The attached patch implements the suggestion by Amit before.
What I'm motivated is to collect extra run-time statistics specific to a particular ForeignScan/CustomScan, not only the standard Instrumentation; like DMA transfer rate or execution time of GPU kernels in my case.
Per-node DSM toc is one of the best way to return run-time statistics to the master backend, because FDW/CSP can assign arbitrary length of the region according to its needs. It is quite easy to require. However, one problem is, the per-node DSM toc is already released when ExecEndNode() is called on the child node of Gather.
This patch allows extensions to get control on the master backend's context when all the worker node gets finished but prior to release of the DSM segment. If FDW/CSP has its special statistics on the segment, it can move to the private memory area for EXPLAIN output or something other purpose.
One design consideration is whether the hook shall be called from ExecParallelRetrieveInstrumentation() or ExecParallelFinish(). The former is a function to retrieve the standard Instrumentation information, thus, it is valid only if EXPLAIN ANALYZE. On the other hands, if we put entrypoint at ExecParallelFinish(), extension can get control regardless of EXPLAIN ANALYZE, however, it also needs an extra planstate_tree_walker().
Right now, we don't assume anything onto the requirement by FDW/CSP. It may want run-time statistics regardless of EXPLAIN ANALYZE, thus, hook shall be invoked always when Gather node confirmed termination of the worker processes.