Fix bug #31604: server crash when setting slow_query_log_file/global general_log_file variable

Problem: we don't evaluate given expression checking values of the
slow_query_log_file/general_log_file, don't check it for NULL.

Fix: evaluate the expression, check result returned.

mysql-test/r/log_state.result

@@ -175,3 +175,16 @@ SET GLOBAL slow_query_log = ON;
 SET GLOBAL READ_ONLY = OFF;
 SET GLOBAL general_log = @old_general_log_state;
 SET GLOBAL slow_query_log = @old_slow_log_state;
+set @old_general_log_file= @@global.general_log_file;
+set @old_slow_query_log_file= @@global.slow_query_log_file;
+set global general_log_file= concat('/not exiting path/log.maste', 'r');
+ERROR 42000: Variable 'general_log_file' can't be set to the value of '/not exiting path/log.master'
+set global general_log_file= NULL;
+ERROR 42000: Variable 'general_log_file' can't be set to the value of 'NULL'
+set global slow_query_log_file= concat('/not exiting path/log.maste', 'r');
+ERROR 42000: Variable 'slow_query_log_file' can't be set to the value of '/not exiting path/log.master'
+set global slow_query_log_file= NULL;
+ERROR 42000: Variable 'slow_query_log_file' can't be set to the value of 'NULL'
+set global general_log_file= @old_general_log_file;
+set global slow_query_log_file= @old_slow_query_log_file;
+End of 5.1 tests

mysql-test/t/log_state.test

@@ -189,3 +189,23 @@ disconnect con1;
 # Remove the log files that was created in the "default location"
 # i.e var/run
 --remove_file $MYSQLTEST_VARDIR/run/master.log
+
+#
+# Bug #31604: server crash when setting slow_query_log_file/general_log_file
+#
+set @old_general_log_file= @@global.general_log_file;
+set @old_slow_query_log_file= @@global.slow_query_log_file;
+
+--error 1231
+set global general_log_file= concat('/not exiting path/log.maste', 'r');
+--error 1231
+set global general_log_file= NULL;
+--error 1231
+set global slow_query_log_file= concat('/not exiting path/log.maste', 'r');
+--error 1231
+set global slow_query_log_file= NULL;
+
+set global general_log_file= @old_general_log_file;
+set global slow_query_log_file= @old_slow_query_log_file;
+
+--echo End of 5.1 tests

sql/set_var.cc

@@ -2102,18 +2102,24 @@ void sys_var_log_state::set_default(THD *thd, enum_var_type type)
 
 static int  sys_check_log_path(THD *thd,  set_var *var)
 {
-  char path[FN_REFLEN];
+  char path[FN_REFLEN], buff[FN_REFLEN];
   MY_STAT f_stat;
-  const char *var_path= var->value->str_value.ptr();
+  String str(buff, sizeof(buff), system_charset_info), *res;
+  const char *log_file_str;
+      
+  if (!(res= var->value->val_str(&str)))
+    goto err;
+
+  log_file_str= res->c_ptr();
   bzero(&f_stat, sizeof(MY_STAT));
 
-  (void) unpack_filename(path, var_path);
+  (void) unpack_filename(path, log_file_str);
   if (my_stat(path, &f_stat, MYF(0)))
   {
     /* Check if argument is a file and we have 'write' permission */
     if (!MY_S_ISREG(f_stat.st_mode) ||
         !(f_stat.st_mode & MY_S_IWRITE))
-      return -1;
+      goto err;
   }
   else
   {
@@ -2122,11 +2128,16 @@ static int  sys_check_log_path(THD *thd,  set_var *var)
       Check if directory exists and 
       we have permission to create file & write to file
     */
-    (void) dirname_part(path, var_path, &path_length);
+    (void) dirname_part(path, log_file_str, &path_length);
     if (my_access(path, (F_OK|W_OK)))
-      return -1;
+      goto err;
   }
   return 0;
+
+err:
+  my_error(ER_WRONG_VALUE_FOR_VAR, MYF(0), var->var->name, 
+           res ? log_file_str : "NULL");
+  return 1;
 }