COPY into partitioned table with cstore_fdw partitions crashes

[marcocitus]

Postgres 11 adds a facility for copying into a partitioned table which has foreign table partitions. The logic calls BeginForeignInsert-ExecForeignInsert-EndForeignInsert, but cstore_fdw expects BeginForeignModify to be called prior to ExecForeignInsert. This results in:

TRAP: FailedAssertion("!(writeState != ((void *)0))", File: "cstore_fdw.c", Line: 2275)
2018-12-04 00:26:56.005 CET [179198] LOG:  server process (PID 179209) was terminated by signal 6: Aborted

[marcocitus]

The same happens when trying to COPY into a cstore_fdw without shared_preload_libraries set.