@@ -15,31 +15,33 @@
 set @max_allowed_packet=@@global.max_allowed_packet;
 set @net_buffer_length=@@global.net_buffer_length;
 
-connect (con1,localhost,root,,);
-connection con1;
+
+# setting values below minimum threshold of 1024 will cause truncating
 set global max_allowed_packet=100;
-set max_allowed_packet=100;
 set global net_buffer_length=100;
-set net_buffer_length=100;
-# Have to be > 1024 as min value of net_buffer_length is 1024
+
+# is not yet in effect
 SELECT length("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa") as len;
-# Should return NULL as 2000 is bigger than max_allowed_packet
 select repeat('a',2000);
-connection default;
-disconnect con1;
 
 #
-# Connection 2 should get error for too big packets
+# Connection 1 should get error for too big packets
 #
-connect (con2,localhost,root,,);
-connection con2;
+connect (con1,localhost,root,,);
+connection con1;
 select @@net_buffer_length, @@max_allowed_packet;
 --error ER_NET_PACKET_TOO_LARGE
 SELECT length("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa") as len;
+connection default;
+disconnect con1;
+
+#
+# Reset to default values and reconnect
+#
 set global max_allowed_packet=default;
-set max_allowed_packet=default;
 set global net_buffer_length=default;
-set net_buffer_length=default;
+connect (con2,localhost,root,,);
+connection con2;
 SELECT length("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa") as len;
 select length(repeat('a',2000));
 connection default;

mysql-test/t/variables.test

@@ -131,17 +131,22 @@ show global variables like 'myisam_max_sort_file_size';
 
 set global net_retry_count=10, session net_retry_count=10;
 set global net_buffer_length=1024, net_write_timeout=200, net_read_timeout=300;
-set session net_buffer_length=2048, net_write_timeout=500, net_read_timeout=600;
+set session net_write_timeout=500, net_read_timeout=600;
 show global variables like 'net_%';
 show session variables like 'net_%';
-set session net_buffer_length=8000, global net_read_timeout=900, net_write_timeout=1000;
+set global net_buffer_length=8000, net_read_timeout=900, net_write_timeout=1000;
 show global variables like 'net_%';
 show session variables like 'net_%';
-set net_buffer_length=1;
-show variables like 'net_buffer_length';
+set global net_buffer_length=1;
+show global variables like 'net_buffer_length';
 #warning 1292
-set net_buffer_length=2000000000;
-show variables like 'net_buffer_length';
+set global net_buffer_length=2000000000;
+show global variables like 'net_buffer_length';
+
+--error ER_UNKNOWN_ERROR
+set session net_buffer_length=1024;
+--error ER_UNKNOWN_ERROR
+set session max_allowed_packet=1024;
 
 set character set cp1251_koi8;
 show variables like "character_set_client";
@@ -272,7 +277,7 @@ set last_insert_id=1;
 set global local_infile=1;
 set long_query_time=100;
 set low_priority_updates=1;
-set max_allowed_packet=100;
+set global max_allowed_packet=100;
 set global max_binlog_cache_size=100;
 set global max_binlog_size=100;
 set global max_connect_errors=100;
@@ -286,7 +291,7 @@ set global max_user_connections=100;
 select @@max_user_connections;
 set global max_write_lock_count=100;
 set myisam_sort_buffer_size=100;
-set net_buffer_length=100;
+set global net_buffer_length=100;
 set net_read_timeout=100;
 set net_write_timeout=100;
 set global query_cache_limit=100;

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;

sql/set_var.h

@@ -873,6 +873,28 @@ class sys_var_max_user_conn : public sys_var_thd
   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() */