From b0c7b929fdc78db4252af5f5ef5e1814cb441f59 Mon Sep 17 00:00:00 2001
From: amit <amitlangote09@gmail.com>
Date: Mon, 26 Jun 2017 15:23:34 +0900
Subject: [PATCH 3/5] Set pd_lower correctly in the SP-GiST index metapage

Also tell xlog.c to treat the metapage like a standard page, so any
hole in it is compressed.
---
 src/backend/access/spgist/spginsert.c |  4 ++--
 src/backend/access/spgist/spgutils.c  | 17 +++++++++++++++++
 src/backend/access/spgist/spgxlog.c   |  7 ++++---
 3 files changed, 23 insertions(+), 5 deletions(-)

diff --git a/src/backend/access/spgist/spginsert.c b/src/backend/access/spgist/spginsert.c
index e4b2c29b0e..80b82e1602 100644
--- a/src/backend/access/spgist/spginsert.c
+++ b/src/backend/access/spgist/spginsert.c
@@ -110,7 +110,7 @@ spgbuild(Relation heap, Relation index, IndexInfo *indexInfo)
 		 * Replay will re-initialize the pages, so don't take full pages
 		 * images.  No other data to log.
 		 */
-		XLogRegisterBuffer(0, metabuffer, REGBUF_WILL_INIT);
+		XLogRegisterBuffer(0, metabuffer, REGBUF_WILL_INIT | REGBUF_STANDARD);
 		XLogRegisterBuffer(1, rootbuffer, REGBUF_WILL_INIT | REGBUF_STANDARD);
 		XLogRegisterBuffer(2, nullbuffer, REGBUF_WILL_INIT | REGBUF_STANDARD);
 
@@ -173,7 +173,7 @@ spgbuildempty(Relation index)
 	smgrwrite(index->rd_smgr, INIT_FORKNUM, SPGIST_METAPAGE_BLKNO,
 			  (char *) page, true);
 	log_newpage(&index->rd_smgr->smgr_rnode.node, INIT_FORKNUM,
-				SPGIST_METAPAGE_BLKNO, page, false);
+				SPGIST_METAPAGE_BLKNO, page, true);
 
 	/* Likewise for the root page. */
 	SpGistInitPage(page, SPGIST_LEAF);
diff --git a/src/backend/access/spgist/spgutils.c b/src/backend/access/spgist/spgutils.c
index 22f64b0103..9e47883ef6 100644
--- a/src/backend/access/spgist/spgutils.c
+++ b/src/backend/access/spgist/spgutils.c
@@ -256,15 +256,24 @@ SpGistUpdateMetaPage(Relation index)
 	if (cache != NULL)
 	{
 		Buffer		metabuffer;
+		Page		metapage;
 		SpGistMetaPageData *metadata;
 
 		metabuffer = ReadBuffer(index, SPGIST_METAPAGE_BLKNO);
+		metapage = BufferGetPage(metabuffer);
 
 		if (ConditionalLockBuffer(metabuffer))
 		{
 			metadata = SpGistPageGetMeta(BufferGetPage(metabuffer));
 			metadata->lastUsedPages = cache->lastUsedPages;
 
+			/*
+			 * Set pd_lower just past the end of the metadata.  This is
+			 * essential, because without doing so, metadata will be lost if
+			 * xlog.c compresses the page.
+			 */
+			((PageHeader) metapage)->pd_lower = ((char *) metadata +
+							sizeof(SpGistMetaPageData)) - (char *) metapage;
 			MarkBufferDirty(metabuffer);
 			UnlockReleaseBuffer(metabuffer);
 		}
@@ -534,6 +543,14 @@ SpGistInitMetapage(Page page)
 	/* initialize last-used-page cache to empty */
 	for (i = 0; i < SPGIST_CACHED_PAGES; i++)
 		metadata->lastUsedPages.cachedPage[i].blkno = InvalidBlockNumber;
+
+	/*
+	 * Set pd_lower just past the end of the metadata.  This is essential,
+	 * because without doing so, metadata will be lost if xlog.c compresses
+	 * the page.
+	 */
+	((PageHeader) page)->pd_lower =
+		((char *) metadata + sizeof(SpGistMetaPageData)) - (char *) page;
 }
 
 /*
diff --git a/src/backend/access/spgist/spgxlog.c b/src/backend/access/spgist/spgxlog.c
index 87def79ee5..539ce8eb8c 100644
--- a/src/backend/access/spgist/spgxlog.c
+++ b/src/backend/access/spgist/spgxlog.c
@@ -1033,15 +1033,16 @@ void
 spg_mask(char *pagedata, BlockNumber blkno)
 {
 	Page		page = (Page) pagedata;
+	PageHeader	pagehdr = (PageHeader) page;
 
 	mask_page_lsn_and_checksum(page);
 
 	mask_page_hint_bits(page);
 
 	/*
-	 * Any SpGist page other than meta contains unused space which needs to be
-	 * masked.
+	 * Mask the unused space, only if the page's pd_lower appears to have been
+	 * set correctly.
 	 */
-	if (!SpGistPageIsMeta(page))
+	if (pagehdr->pd_lower > SizeOfPageHeaderData)
 		mask_unused_space(page);
 }
-- 
2.11.0