[PATCH 3/3] When remapping paths, only match whole path components

[Ximin Luo <infinity0@pwned.gg>]

Change the remapping algorithm so that each old_prefix only matches paths that
have old_prefix as a whole path component prefix.  (A whole path component is a
part of a path that begins and ends at a directory separator or at either end
of the path string.)

This remapping algorithm is more predictable than the old algorithm, because
there is no chance of mappings for one directory interfering with mappings for
other directories.  It contains less corner cases and therefore it is easier
for users to figure out how to set the mapping appropriately.  Therefore, I
believe it is better as a standardised algorithm that other build tools might
like to adopt, and so in our BUILD_PATH_PREFIX_MAP specification we recommend
this algorithm - though we allow others, and explicitly mention GCC's current
algorithm.  But it would be good for GCC to adopt this newer and cleaner one.

(The original idea came from discussions with rustc developers on this topic.)

This does technically break backwards-compatibility, but I was under the
impression that this option was not seen as such a critical feature, that this
would be too important.  Nevertheless, this part is totally independent from
the other patches and may be included or excluded as GCC maintainers desire.

Acknowledgements
----------------

Discussions with Michael Woerister and other members of the Rust compiler team
on Github, and discussions with Daniel Shahaf on the rb-general@ mailing list
on lists.reproducible-builds.org.

ChangeLogs
----------

libiberty/ChangeLog:

2017-07-21  Ximin Luo  <infinity0@pwned.gg>

	* prefix-map.c: When remapping paths, only match whole path components.

Index: gcc-8-20170716/libiberty/prefix-map.c
===================================================================
--- gcc-8-20170716.orig/libiberty/prefix-map.c
+++ gcc-8-20170716/libiberty/prefix-map.c
@@ -87,12 +87,22 @@ struct prefix_map *
 prefix_map_find (struct prefix_map *map, const char *old_name,
 		 const char **suffix, size_t *suf_len)
 {
+  size_t len;
+
   for (; map; map = map->next)
-    if (filename_ncmp (old_name, map->old_prefix, map->old_len) == 0)
-      {
-	*suf_len = strlen (*suffix = old_name + map->old_len);
-	break;
-      }
+    {
+      len = map->old_len;
+      /* Ignore trailing path separators at the end of old_prefix */
+      while (len > 0 && IS_DIR_SEPARATOR (map->old_prefix[len-1])) len--;
+      /* Check if old_name matches old_prefix at a path component boundary */
+      if (! filename_ncmp (old_name, map->old_prefix, len)
+	  && (IS_DIR_SEPARATOR (old_name[len])
+	      || old_name[len] == '\0'))
+	{
+	  *suf_len = strlen (*suffix = old_name + len);
+	  break;
+	}
+    }
 
   return map;
 }