From acbffaa77d0c4846f8749daa45fde507be1e969b Mon Sep 17 00:00:00 2001
From: Heikki Linnakangas <heikki.linnakangas@iki.fi>
Date: Tue, 16 Nov 2010 11:02:11 +0200
Subject: [PATCH] The GiST scan algorithm uses LSNs to detect concurrent pages
 splits, but temporary indexes are not WAL-logged. We used a constant LSN for
 temporary indexes, on the assumption that we don't need to worry about
 concurrent page splits in temporary indexes because they're only visible to
 the current session. But that assumption is wrong, it's possible to insert
 rows and split pages in the same session, while a scan is in progress. For
 example, by opening a cursor and fetching some rows, and INSERTing new rows
 before fetching some more.

Fix by generating fake increasing LSNs, used in place of real LSNs in
temporary GiST indexes.
---
 src/backend/access/gist/gist.c       | 10 ++++------
 src/backend/access/gist/gistutil.c   | 21 +++++++++++++++++++++
 src/backend/access/gist/gistvacuum.c |  6 +++---
 src/include/access/gist_private.h    |  3 ++-
 4 files changed, 30 insertions(+), 10 deletions(-)

diff --git a/src/backend/access/gist/gist.c b/src/backend/access/gist/gist.c
index 00b1620ab00..fbc828e7832 100644
--- a/src/backend/access/gist/gist.c
+++ b/src/backend/access/gist/gist.c
@@ -23,8 +23,6 @@
 #include "miscadmin.h"
 #include "utils/memutils.h"
 
-const XLogRecPtr XLogRecPtrForTemp = {1, 1};
-
 /* Working state for gistbuild and its callback */
 typedef struct
 {
@@ -127,7 +125,7 @@ gistbuild(PG_FUNCTION_ARGS)
 		END_CRIT_SECTION();
 	}
 	else
-		PageSetLSN(BufferGetPage(buffer), XLogRecPtrForTemp);
+		PageSetLSN(BufferGetPage(buffer), GetXLogRecPtrForTemp());
 	LockBuffer(buffer, GIST_UNLOCK);
 	WriteBuffer(buffer);
 
@@ -356,7 +354,7 @@ gistplacetopage(GISTInsertState *state, GISTSTATE *giststate)
 			ptr = dist;
 			while (ptr)
 			{
-				PageSetLSN(BufferGetPage(ptr->buffer), XLogRecPtrForTemp);
+				PageSetLSN(BufferGetPage(ptr->buffer), GetXLogRecPtrForTemp());
 				ptr = ptr->next;
 			}
 		}
@@ -475,7 +473,7 @@ gistplacetopage(GISTInsertState *state, GISTSTATE *giststate)
 			END_CRIT_SECTION();
 		}
 		else
-			PageSetLSN(state->stack->page, XLogRecPtrForTemp);
+			PageSetLSN(state->stack->page, GetXLogRecPtrForTemp());
 
 		if (state->stack->blkno == GIST_ROOT_BLKNO)
 			state->needInsertComplete = false;
@@ -1206,7 +1204,7 @@ gistnewroot(Relation r, Buffer buffer, IndexTuple *itup, int len, ItemPointer ke
 		END_CRIT_SECTION();
 	}
 	else
-		PageSetLSN(page, XLogRecPtrForTemp);
+		PageSetLSN(page, GetXLogRecPtrForTemp());
 }
 
 void
diff --git a/src/backend/access/gist/gistutil.c b/src/backend/access/gist/gistutil.c
index c0d904c35a4..9f43b4520c9 100644
--- a/src/backend/access/gist/gistutil.c
+++ b/src/backend/access/gist/gistutil.c
@@ -977,3 +977,24 @@ gistNewBuffer(Relation r)
 
 	return buffer;
 }
+
+/*
+ * Temporary GiST indexes are not WAL-logged, but we need LSNs to detect
+ * concurrent page splits anyway. GetXLogRecPtrForTemp() provides a fake
+ * sequence of LSNs for that purpose. Each call generates an LSN that is
+ * greater than any previous value returned by this function in the same
+ * session.
+ */
+XLogRecPtr
+GetXLogRecPtrForTemp(void)
+{
+	static XLogRecPtr counter = {0, 1};
+
+	counter.xrecoff++;
+	if (counter.xrecoff == 0)
+	{
+		counter.xlogid++;
+		counter.xrecoff++;
+	}
+	return counter;
+}
diff --git a/src/backend/access/gist/gistvacuum.c b/src/backend/access/gist/gistvacuum.c
index 9d02ea7b46f..fa545f74683 100644
--- a/src/backend/access/gist/gistvacuum.c
+++ b/src/backend/access/gist/gistvacuum.c
@@ -201,7 +201,7 @@ gistVacuumUpdate(GistVacuum *gv, BlockNumber blkno, bool needunion)
 					ptr = dist;
 					while (ptr)
 					{
-						PageSetLSN(BufferGetPage(ptr->buffer), XLogRecPtrForTemp);
+						PageSetLSN(BufferGetPage(ptr->buffer), GetXLogRecPtrForTemp());
 						ptr = ptr->next;
 					}
 				}
@@ -306,7 +306,7 @@ gistVacuumUpdate(GistVacuum *gv, BlockNumber blkno, bool needunion)
 			pfree(rdata);
 		}
 		else
-			PageSetLSN(page, XLogRecPtrForTemp);
+			PageSetLSN(page, GetXLogRecPtrForTemp());
 		WriteBuffer(buffer);
 	}
 	else
@@ -589,7 +589,7 @@ gistbulkdelete(PG_FUNCTION_ARGS)
 					pfree(rdata);
 				}
 				else
-					PageSetLSN(page, XLogRecPtrForTemp);
+					PageSetLSN(page, GetXLogRecPtrForTemp());
 				WriteNoReleaseBuffer(buffer);
 			}
 		}
diff --git a/src/include/access/gist_private.h b/src/include/access/gist_private.h
index 1668f80a9c9..18c766c7891 100644
--- a/src/include/access/gist_private.h
+++ b/src/include/access/gist_private.h
@@ -91,7 +91,6 @@ typedef struct GISTScanOpaqueData
 typedef GISTScanOpaqueData *GISTScanOpaque;
 
 /* XLog stuff */
-extern const XLogRecPtr XLogRecPtrForTemp;
 
 #define XLOG_GIST_ENTRY_UPDATE	0x00
 #define XLOG_GIST_ENTRY_DELETE	0x10
@@ -318,6 +317,8 @@ extern void gistdentryinit(GISTSTATE *giststate, int nkey, GISTENTRY *e,
 void gistUserPicksplit(Relation r, GistEntryVector *entryvec, GIST_SPLITVEC *v,
 				  IndexTuple *itup, int len, GISTSTATE *giststate);
 
+extern XLogRecPtr GetXLogRecPtrForTemp(void);
+
 /* gistvacuum.c */
 extern Datum gistbulkdelete(PG_FUNCTION_ARGS);
 extern Datum gistvacuumcleanup(PG_FUNCTION_ARGS);