Add new FDW API to test for parallel-safety.

This is basically a bug fix; the old code assumes that a ForeignScan
is always parallel-safe, but for postgres_fdw, for example, this is
definitely false.  It should be true for file_fdw, though, since a
worker can read a file from the filesystem just as well as any other
backend process.

Original patch by Thomas Munro.  Documentation, and changes to the
comments, by me.

src/backend/optimizer/path/allpaths.c

@@ -527,6 +527,23 @@ set_rel_consider_parallel(PlannerInfo *root, RelOptInfo *rel,
 					return;
 				return;
 			}
+
+			/*
+			 * Ask FDWs whether they can support performing a ForeignScan
+			 * within a worker.  Most often, the answer will be no.  For
+			 * example, if the nature of the FDW is such that it opens a TCP
+			 * connection with a remote server, each parallel worker would end
+			 * up with a separate connection, and these connections might not
+			 * be appropriately coordinated between workers and the leader.
+			 */
+			if (rte->relkind == RELKIND_FOREIGN_TABLE)
+			{
+				Assert(rel->fdwroutine);
+				if (!rel->fdwroutine->IsForeignScanParallelSafe)
+					return;
+				if (!rel->fdwroutine->IsForeignScanParallelSafe(root, rel, rte))
+					return;
+			}
 			break;
 
 		case RTE_SUBQUERY:

src/include/foreign/fdwapi.h

@@ -131,6 +131,10 @@ typedef void (*InitializeDSMForeignScan_function) (ForeignScanState *node,
 typedef void (*InitializeWorkerForeignScan_function) (ForeignScanState *node,
 													  shm_toc *toc,
 													  void *coordinate);
+typedef bool (*IsForeignScanParallelSafe_function) (PlannerInfo *root,
+															 RelOptInfo *rel,
+														 RangeTblEntry *rte);
+
 /*
  * FdwRoutine is the struct returned by a foreign-data wrapper's handler
  * function.  It provides pointers to the callback functions needed by the
@@ -188,6 +192,7 @@ typedef struct FdwRoutine
 	ImportForeignSchema_function ImportForeignSchema;
 
 	/* Support functions for parallelism under Gather node */
+	IsForeignScanParallelSafe_function IsForeignScanParallelSafe;
 	EstimateDSMForeignScan_function EstimateDSMForeignScan;
 	InitializeDSMForeignScan_function InitializeDSMForeignScan;
 	InitializeWorkerForeignScan_function InitializeWorkerForeignScan;