It's not safe for nbtree VACUUM to attempt to delete a target page whose right sibling is already half-dead, since that would fail the cross-check when VACUUM attempts to re-find a downlink to the right sibling in the parent page. Logic to prevent this from happening was added by commit 8da31837803, which addressed a bug in the overhaul of page deletion that went into PostgreSQL 9.4 (commit efada2b8e92).
VACUUM was made to check the right sibling page, and back off when it happened to be half-dead already.
However, it is only truly necessary to do the right sibling check on the leaf level, since that transitively determines if the deletion target's parent's right sibling page is itself undergoing deletion. Remove the internal page level check, and add a comment explaining why the leaf level check alone suffices.
The extra check is also unnecessary due to the fact that internal pages that are marked half-dead are generally considered corrupt. Commit efada2b8e92 established the principle that there should never be half-dead internal pages (internal pages pending deletion are possible, but that status is never directly represented in the internal page).
VACUUM will complain about corruption when it encounters half-dead internal pages, so VACUUM is bound to raise an error one way or another when an nbtree index has a half-dead internal page (contrib/amcheck will also report that the page is corrupt).
It's possible that a pg_upgrade'd 9.3 database will still have half-dead internal pages, so it may seem like there is an argument for leaving the check in place to reliably get a cleaner error message that advises the user to REINDEX. However, leaf pages are also deleted in the first phase of deletion prior to PostgreSQL 9.4, so I believe we won't even attempt to re-find the parent page anyway (we won't have the fully deleted leaf page as the right sibling of our target page, so we won't even try to find a downlink for it).
3f58cc6dd8 Remove extra nbtree half-dead internal page check.
src/backend/access/nbtree/README | 14 +++++++++-----
src/backend/access/nbtree/nbtpage.c | 23 +++++++++++------------
2 files changed, 20 insertions(+), 17 deletions(-)