CustomScan and readfuncs.c

Hello,

Under the investigation of ParallelAppend, I noticed here is a few
problems in CustomScan, that prevents to reproduce an equivalent
plan node on the background worker from serialized string.

1. CustomScanMethods->TextOutCustomScan callback
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This callback allows to output custom information on nodeToString.
Originally, we intend to use this callback for debug only, because
CustomScan must be copyObject() safe, thus, all the private data
also must be stored in custom_exprs or custom_private.
However, it will lead another problem when we try to reproduce
CustomScan node from the string form generated by outfuncs.c.
If TextOutCustomScan prints something, upcoming _readCustomScan
has to deal with unexpected number of tokens in unexpected format.
I'd like to propose to omit this callback prior to v9.5 release,
for least compatibility issues.

2. Reproduce method table on background worker
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The method field of CustomPath/Scan/ScanState is expected to be
a reference to a static structure. Thus, copyObject() does not
copy the entire table, but only pointers.
However, we have no way to guarantee the callback functions have
same entrypoint addressed on background workers. So, we may need
an infrastructure to reproduce same CustomScan node with same
callback function tables, probably, identified by name.
We may have a few ways to solve the problem.

* Add system catalog, function returns pointer
The simplest way, like FDW. System catalog has a name and function
to return callback pointers. It also needs SQL statement support,
even a little down side.

* Registered by name, during shared_preload_libraries only
Like an early version of CustomScan interface, it requires custom-
scan providers to register a pair of name and callbacks, but only
when shared_preload_libraries is processed, to guarantee the callbacks
are also registered in the background workers also.
(Is this assumption right on windows?)

Thanks,
--
NEC Business Creation Division / PG-Strom Project
KaiGai Kohei <kaigai(at)ak(dot)jp(dot)nec(dot)com>