pgsql: postgres_fdw: Inherit the local transaction's access/deferrable

postgres_fdw: Inherit the local transaction's access/deferrable modes.

READ ONLY transactions should prevent modifications to foreign data as
well as local data, but postgres_fdw transactions declared as READ ONLY
that reference foreign tables mapped to a remote view executing volatile
functions would modify data on remote servers, as it would open remote
transactions in READ WRITE mode.

Similarly, DEFERRABLE transactions should not abort due to a
serialization failure even when accessing foreign data, but postgres_fdw
transactions declared as DEFERRABLE would abort due to that failure in a
remote server, as it would open remote transactions in NOT DEFERRABLE
mode.

To fix, modify postgres_fdw to open remote transactions in the same
access/deferrable modes as the local transaction. This commit also
modifies it to open remote subtransactions in the same access mode as
the local subtransaction.

This commit changes the behavior of READ ONLY/DEFERRABLE transactions
using postgres_fdw; in particular, it doesn't allow the READ ONLY
transactions to modify data on remote servers anymore, so such
transactions should be redeclared as READ WRITE or rewritten using other
tools like dblink. The release notes should note this as an
incompatibility.

These issues exist since the introduction of postgres_fdw, but to avoid
the incompatibility in the back branches, fix them in master only.

Author: Etsuro Fujita <etsuro(dot)fujita(at)gmail(dot)com>
Reviewed-by: Ashutosh Bapat <ashutosh(dot)bapat(dot)oss(at)gmail(dot)com>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Reviewed-by: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAPmGK16n_hcUUWuOdmeUS%2Bw4Q6dZvTEDHb%3DOP%3D5JBzo-M3QmpQ%40mail.gmail.com
Discussion: https://postgr.es/m/E1uLe9X-000zsY-2g%40gemulon.postgresql.org

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/de28140ded8d4ba00faf905ec3530ffeb8a34a53

Modified Files
--------------
contrib/postgres_fdw/connection.c | 124 ++++++++++++++++++++--
contrib/postgres_fdw/expected/postgres_fdw.out | 136 +++++++++++++++++++++++++
contrib/postgres_fdw/sql/postgres_fdw.sql | 80 +++++++++++++++
doc/src/sgml/postgres-fdw.sgml | 17 ++++
src/backend/access/transam/xact.c | 28 +++++
src/include/access/xact.h | 1 +
6 files changed, 378 insertions(+), 8 deletions(-)