empty mergeinfo produced by svn_mergeinfo_inheritable

I've been hunting down a "Mergeinfo for '...' maps to an empty
revision range" problem, and after cooking up a bunch of gdb macros
and tracing through
svn_fs_get_mergeinfo I think I have a handle on it. (I haven't
produced a stripped down case yet, I've been working with a copy of
our 17G 86000rev repository, though I'll attempt that next - I (and my
coworkers) wanted a second opinion on the analysis, though.)

The actual failure occurs in either an attempted merge or a "log -g";
the latter was easier to work with... do_logs calls
get_combined_mergeinfo_changes, which then calls
svn_fs_get_mergeinfo, eventually calling get_mergeinfo_for_path to
actually resolve the inherited mergeinfo. It finds some mergeinfo
higher up the tree, calls svn_mergeinfo_inheritable to filter it
(which produces an svn_mergeinfo_t with paths that have empty
rangelists.) That's where the problem appears; the empty rangelists
eventually get unparsed into a string that gets reparsed by
svn_parse_mergeinfo, which blows up on the empty values.

The workaround I'm looking at is to simply filter the empty mergeinfo
in append_to_merged_froms (patch below.) It might be more correct to
prevent svn_mergeinfo_inheritable from generating them in the first
place - *is* there a general rule about where producing/discarding
empty mergeinfo should happen? For example, svn_rangelist_diff talks
about permitting empty rangelists in their arguments, so they're not
completely forbidden...

Index: subversion/libsvn_fs_fs/tree.c
===================================================================
--- subversion/libsvn_fs_fs/tree.c (revision 87267)
+++ subversion/libsvn_fs_fs/tree.c (working copy)
@@ -3502,6 +3504,10 @@
       char *newpath;

       apr_hash_this(hi, &key, NULL, &val);
+ if (((apr_array_header_t *)val)->nelts == 0) {
+ /* [eichin:20081024T1511-04] don't propagate an empty one! */
+ continue;
+ }
       newpath = svn_path_join((const char *) key, path_piece, pool);
       apr_hash_set(*output, newpath, APR_HASH_KEY_STRING,
                    svn_rangelist_dup((apr_array_header_t *) val, pool));

(ignore the revision numbers, they're relative to a local repository,
though the function in question appears unchanged in 1.5.4, and 1.5.4
does exhibit the original failure.)

-- 
_Mark_ <eichin_at_metacarta.com> <eichin_at_gmail.com>
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