Turns out RB_INSERT() has a return value which I overlooked.
This return value can tell us whether an element we are trying
to insert is already present in the RB tree.

The patch below simply raises an error in such situations. Our existing
code should never allow this condition to occur. If we hit this error
then there is a bug that must fixed.

In the future, we could use this to optimize code which performs
a presence check before doing an insertion.
For example, assuming we propagate RB_INSERT() errors up via the
function got_object_idset_add(), instead of having code which does:

	if (!got_object_idset_contains(idset, id)) {
		err = got_object_idset_add(idset, id, NULL);
		if (err)
			goto done;
	}

... we could have code which does the same thing with just
a single walk of the RB tree instead of two walks:

	err = got_object_idset_add(idset, id, NULL);
	if (err && err->code != GOT_ERR_OBJ_EXISTS)
		goto done;

I have a WIP patch which makes some changes of the above nature
in pack_create.c, but it does not help with performance (yet?).
It is possible that we spend so much time looking things up that
the overhead related to additions is too small to make a difference.
In principle it is always a good idea to avoid doing any extra
work which we can avoid doing.

In any case, as a first step, we should check the return value.

ok?

diff 3d1a1e4cbc91e39df83eb35eb3df3cc87719a0b5 096ea7a61d69620491f66b05fccd7e0e72f61f2b
blob - 15a27715685e7d137844ebe2be87a33d550d6f96
blob + 8ae16da40c07f4dcb3133f9ff08328a10605722b
--- include/got_error.h
+++ include/got_error.h
@@ -167,6 +167,7 @@
 #define GOT_ERR_NO_PATCH	149
 #define GOT_ERR_HUNK_FAILED	150
 #define GOT_ERR_PATCH_FAILED	151
+#define GOT_ERR_FILEIDX_DUP_ENTRY 152
 
 struct got_error {
         int code;
blob - 19d6e558696c99c50807ee4f730af001911d1f1f
blob + 29541c0234b7f96a4638157fdd9017888571a76c
--- lib/error.c
+++ lib/error.c
@@ -215,6 +215,7 @@ static const struct got_error got_errors[] = {
 	{ GOT_ERR_NO_PATCH, "no patch found" },
 	{ GOT_ERR_HUNK_FAILED, "hunk failed to apply" },
 	{ GOT_ERR_PATCH_FAILED, "patch failed to apply" },
+	{ GOT_ERR_FILEIDX_DUP_ENTRY, "duplicate file index entry" },
 };
 
 static struct got_custom_error {
blob - 6ea3deaf8e0753eaf31a2c0f81af1a25b313be30
blob + b05a8137d02606a3c4fee8f04809b0ac5cd71e8d
--- lib/fileindex.c
+++ lib/fileindex.c
@@ -265,7 +265,9 @@ add_entry(struct got_fileindex *fileindex, struct got_
 	if (fileindex->nentries >= GOT_FILEIDX_MAX_ENTRIES)
 		return got_error(GOT_ERR_NO_SPACE);
 
-	RB_INSERT(got_fileindex_tree, &fileindex->entries, ie);
+	if (RB_INSERT(got_fileindex_tree, &fileindex->entries, ie) != NULL)
+		return got_error_path(ie->path, GOT_ERR_FILEIDX_DUP_ENTRY);
+
 	fileindex->nentries++;
 	return NULL;
 }
blob - 37ea8e262784f9521e53f7db7f9ff11232dce3d6
blob + aee8d02645d4856a33f795cd0888ee6ae12d64ac
--- lib/object_idset.c
+++ lib/object_idset.c
@@ -102,7 +102,9 @@ got_object_idset_add(struct got_object_idset *set, str
 	memcpy(&new->id, id, sizeof(new->id));
 	new->data = data;
 
-	RB_INSERT(got_object_idset_tree, &set->entries, new);
+	if (RB_INSERT(got_object_idset_tree, &set->entries, new) != NULL)
+		return got_error(GOT_ERR_OBJ_EXISTS);
+
 	set->totelem++;
 	return NULL;
 }