Fix modify_defaults_file as agreed with monty (correct all occurences of an option in the section)

mysys/default_modify.c:
  Correct all occurences of an option in the section. To avoid memory problems preallocate reserve of 1K, and
  realloc it later if needed (almost never).

mysys/default_modify.c

@@ -20,6 +20,8 @@
 #include <my_dir.h>
 
 #define BUFF_SIZE 1024
+/* should be big enough to handle at least one line */
+#define RESERVE 1024
 
 #ifdef __WIN__
 #define NEWLINE "\r\n"
@@ -66,8 +68,10 @@ int modify_defaults_file(const char *file_location, const char *option,
   FILE *cnf_file;
   MY_STAT file_stat;
   char linebuff[BUFF_SIZE], *src_ptr, *dst_ptr, *file_buffer;
-  uint opt_len, optval_len, sect_len, nr_newlines= 0;
+  uint opt_len, optval_len, sect_len, nr_newlines= 0, buffer_size;
   my_bool in_section= FALSE, opt_applied= 0;
+  int reserve_occupied= 0, reserve_extended= 1, old_opt_len;
+  int new_opt_len= opt_len + 1 + optval_len + NEWLINE_LEN;
   DBUG_ENTER("modify_defaults_file");
 
   if (!(cnf_file= my_fopen(file_location, O_RDWR | O_BINARY, MYF(0))))
@@ -80,23 +84,26 @@ int modify_defaults_file(const char *file_location, const char *option,
   opt_len= (uint) strlen(option);
   optval_len= (uint) strlen(option_value);
 
+  /* calculate the size of the buffer we need */
+  buffer_size= sizeof(char) * (file_stat.st_size +
+                               /* option name len */
+                               opt_len +
+                               /* reserve for '=' char */
+                               1 +
+                               /* option value len */
+                               optval_len +
+                               /* reserve space for newline */
+                               NEWLINE_LEN +
+                               /* The ending zero */
+                               1 +
+                               /* reserve some additional space */
+                               RESERVE);
+
   /*
     Reserve space to read the contents of the file and some more
     for the option we want to add.
   */
-  if (!(file_buffer= (char*) my_malloc(sizeof(char) *
-                                       (file_stat.st_size +
-                                        /* option name len */
-                                        opt_len +
-                                        /* reserve space for newline */
-                                        NEWLINE_LEN +
-                                        /* reserve for '=' char */
-                                        1 +
-                                        /* option value len */
-                                        optval_len +
-                                        /* The ending zero */
-                                        1), MYF(MY_WME))))
-
+  if (!(file_buffer= (char*) my_malloc(buffer_size, MYF(MY_WME))))
     goto malloc_err;
 
   sect_len= (uint) strlen(section_name);
@@ -115,13 +122,31 @@ int modify_defaults_file(const char *file_location, const char *option,
     }
 
     /* correct the option */
-    if (!opt_applied && in_section && !strncmp(src_ptr, option, opt_len) &&
+    if (in_section && !strncmp(src_ptr, option, opt_len) &&
         (*(src_ptr + opt_len) == '=' ||
          my_isspace(&my_charset_latin1, *(src_ptr + opt_len)) ||
          *(src_ptr + opt_len) == '\0'))
     {
+      /*
+        we should change all options. If opt_applied is set, we are running
+        into reserved memory area. Hence we should check for overruns.
+      */
+      if (opt_applied)
+      {
+        old_opt_len= strlen(linebuff);
+        /* could be negative */
+        reserve_occupied+= new_opt_len - old_opt_len;
+        if (reserve_occupied > RESERVE*reserve_extended)
+        {
+          file_buffer= (char*) my_realloc(file_buffer, buffer_size +
+                                          RESERVE*reserve_extended,
+                                          MYF(MY_WME));
+          reserve_extended++;
+        }
+      }
+      else
+        opt_applied= 1;
       dst_ptr= add_option(dst_ptr, option_value, option, remove_option);
-      opt_applied= 1;
     }
     else
     {