BUG#5964 - 4.1 MERGE tables regression from 4.0

Changed the creation of the .MRG file so that only the table  name
is written when the MyISAM table is in the same database as the
MERGE table, a relative path is used in other cases in mysqld,
and possibly an absolute path is used in an embedded server.
No test case is added as the external behaviour is unchanged.
Only the file names within the .MRG file are changed.

include/my_sys.h

@@ -609,6 +609,7 @@ extern uint dirname_part(my_string to,const char *name);
 extern uint dirname_length(const char *name);
 #define base_name(A) (A+dirname_length(A))
 extern int test_if_hard_path(const char *dir_name);
+extern my_bool has_path(const char *name);
 extern char *convert_dirname(char *to, const char *from, const char *from_end);
 extern void to_unix_path(my_string name);
 extern my_string fn_ext(const char *name);

myisammrg/myrg_open.c

@@ -80,7 +80,7 @@ MYRG_INFO *myrg_open(const char *name, int mode, int handle_locking)
       continue;		/* Skip comments */
     }
 
-    if (!test_if_hard_path(buff))
+    if (!has_path(buff))
     {
       VOID(strmake(name_buff+dir_length,buff,
                    sizeof(name_buff)-1-dir_length));

mysys/my_getwd.c

@@ -192,3 +192,25 @@ int test_if_hard_path(register const char *dir_name)
   return FALSE;
 #endif
 } /* test_if_hard_path */
+
+
+/*
+  Test if a name contains an (absolute or relative) path.
+
+  SYNOPSIS
+    has_path()
+    name                The name to test.
+
+  RETURN
+    TRUE        name contains a path.
+    FALSE       name does not contain a path.
+*/
+
+my_bool has_path(const char *name)
+{
+  return test(strchr(name, FN_LIBCHAR))
+#ifdef FN_DEVCHAR
+    || test(strchr(name, FN_DEVCHAR))
+#endif
+    ;
+}

sql/ha_myisammrg.cc

@@ -381,6 +381,7 @@ int ha_myisammrg::create(const char *name, register TABLE *form,
   char buff[FN_REFLEN],**table_names,**pos;
   TABLE_LIST *tables= (TABLE_LIST*) create_info->merge_list.first;
   THD *thd= current_thd;
+  uint dirlgt= dirname_length(name);
   DBUG_ENTER("ha_myisammrg::create");
 
   if (!(table_names= (char**) thd->alloc((create_info->merge_list.elements+1)*
@@ -394,11 +395,30 @@ int ha_myisammrg::create(const char *name, register TABLE *form,
       tbl= find_temporary_table(thd, tables->db, tables->real_name);
     if (!tbl)
     {
-      uint length= my_snprintf(buff,FN_REFLEN,"%s%s/%s",
-			       mysql_real_data_home,
+      /*
+        Construct the path to the MyISAM table. Try to meet two conditions:
+        1.) Allow to include MyISAM tables from different databases, and
+        2.) allow for moving DATADIR around in the file system.
+        The first means that we need paths in the .MRG file. The second
+        means that we should not have absolute paths in the .MRG file.
+        The best, we can do, is to use 'mysql_data_home', which is '.'
+        in mysqld and may be an absolute path in an embedded server.
+        This means that it might not be possible to move the DATADIR of
+        an embedded server without changing the paths in the .MRG file.
+      */
+      uint length= my_snprintf(buff, FN_REFLEN, "%s/%s/%s", mysql_data_home,
 			       tables->db, tables->real_name);
-      if (!(table_name= thd->strmake(buff, length)))
-	DBUG_RETURN(HA_ERR_OUT_OF_MEM);
+      /*
+        If a MyISAM table is in the same directory as the MERGE table,
+        we use the table name without a path. This means that the
+        DATADIR can easily be moved even for an embedded server as long
+        as the MyISAM tables are from the same database as the MERGE table.
+      */
+      if ((dirname_length(buff) == dirlgt) && ! memcmp(buff, name, dirlgt))
+        table_name= tables->real_name;
+      else
+        if (! (table_name= thd->strmake(buff, length)))
+          DBUG_RETURN(HA_ERR_OUT_OF_MEM);
     }
     else
       table_name=(*tbl)->path;