Add LoadShardPlacement UDF

This UDF returns a shard placement from cache given shard id and placement id. At the
moment it iterates over all shard placements of given shard by ShardPlacementList and
searches given placement id in that list, which is not a good solution performance-wise.
However, currently, this function will be used only when there is a failed transaction.
If a need arises we can optimize this function in the future.

src/backend/distributed/utils/metadata_cache.c

@@ -245,6 +245,48 @@ LoadShardInterval(uint64 shardId)
 }
 
 
+/*
+ * LoadShardPlacement returns the, cached, metadata about a shard placement.
+ *
+ * The return value is a copy of the cached ShardPlacement struct and may
+ * therefore be modified and/or freed.
+ */
+ShardPlacement *
+LoadShardPlacement(uint64 shardId, uint64 placementId)
+{
+	ShardCacheEntry *shardEntry = NULL;
+	DistTableCacheEntry *tableEntry = NULL;
+
+	ShardPlacement *placementArray = NULL;
+	int numberOfPlacements = 0;
+
+	int i = 0;
+
+	shardEntry = LookupShardCacheEntry(shardId);
+	tableEntry = shardEntry->tableEntry;
+
+	/* the offset better be in a valid range */
+	Assert(shardEntry->shardIndex < tableEntry->shardIntervalArrayLength);
+
+	placementArray = tableEntry->arrayOfPlacementArrays[shardEntry->shardIndex];
+	numberOfPlacements = tableEntry->arrayOfPlacementArrayLengths[shardEntry->shardIndex];
+
+	for (i = 0; i < numberOfPlacements; i++)
+	{
+		if (placementArray[i].placementId == placementId)
+		{
+			ShardPlacement *shardPlacement = CitusMakeNode(ShardPlacement);
+			CopyShardPlacement(&placementArray[i], shardPlacement);
+
+			return shardPlacement;
+		}
+	}
+
+	ereport(ERROR, (errmsg("could not find valid entry for shard placement "
+						   UINT64_FORMAT, placementId)));
+}
+
+
 /*
  * ShardPlacementList returns the list of placements for the given shard from
  * the cache.

src/include/distributed/metadata_cache.h

@@ -59,6 +59,7 @@ typedef struct
 extern bool IsDistributedTable(Oid relationId);
 extern List * DistributedTableList(void);
 extern ShardInterval * LoadShardInterval(uint64 shardId);
+extern ShardPlacement * LoadShardPlacement(uint64 shardId, uint64 placementId);
 extern DistTableCacheEntry * DistributedTableCacheEntry(Oid distributedRelationId);
 extern int GetLocalGroupId(void);
 extern List * DistTableOidList(void);