Allow parallel custom and foreign scans.

This patch doesn't put the new infrastructure to use anywhere, and
indeed it's not clear how it could ever be used for something like
postgres_fdw which has to send an SQL query and wait for a reply,
but there might be FDWs or custom scan providers that are CPU-bound,
so let's give them a way to join club parallel.

KaiGai Kohei, reviewed by me.

doc/src/sgml/custom-scan.sgml

@@ -303,6 +303,43 @@ void (*RestrPosCustomScan) (CustomScanState *node);
 
    <para>
 <programlisting>
+Size (*EstimateDSMCustomScan) (CustomScanState *node,
+                               ParallelContext *pcxt);
+</programlisting>
+    Estimate the amount of dynamic shared memory that will be required
+    for parallel operation.  This may be higher than the amount that will
+    actually be used, but it must not be lower.  The return value is in bytes.
+    This callback is optional, and need only be supplied if this custom
+    scan provider supports parallel execution.
+   </para>
+
+   <para>
+<programlisting>
+void (*InitializeDSMCustomScan) (CustomScanState *node,
+                                 ParallelContext *pcxt,
+                                 void *coordinate);
+</programlisting>
+    Initialize the dynamic shared memory that will be required for parallel
+    operation; <literal>coordinate</> points to an amount of allocated space
+    equal to the return value of <function>EstimateDSMCustomScan</>.
+    This callback is optional, and need only be supplied if this custom
+    scan provider supports parallel execution.
+   </para>
+
+   <para>
+<programlisting>
+void (*InitializeWorkerCustomScan) (CustomScanState *node,
+                                    shm_toc *toc,
+                                    void *coordinate);
+</programlisting>
+    Initialize a parallel worker's custom state based on the shared state
+    set up in the leader by <literal>InitializeDSMCustomScan</>.
+    This callback is optional, and needs only be supplied if this
+    custom path supports parallel execution.
+   </para>
+
+   <para>
+<programlisting>
 void (*ExplainCustomScan) (CustomScanState *node,
                            List *ancestors,
                            ExplainState *es);

doc/src/sgml/fdwhandler.sgml

@@ -955,6 +955,53 @@ ImportForeignSchema (ImportForeignSchemaStmt *stmt, Oid serverOid);
 
    </sect2>
 
+   <sect2 id="fdw-callbacks-parallel">
+    <title>FDW Routines for Parallel Execution</title>
+    <para>
+     A <structname>ForeignScan</> node can, optionally, support parallel
+     execution.  A parallel <structname>ForeignScan</> will be executed
+     in multiple processes and should return each row only once across
+     all cooperating processes.  To do this, processes can coordinate through
+     fixed size chunks of dynamic shared memory.  This shared memory is not
+     guaranteed to be mapped at the same address in every process, so pointers
+     may not be used.  The following callbacks are all optional in general,
+     but required if parallel execution is to be supported.
+    </para>
+
+    <para>
+<programlisting>
+Size
+EstimateDSMForeignScan(ForeignScanState *node, ParallelContext *pcxt);
+</programlisting>
+    Estimate the amount of dynamic shared memory that will be required
+    for parallel operation.  This may be higher than the amount that will
+    actually be used, but it must not be lower.  The return value is in bytes.
+    </para>
+
+    <para>
+<programlisting>
+void
+InitializeDSMForeignScan(ForeignScanState *node, ParallelContext *pcxt,
+                         void *coordinate);
+</programlisting>
+    Initialize the dynamic shared memory that will be required for parallel
+    operation; <literal>coordinate</> points to an amount of allocated space
+    equal to the return value of <function>EstimateDSMForeignScan</>.
+   </para>
+
+   <para>
+<programlisting>
+void
+InitializeWorkerForeignScan(ForeignScanState *node, shm_toc *toc,
+                            void *coordinate);
+</programlisting>
+    Initialize a parallel worker's custom state based on the shared state
+    set up in the leader by <literal>InitializeDSMForeignScan</>.
+    This callback is optional, and needs only be supplied if this
+    custom path supports parallel execution.
+   </para>
+   </sect2>
+
    </sect1>
 
    <sect1 id="fdw-helpers">