Separate reinitialization of shared parallel-scan state from ExecReScan.

Previously, the parallel executor logic did reinitialization of shared
state within the ExecReScan code for parallel-aware scan nodes.  This is
problematic, because it means that the ExecReScan call has to occur
synchronously (ie, during the parent Gather node's ReScan call).  That is
swimming very much against the tide so far as the ExecReScan machinery is
concerned; the fact that it works at all today depends on a lot of fragile
assumptions, such as that no plan node between Gather and a parallel-aware
scan node is parameterized.  Another objection is that because ExecReScan
might be called in workers as well as the leader, hacky extra tests are
needed in some places to prevent unwanted shared-state resets.

Hence, let's separate this code into two functions, a ReInitializeDSM
call and the ReScan call proper.  ReInitializeDSM is called only in
the leader and is guaranteed to run before we start new workers.
ReScan is returned to its traditional function of resetting only local
state, which means that ExecReScan's usual habits of delaying or
eliminating child rescan calls are safe again.

As with the preceding commit 7df2c1f8d, it doesn't seem to be necessary
to make these changes in 9.6, which is a good thing because the FDW and
CustomScan APIs are impacted.

Discussion: https://postgr.es/m/CAA4eK1JkByysFJNh9M349u_nNjqETuEnY_y1VUc_kJiU0bxtaQ@mail.gmail.com

8 files changed, 13 insertions, 1 deletions

diff --git a/src/include/executor/execParallel.h b/src/include/executor/execParallel.h
index 79b886706f7..1cb895d8984 100644
--- a/src/include/executor/execParallel.h
+++ b/src/include/executor/execParallel.h
@@ -36,7 +36,8 @@ extern ParallelExecutorInfo *ExecInitParallelPlan(PlanState *planstate,
 					 EState *estate, int nworkers, int64 tuples_needed);
 extern void ExecParallelFinish(ParallelExecutorInfo *pei);
 extern void ExecParallelCleanup(ParallelExecutorInfo *pei);
-extern void ExecParallelReinitialize(ParallelExecutorInfo *pei);
+extern void ExecParallelReinitialize(PlanState *planstate,
+						 ParallelExecutorInfo *pei);
 
 extern void ParallelQueryMain(dsm_segment *seg, shm_toc *toc);
 
diff --git a/src/include/executor/nodeBitmapHeapscan.h b/src/include/executor/nodeBitmapHeapscan.h
index c77694cf22f..10844a405a5 100644
--- a/src/include/executor/nodeBitmapHeapscan.h
+++ b/src/include/executor/nodeBitmapHeapscan.h
@@ -24,6 +24,8 @@ extern void ExecBitmapHeapEstimate(BitmapHeapScanState *node,
 					   ParallelContext *pcxt);
 extern void ExecBitmapHeapInitializeDSM(BitmapHeapScanState *node,
 							ParallelContext *pcxt);
+extern void ExecBitmapHeapReInitializeDSM(BitmapHeapScanState *node,
+							  ParallelContext *pcxt);
 extern void ExecBitmapHeapInitializeWorker(BitmapHeapScanState *node,
 							   shm_toc *toc);
 
diff --git a/src/include/executor/nodeCustom.h b/src/include/executor/nodeCustom.h
index a1cc63ae1f3..25767b6a4a5 100644
--- a/src/include/executor/nodeCustom.h
+++ b/src/include/executor/nodeCustom.h
@@ -34,6 +34,8 @@ extern void ExecCustomScanEstimate(CustomScanState *node,
 					   ParallelContext *pcxt);
 extern void ExecCustomScanInitializeDSM(CustomScanState *node,
 							ParallelContext *pcxt);
+extern void ExecCustomScanReInitializeDSM(CustomScanState *node,
+							  ParallelContext *pcxt);
 extern void ExecCustomScanInitializeWorker(CustomScanState *node,
 							   shm_toc *toc);
 extern void ExecShutdownCustomScan(CustomScanState *node);
diff --git a/src/include/executor/nodeForeignscan.h b/src/include/executor/nodeForeignscan.h
index 0b662597d8f..0354c2c4308 100644
--- a/src/include/executor/nodeForeignscan.h
+++ b/src/include/executor/nodeForeignscan.h
@@ -25,6 +25,8 @@ extern void ExecForeignScanEstimate(ForeignScanState *node,
 						ParallelContext *pcxt);
 extern void ExecForeignScanInitializeDSM(ForeignScanState *node,
 							 ParallelContext *pcxt);
+extern void ExecForeignScanReInitializeDSM(ForeignScanState *node,
+							   ParallelContext *pcxt);
 extern void ExecForeignScanInitializeWorker(ForeignScanState *node,
 								shm_toc *toc);
 extern void ExecShutdownForeignScan(ForeignScanState *node);
diff --git a/src/include/executor/nodeIndexonlyscan.h b/src/include/executor/nodeIndexonlyscan.h
index c8a709c26ed..690b5dbfe59 100644
--- a/src/include/executor/nodeIndexonlyscan.h
+++ b/src/include/executor/nodeIndexonlyscan.h
@@ -28,6 +28,8 @@ extern void ExecIndexOnlyScanEstimate(IndexOnlyScanState *node,
 						  ParallelContext *pcxt);
 extern void ExecIndexOnlyScanInitializeDSM(IndexOnlyScanState *node,
 							   ParallelContext *pcxt);
+extern void ExecIndexOnlyScanReInitializeDSM(IndexOnlyScanState *node,
+								 ParallelContext *pcxt);
 extern void ExecIndexOnlyScanInitializeWorker(IndexOnlyScanState *node,
 								  shm_toc *toc);
 
diff --git a/src/include/executor/nodeIndexscan.h b/src/include/executor/nodeIndexscan.h
index 1668e347eef..0670e87e395 100644
--- a/src/include/executor/nodeIndexscan.h
+++ b/src/include/executor/nodeIndexscan.h
@@ -24,6 +24,7 @@ extern void ExecIndexRestrPos(IndexScanState *node);
 extern void ExecReScanIndexScan(IndexScanState *node);
 extern void ExecIndexScanEstimate(IndexScanState *node, ParallelContext *pcxt);
 extern void ExecIndexScanInitializeDSM(IndexScanState *node, ParallelContext *pcxt);
+extern void ExecIndexScanReInitializeDSM(IndexScanState *node, ParallelContext *pcxt);
 extern void ExecIndexScanInitializeWorker(IndexScanState *node, shm_toc *toc);
 
 /*
diff --git a/src/include/executor/nodeSeqscan.h b/src/include/executor/nodeSeqscan.h
index 0fba79f8de6..eb96799cade 100644
--- a/src/include/executor/nodeSeqscan.h
+++ b/src/include/executor/nodeSeqscan.h
@@ -24,6 +24,7 @@ extern void ExecReScanSeqScan(SeqScanState *node);
 /* parallel scan support */
 extern void ExecSeqScanEstimate(SeqScanState *node, ParallelContext *pcxt);
 extern void ExecSeqScanInitializeDSM(SeqScanState *node, ParallelContext *pcxt);
+extern void ExecSeqScanReInitializeDSM(SeqScanState *node, ParallelContext *pcxt);
 extern void ExecSeqScanInitializeWorker(SeqScanState *node, shm_toc *toc);
 
 #endif							/* NODESEQSCAN_H */
diff --git a/src/include/executor/nodeSort.h b/src/include/executor/nodeSort.h
index 77ac06597ff..1ab8f767210 100644
--- a/src/include/executor/nodeSort.h
+++ b/src/include/executor/nodeSort.h
@@ -26,6 +26,7 @@ extern void ExecReScanSort(SortState *node);
 /* parallel instrumentation support */
 extern void ExecSortEstimate(SortState *node, ParallelContext *pcxt);
 extern void ExecSortInitializeDSM(SortState *node, ParallelContext *pcxt);
+extern void ExecSortReInitializeDSM(SortState *node, ParallelContext *pcxt);
 extern void ExecSortInitializeWorker(SortState *node, shm_toc *toc);
 extern void ExecSortRetrieveInstrumentation(SortState *node);