pgsql: Fix lookup code for REINDEX INDEX.

Fix lookup code for REINDEX INDEX.

This commit adjusts RangeVarCallbackForReindexIndex() to handle an
extremely unlikely race condition involving concurrent OID reuse.
In short, if REINDEX INDEX is executed at the same time that the
index is re-created with the same name and OID but a different
parent table OID, we might lock the wrong parent table. To fix,
simply detect when this happens and emit an ERROR. Unfortunately,
we can't gracefully handle this situation because we will have
already locked the index, and we must lock the parent table before
the index to avoid deadlocks.

While at it, I've replaced all but one early return in this
callback function with ERRORs that should be unreachable. While I
haven't verified the presence of a live bug, the checks in question
appear to be unnecessary, and the early returns seem prone to
breaking the parent table locking code in subtle ways. If nothing
else, this simplifies the code a bit.

This is a bug fix and could be back-patched, but given the presumed
rarity of the race condition and the lack of reports, I'm not going
to bother.

Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Reviewed-by: Jeff Davis <pgsql(at)j-davis(dot)com>
Discussion: https://postgr.es/m/Z8zwVmGzXyDdkAXj%40nathan

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/079480dc20223dfe03d60fcabda9c0a65e30d0d5

Modified Files
--------------
src/backend/commands/indexcmds.c | 50 ++++++++++++++++++++--------------------
1 file changed, 25 insertions(+), 25 deletions(-)