Change the PageGetContents() macro to guarantee its result is maxalign'd,

thereby forestalling any problems with alignment of the data structure placed
there.  Since SizeOfPageHeaderData is maxalign'd anyway in 8.3 and HEAD, this
does not actually change anything right now, but it is foreseeable that the
header size will change again someday.  I had to fix a couple of places that
were assuming that the content offset is just SizeOfPageHeaderData rather than
MAXALIGN(SizeOfPageHeaderData).  Per discussion of Zdenek's page-macros patch.

src/backend/storage/page/bufpage.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/storage/page/bufpage.c,v 1.79 2008/05/13 15:44:08 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/storage/page/bufpage.c,v 1.80 2008/07/13 21:50:04 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -260,7 +260,6 @@ Page
 PageGetTempPage(Page page, Size specialSize)
 {
 	Size		pageSize;
-	Size		size;
 	Page		temp;
 	PageHeader	thdr;
 
@@ -271,15 +270,13 @@ PageGetTempPage(Page page, Size specialSize)
 	/* copy old page in */
 	memcpy(temp, page, pageSize);
 
-	/* clear out the middle */
-	size = pageSize - SizeOfPageHeaderData;
-	size -= MAXALIGN(specialSize);
-	MemSet(PageGetContents(thdr), 0, size);
-
 	/* set high, low water marks */
 	thdr->pd_lower = SizeOfPageHeaderData;
 	thdr->pd_upper = pageSize - MAXALIGN(specialSize);
 
+	/* clear out the middle */
+	MemSet((char *) temp + thdr->pd_lower, 0, thdr->pd_upper - thdr->pd_lower);
+
 	return temp;
 }
 

src/include/access/gin.h

@@ -4,7 +4,7 @@
  *
  *	Copyright (c) 2006-2008, PostgreSQL Global Development Group
  *
- *	$PostgreSQL: pgsql/src/include/access/gin.h,v 1.23 2008/07/11 21:06:29 tgl Exp $
+ *	$PostgreSQL: pgsql/src/include/access/gin.h,v 1.24 2008/07/13 21:50:04 tgl Exp $
  *--------------------------------------------------------------------------
  */
 
@@ -121,17 +121,19 @@ typedef struct
 /*
  * Data (posting tree) pages
  */
+#define GinDataPageGetRightBound(page)	((ItemPointer) PageGetContents(page))
 #define GinDataPageGetData(page)	\
-	(PageGetContents(page)+MAXALIGN(sizeof(ItemPointerData)))
-#define GinDataPageGetRightBound(page)	((ItemPointer)PageGetContents(page))
-#define GinSizeOfItem(page) ( (GinPageIsLeaf(page)) ? sizeof(ItemPointerData) : sizeof(PostingItem) )
-#define GinDataPageGetItem(page,i) ( GinDataPageGetData(page) + ((i)-1) * GinSizeOfItem(page) )
+	(PageGetContents(page) + MAXALIGN(sizeof(ItemPointerData)))
+#define GinSizeOfItem(page)	\
+	(GinPageIsLeaf(page) ? sizeof(ItemPointerData) : sizeof(PostingItem))
+#define GinDataPageGetItem(page,i)	\
+	(GinDataPageGetData(page) + ((i)-1) * GinSizeOfItem(page))
 
 #define GinDataPageGetFreeSpace(page)	\
-	( BLCKSZ - SizeOfPageHeaderData - MAXALIGN(sizeof(GinPageOpaqueData)) - \
-		GinPageGetOpaque(page)->maxoff * GinSizeOfItem(page) - \
-		MAXALIGN(sizeof(ItemPointerData)))
-
+	(BLCKSZ - MAXALIGN(SizeOfPageHeaderData) \
+	 - MAXALIGN(sizeof(ItemPointerData)) \
+	 - GinPageGetOpaque(page)->maxoff * GinSizeOfItem(page) \
+	 - MAXALIGN(sizeof(GinPageOpaqueData)))
 
 
 #define GIN_UNLOCK	BUFFER_LOCK_UNLOCK

src/include/storage/bufpage.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/storage/bufpage.h,v 1.81 2008/06/08 22:00:48 alvherre Exp $
+ * $PostgreSQL: pgsql/src/include/storage/bufpage.h,v 1.82 2008/07/13 21:50:04 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -206,9 +206,13 @@ typedef PageHeaderData *PageHeader;
 /*
  * PageGetContents
  *		To be used in case the page does not contain item pointers.
+ *
+ * Note: prior to 8.3 this was not guaranteed to yield a MAXALIGN'd result.
+ * Now it is.  Beware of old code that might think the offset to the contents
+ * is just SizeOfPageHeaderData rather than MAXALIGN(SizeOfPageHeaderData).
  */
 #define PageGetContents(page) \
-	((char *) (&((PageHeader) (page))->pd_linp[0]))
+	((char *) (page) + MAXALIGN(SizeOfPageHeaderData))
 
 /* ----------------
  *		macros to access page size info