Re: Bug in merge-recursive in virtual commit corner case

Shawn Pearce [off-list ref] writes:

I found the above error message in tree-diff.c's diff_tree_sha1
function.  I threw in debugging and found that the new tree was
the root tree of one branch and the base was the root tree of some
other revision.

Apparently the empty tree is being created in merge-recursive.c:

   1219     if (merged_common_ancestors == NULL) {
   1220         /* if there is no common ancestor, make an empty tree */
   1221         struct tree *tree = xcalloc(1, sizeof(struct tree));
   1222
   1223         tree->object.parsed = 1;
   1224         tree->object.type = OBJ_TREE;
   1225         hash_sha1_file(NULL, 0, tree_type, tree->object.sha1);
   1226         merged_common_ancestors = make_virtual_commit(tree, "ancestor");
   1227     }

So basically this code crashes if its ever used in a repository
that hasn't had a need for the empty tree before.  :-(

I hit the same issue when I integrated Johannes's in-core merge;
I originally used hash_sha1_file() but that results in objects
that are supposed to be in the virtual parent unreadable when
merging the real children.  The key is to use write_sha1_file()
to actually create the needed objects, and trust later prune to
remove them.

Replace it with write_sha1_file() and you should be fine, I
think.