From c51791a4e0effe9c5b31af11ed07f8c49cd0014e Mon Sep 17 00:00:00 2001
From: amit <amitlangote09@gmail.com>
Date: Mon, 26 Jun 2017 15:13:32 +0900
Subject: [PATCH 2/3] Set pd_lower correctly in the BRIN index metapage

Also tell xlog.c to treat the metapage like a standard page, so any
hole in it is compressed.
---
 src/backend/access/brin/brin.c         |  4 ++--
 src/backend/access/brin/brin_pageops.c |  9 +++++++++
 src/backend/access/brin/brin_revmap.c  | 11 ++++++++++-
 src/backend/access/brin/brin_xlog.c    | 19 +++++++++++++++++--
 4 files changed, 38 insertions(+), 5 deletions(-)

diff --git a/src/backend/access/brin/brin.c b/src/backend/access/brin/brin.c
index b3aa6d1ced..e6909d7aea 100644
--- a/src/backend/access/brin/brin.c
+++ b/src/backend/access/brin/brin.c
@@ -685,7 +685,7 @@ brinbuild(Relation heap, Relation index, IndexInfo *indexInfo)
 
 		XLogBeginInsert();
 		XLogRegisterData((char *) &xlrec, SizeOfBrinCreateIdx);
-		XLogRegisterBuffer(0, meta, REGBUF_WILL_INIT);
+		XLogRegisterBuffer(0, meta, REGBUF_WILL_INIT | REGBUF_STANDARD);
 
 		recptr = XLogInsert(RM_BRIN_ID, XLOG_BRIN_CREATE_INDEX);
 
@@ -742,7 +742,7 @@ brinbuildempty(Relation index)
 	brin_metapage_init(BufferGetPage(metabuf), BrinGetPagesPerRange(index),
 					   BRIN_CURRENT_VERSION);
 	MarkBufferDirty(metabuf);
-	log_newpage_buffer(metabuf, false);
+	log_newpage_buffer(metabuf, true);
 	END_CRIT_SECTION();
 
 	UnlockReleaseBuffer(metabuf);
diff --git a/src/backend/access/brin/brin_pageops.c b/src/backend/access/brin/brin_pageops.c
index 80f803e438..92903f38c7 100644
--- a/src/backend/access/brin/brin_pageops.c
+++ b/src/backend/access/brin/brin_pageops.c
@@ -491,6 +491,15 @@ brin_metapage_init(Page page, BlockNumber pagesPerRange, uint16 version)
 	 * revmap page to be created when the index is.
 	 */
 	metadata->lastRevmapPage = 0;
+
+	/*
+	 * Set pd_lower just past the end of the metadata.  This is not essential
+	 * but it makes the page look compressible to xlog.c, because we pass the
+	 * buffer containing this page to XLogRegisterBuffer() as a page with
+	 * standard layout.
+	 */
+	((PageHeader) page)->pd_lower =
+		((char *) metadata + sizeof(BrinMetaPageData)) - (char *) page;
 }
 
 /*
diff --git a/src/backend/access/brin/brin_revmap.c b/src/backend/access/brin/brin_revmap.c
index 22f2076887..4b056c68a2 100644
--- a/src/backend/access/brin/brin_revmap.c
+++ b/src/backend/access/brin/brin_revmap.c
@@ -624,6 +624,15 @@ revmap_physical_extend(BrinRevmap *revmap)
 	MarkBufferDirty(buf);
 
 	metadata->lastRevmapPage = mapBlk;
+
+	/*
+	 * Set pd_lower just past the end of the metadata.  This is not essential
+	 * but it makes the page look compressible to xlog.c, because we pass the
+	 * buffer containing this page to XLogRegisterBuffer() as a page with
+	 * standard layout.
+	 */
+	((PageHeader) metapage)->pd_lower =
+		((char *) metadata + sizeof(BrinMetaPageData)) - (char *) metapage;
 	MarkBufferDirty(revmap->rm_metaBuf);
 
 	if (RelationNeedsWAL(revmap->rm_irel))
@@ -635,7 +644,7 @@ revmap_physical_extend(BrinRevmap *revmap)
 
 		XLogBeginInsert();
 		XLogRegisterData((char *) &xlrec, SizeOfBrinRevmapExtend);
-		XLogRegisterBuffer(0, revmap->rm_metaBuf, 0);
+		XLogRegisterBuffer(0, revmap->rm_metaBuf, REGBUF_STANDARD);
 
 		XLogRegisterBuffer(1, buf, REGBUF_WILL_INIT);
 
diff --git a/src/backend/access/brin/brin_xlog.c b/src/backend/access/brin/brin_xlog.c
index dff7198a39..a9c7d909f6 100644
--- a/src/backend/access/brin/brin_xlog.c
+++ b/src/backend/access/brin/brin_xlog.c
@@ -234,6 +234,15 @@ brin_xlog_revmap_extend(XLogReaderState *record)
 		metadata->lastRevmapPage = xlrec->targetBlk;
 
 		PageSetLSN(metapg, lsn);
+
+		/*
+		 * Set pd_lower just past the end of the metadata.  This is not
+		 * essential but it makes the page look compressible to xlog.c, because
+		 * we pass the buffer containing this page to XLogRegisterBuffer() as a
+		 * page with standard layout.
+		 */
+		((PageHeader) metapg)->pd_lower =
+			((char *) metadata + sizeof(BrinMetaPageData)) - (char *) metapg;
 		MarkBufferDirty(metabuf);
 	}
 
@@ -331,14 +340,20 @@ void
 brin_mask(char *pagedata, BlockNumber blkno)
 {
 	Page		page = (Page) pagedata;
+	PageHeader	pagehdr = (PageHeader) page;
 
 	mask_page_lsn(page);
 
 	mask_page_hint_bits(page);
 
-	if (BRIN_IS_REGULAR_PAGE(page))
+	/*
+	 * Regular brin pages contain unused space which needs to be masked.
+	 * Similarly for meta pages, but mask it only if pd_lower appears to have
+	 * been set correctly.
+	 */
+	if (BRIN_IS_REGULAR_PAGE(page) ||
+		(BRIN_IS_META_PAGE(page) && pagehdr->pd_lower > SizeOfPageHeaderData))
 	{
-		/* Regular brin pages contain unused space which needs to be masked. */
 		mask_unused_space(page);
 	}
 }
-- 
2.11.0