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.

contrib/file_fdw/file_fdw.c

@@ -131,6 +131,8 @@ static void fileEndForeignScan(ForeignScanState *node);
 static bool fileAnalyzeForeignTable(Relation relation,
 						AcquireSampleRowsFunc *func,
 						BlockNumber *totalpages);
+static bool fileIsForeignScanParallelSafe(PlannerInfo *root, RelOptInfo *rel,
+										  RangeTblEntry *rte);
 
 /*
  * Helper functions
@@ -170,6 +172,7 @@ file_fdw_handler(PG_FUNCTION_ARGS)
 	fdwroutine->ReScanForeignScan = fileReScanForeignScan;
 	fdwroutine->EndForeignScan = fileEndForeignScan;
 	fdwroutine->AnalyzeForeignTable = fileAnalyzeForeignTable;
+	fdwroutine->IsForeignScanParallelSafe = fileIsForeignScanParallelSafe;
 
 	PG_RETURN_POINTER(fdwroutine);
 }
@@ -761,6 +764,18 @@ fileAnalyzeForeignTable(Relation relation,
 	return true;
 }
 
+/*
+ * fileIsForeignScanParallelSafe
+ * 		Reading a file in a parallel worker should work just the same as
+ * 		reading it in the leader, so mark scans safe.
+ */
+static bool
+fileIsForeignScanParallelSafe(PlannerInfo *root, RelOptInfo *rel,
+								  RangeTblEntry *rte)
+{
+	return true;
+}
+
 /*
  * check_selective_binary_conversion
  *

doc/src/sgml/fdwhandler.sgml

@@ -988,6 +988,29 @@ ImportForeignSchema (ImportForeignSchemaStmt *stmt, Oid serverOid);
     <para>
 <programlisting>
 Size
+IsForeignScanParallelSafe(PlannerInfo *root, RelOptInfo *rel,
+                          RangeTblEntry *rte);
+</programlisting>
+    Test whether a scan can be performed within a parallel worker.  This
+    function will only be called when the planner believes that a parallel
+    plan might be possible, and should return true if it is safe for that scan
+    to run within a parallel worker.  This will generally not be the case if
+    the remote data source has transaction semantics, unless the worker's
+    connection to the data can somehow be made to share the same transaction
+    context as the leader.
+    </para>
+
+    <para>
+    If this callback is not defined, it is assumed that the scan must take
+    place within the parallel leader.  Note that returning true does not mean
+    that the scan itself can be done in parallel, only that the scan can be
+    performed within a parallel worker.  Therefore, it can be useful to define
+    this method even when parallel execution is not supported.
+    </para>
+
+    <para>
+<programlisting>
+Size
 EstimateDSMForeignScan(ForeignScanState *node, ParallelContext *pcxt);
 </programlisting>
     Estimate the amount of dynamic shared memory that will be required

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;