The wt_txn_recover function doesn't open an internal session, it opens a normal session:

/* We need a real session for recovery. */
WT_RET(__wt_open_session(conn, NULL, NULL, true, &session));
F_SET(session, WT_SESSION_NO_LOGGING);
r.session = session;

I was looking at this in the context of WT-2140: that ticket concerns freeing temporary allocated memory after an application thread does reconciliation, and the obvious way to do that is based on the WT_SESSION_INTERNAL flag, which recovery won't set. Depending on how many blocks recovery writes, this could potentially slow down recovery.

If this code is correct, I think it deserves a comment, it's the only place in WiredTiger where an internal action opens a "normal" session.