Memory leak in FDW

Foreign data wrapper's IterateForeignScan() function is supposed to be
called in a short-lived memory context, but the memory context is
actually not reset during query execution. That's a pretty bad memory
leak. I've been testing this with file_fdw and a large file, and "SELECT
COUNT(*) FROM foreign_table"

Interestingly, if you add any WHERE clause to it, the memory context is
reset in ExecScan and the leak goes away. This is only a problem with
the fastpath in ExecScan for the case of no quals and no projections.

The trivial fix is to reset the per-tuple memory context between
iterations. I tried to look around for other executor nodes that might
have the same problem. I didn't see any obvious leaks, although index
scan node seems to call AM's getnext without resetting the memory
context in between. That's a pretty well-tested codepath, however, and
there hasn't been any complains of leaks with index scans, so there must
be something that mitigates it.

--
   Heikki Linnakangas
   EnterpriseDB   http://www.enterprisedb.com