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

[Junio C Hamano <junkio@cox.net>]

Shawn Pearce <spearce@spearce.org> 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.