Add extra foreach convenience macros (#3117)

This completely hides `ListCell` to the user of the loop

Example usage:
```c
WorkerNode *workerNode = NULL;

foreach_ptr(workerNode, workerNodeList) {
	// Do stuff with workerNode
}
```

Instead of:
```c
ListCell *workerNodeCell = NULL;

foreach(cell, workerNodeList) {
    WorkerNode *workerNode = lfirst(workerNodeCell);
	// Do stuff with workerNode
}
```

src/backend/distributed/commands/truncate.c

@@ -181,7 +181,7 @@ LockTruncatedRelationMetadataInWorkers(TruncateStmt *truncateStatement)
 		Oid relationId = RangeVarGetRelid(rangeVar, NoLock, false);
 		DistTableCacheEntry *cacheEntry = NULL;
 		List *referencingTableList = NIL;
-		ListCell *referencingTableCell = NULL;
+		Oid referencingRelationId = InvalidOid;
 
 		if (!IsDistributedTable(relationId))
 		{
@@ -199,9 +199,8 @@ LockTruncatedRelationMetadataInWorkers(TruncateStmt *truncateStatement)
 		Assert(cacheEntry != NULL);
 
 		referencingTableList = cacheEntry->referencingRelationsViaForeignKey;
-		foreach(referencingTableCell, referencingTableList)
+		foreach_oid(referencingRelationId, referencingTableList)
 		{
-			Oid referencingRelationId = lfirst_oid(referencingTableCell);
 			distributedRelationList = list_append_unique_oid(distributedRelationList,
 															 referencingRelationId);
 		}
@@ -228,7 +227,7 @@ LockTruncatedRelationMetadataInWorkers(TruncateStmt *truncateStatement)
 static void
 AcquireDistributedLockOnRelations(List *relationIdList, LOCKMODE lockMode)
 {
-	ListCell *relationIdCell = NULL;
+	Oid relationId = InvalidOid;
 	List *workerNodeList = ActivePrimaryNodeList(NoLock);
 	const char *lockModeText = LockModeToLockModeText(lockMode);
 
@@ -241,10 +240,8 @@ AcquireDistributedLockOnRelations(List *relationIdList, LOCKMODE lockMode)
 
 	BeginOrContinueCoordinatedTransaction();
 
-	foreach(relationIdCell, relationIdList)
+	foreach_oid(relationId, relationIdList)
 	{
-		Oid relationId = lfirst_oid(relationIdCell);
-
 		/*
 		 * We only acquire distributed lock on relation if
 		 * the relation is sync'ed between mx nodes.

src/backend/distributed/planner/query_pushdown_planning.c

@@ -24,6 +24,7 @@
 #include "distributed/citus_clauses.h"
 #include "distributed/citus_ruleutils.h"
 #include "distributed/deparse_shard_query.h"
+#include "distributed/listutils.h"
 #include "distributed/metadata_cache.h"
 #include "distributed/multi_logical_optimizer.h"
 #include "distributed/multi_logical_planner.h"
@@ -1061,7 +1062,7 @@ DeferErrorIfUnsupportedTableCombination(Query *queryTree)
 {
 	List *rangeTableList = queryTree->rtable;
 	List *joinTreeTableIndexList = NIL;
-	ListCell *joinTreeTableIndexCell = NULL;
+	int joinTreeTableIndex = 0;
 	bool unsupportedTableCombination = false;
 	char *errorDetail = NULL;
 
@@ -1071,13 +1072,12 @@ DeferErrorIfUnsupportedTableCombination(Query *queryTree)
 	 */
 	ExtractRangeTableIndexWalker((Node *) queryTree->jointree, &joinTreeTableIndexList);
 
-	foreach(joinTreeTableIndexCell, joinTreeTableIndexList)
+	foreach_int(joinTreeTableIndex, joinTreeTableIndexList)
 	{
 		/*
 		 * Join tree's range table index starts from 1 in the query tree. But,
 		 * list indexes start from 0.
 		 */
-		int joinTreeTableIndex = lfirst_int(joinTreeTableIndexCell);
 		int rangeTableListIndex = joinTreeTableIndex - 1;
 
 		RangeTblEntry *rangeTableEntry =

src/backend/distributed/utils/node_metadata.c

@@ -765,8 +765,8 @@ UpdateNodeLocation(int32 nodeId, char *newNodeName, int32 newNodePort)
 Datum
 master_initialize_node_metadata(PG_FUNCTION_ARGS)
 {
-	ListCell *workerNodeCell = NULL;
 	List *workerNodes = NIL;
+	WorkerNode *workerNode = NULL;
 
 	CheckCitusVersion(ERROR);
 
@@ -779,9 +779,8 @@ master_initialize_node_metadata(PG_FUNCTION_ARGS)
 
 	workerNodes = ParseWorkerNodeFileAndRename();
 
-	foreach(workerNodeCell, workerNodes)
+	foreach_ptr(workerNode, workerNodes)
 	{
-		WorkerNode *workerNode = (WorkerNode *) lfirst(workerNodeCell);
 		bool nodeAlreadyExists = false;
 		NodeMetadata nodeMetadata = DefaultNodeMetadata();
 		nodeMetadata.nodeRack = workerNode->workerRack;

src/include/distributed/listutils.h

@@ -20,6 +20,51 @@
 #include "utils/hsearch.h"
 
 
+/*
+ * foreach_ptr -
+ *	  a convenience macro which loops through a pointer list without needing a
+ *	  ListCell, just a declared pointer variable to store the pointer of the
+ *	  cell in.
+ *
+ *   How it works:
+ *	  - A ListCell is declared with the name {var}Cell and used throughout the
+ *	    for loop using ## to concat.
+ *	  - To assign to var it needs to be done in the condition of the for loop,
+ *	    because we cannot use the initializer since a ListCell* variable is
+ *	    declared there.
+ *	  - || true is used to always enter the loop when cell is not null even if
+ *	    var is NULL.
+ */
+#define foreach_ptr(var, l) \
+	for (ListCell *(var ## Cell) = list_head(l); \
+		 (var ## Cell) != NULL && (((var) = lfirst(var ## Cell)) || true); \
+		 var ## Cell = lnext(var ## Cell))
+
+
+/*
+ * foreach_int -
+ *	  a convenience macro which loops through an int list without needing a
+ *	  ListCell, just a declared int variable to store the int of the cell in.
+ *	  For explanation of how it works see foreach_ptr.
+ */
+#define foreach_int(var, l) \
+	for (ListCell *(var ## Cell) = list_head(l); \
+		 (var ## Cell) != NULL && (((var) = lfirst_int(var ## Cell)) || true); \
+		 var ## Cell = lnext(var ## Cell))
+
+
+/*
+ * foreach_oid -
+ *	  a convenience macro which loops through an oid list without needing a
+ *	  ListCell, just a declared Oid variable to store the oid of the cell in.
+ *	  For explanation of how it works see foreach_ptr.
+ */
+#define foreach_oid(var, l) \
+	for (ListCell *(var ## Cell) = list_head(l); \
+		 (var ## Cell) != NULL && (((var) = lfirst_oid(var ## Cell)) || true); \
+		 var ## Cell = lnext(var ## Cell))
+
+
 /* utility functions declaration shared within this module */
 extern List * SortList(List *pointerList,
 					   int (*ComparisonFunction)(const void *, const void *));