Simplify hash_xlog_split_allocate_page()

Instead of complicated pointer arithmetic, overlay a uint32 array and
just access the array members.  That's safe thanks to
XLogRecGetBlockData() returning a MAXALIGNed buffer.

Reviewed-by: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Reviewed-by: Jacob Champion <jacob.champion@enterprisedb.com>
Discussion: https://www.postgresql.org/message-id/flat/aSQy2JawavlVlEB0%40ip-10-97-1-34.eu-west-3.compute.internal

Author: petere

src/backend/access/hash/hash_xlog.c

@@ -314,8 +314,6 @@ hash_xlog_split_allocate_page(XLogReaderState *record)
 	Buffer		oldbuf;
 	Buffer		newbuf;
 	Buffer		metabuf;
-	Size		datalen PG_USED_FOR_ASSERTS_ONLY;
-	char	   *data;
 	XLogRedoAction action;
 
 	/*
@@ -375,41 +373,42 @@ hash_xlog_split_allocate_page(XLogReaderState *record)
 	{
 		Page		page;
 		HashMetaPage metap;
+		Size		datalen;
+		char	   *data;
+		uint32	   *uidata;
+		int			uidatacount;
 
 		page = BufferGetPage(metabuf);
 		metap = HashPageGetMeta(page);
 		metap->hashm_maxbucket = xlrec->new_bucket;
 
 		data = XLogRecGetBlockData(record, 2, &datalen);
 
+		/*
+		 * This cast is ok because XLogRecGetBlockData() returns a MAXALIGNed
+		 * buffer.
+		 */
+		uidata = (uint32 *) data;
+		uidatacount = 0;
+
 		if (xlrec->flags & XLH_SPLIT_META_UPDATE_MASKS)
 		{
-			uint32		lowmask;
-			uint32	   *highmask;
-
-			/* extract low and high masks. */
-			memcpy(&lowmask, data, sizeof(uint32));
-			highmask = (uint32 *) ((char *) data + sizeof(uint32));
+			uint32		lowmask = uidata[uidatacount++];
+			uint32		highmask = uidata[uidatacount++];
 
 			/* update metapage */
 			metap->hashm_lowmask = lowmask;
-			metap->hashm_highmask = *highmask;
-
-			data += sizeof(uint32) * 2;
+			metap->hashm_highmask = highmask;
 		}
 
 		if (xlrec->flags & XLH_SPLIT_META_UPDATE_SPLITPOINT)
 		{
-			uint32		ovflpoint;
-			uint32	   *ovflpages;
-
-			/* extract information of overflow pages. */
-			memcpy(&ovflpoint, data, sizeof(uint32));
-			ovflpages = (uint32 *) ((char *) data + sizeof(uint32));
+			uint32		ovflpoint = uidata[uidatacount++];
+			uint32		ovflpages = uidata[uidatacount++];
 
 			/* update metapage */
-			metap->hashm_spares[ovflpoint] = *ovflpages;
 			metap->hashm_ovflpoint = ovflpoint;
+			metap->hashm_spares[ovflpoint] = ovflpages;
 		}
 
 		MarkBufferDirty(metabuf);