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.

4 files changed, 28 insertions, 8 deletions

diff --git a/src/backend/access/gist/gist.c b/src/backend/access/gist/gist.c
index 3054f98c9e8..8c2dbc940f4 100644
--- a/src/backend/access/gist/gist.c
+++ b/src/backend/access/gist/gist.c
@@ -22,8 +22,6 @@
 #include "storage/indexfsm.h"
 #include "utils/memutils.h"
 
-const XLogRecPtr XLogRecPtrForTemp = {1, 1};
-
 /* Working state for gistbuild and its callback */
 typedef struct
 {
@@ -132,7 +130,7 @@ gistbuild(PG_FUNCTION_ARGS)
 		PageSetTLI(page, ThisTimeLineID);
 	}
 	else
-		PageSetLSN(page, XLogRecPtrForTemp);
+		PageSetLSN(page, GetXLogRecPtrForTemp());
 
 	UnlockReleaseBuffer(buffer);
 
@@ -423,7 +421,7 @@ gistplacetopage(GISTInsertState *state, GISTSTATE *giststate)
 		{
 			for (ptr = dist; ptr; ptr = ptr->next)
 			{
-				PageSetLSN(ptr->page, XLogRecPtrForTemp);
+				PageSetLSN(ptr->page, GetXLogRecPtrForTemp());
 			}
 		}
 
@@ -491,7 +489,7 @@ gistplacetopage(GISTInsertState *state, GISTSTATE *giststate)
 			PageSetTLI(state->stack->page, ThisTimeLineID);
 		}
 		else
-			PageSetLSN(state->stack->page, XLogRecPtrForTemp);
+			PageSetLSN(state->stack->page, GetXLogRecPtrForTemp());
 
 		if (state->stack->blkno == GIST_ROOT_BLKNO)
 			state->needInsertComplete = false;
@@ -1027,7 +1025,7 @@ gistnewroot(Relation r, Buffer buffer, IndexTuple *itup, int len, ItemPointer ke
 		PageSetTLI(page, ThisTimeLineID);
 	}
 	else
-		PageSetLSN(page, XLogRecPtrForTemp);
+		PageSetLSN(page, GetXLogRecPtrForTemp());
 
 	END_CRIT_SECTION();
 }
diff --git a/src/backend/access/gist/gistutil.c b/src/backend/access/gist/gistutil.c
index 28ab575425e..d488bbb4200 100644
--- a/src/backend/access/gist/gistutil.c
+++ b/src/backend/access/gist/gistutil.c
@@ -677,3 +677,24 @@ gistoptions(PG_FUNCTION_ARGS)
 		PG_RETURN_BYTEA_P(result);
 	PG_RETURN_NULL();
 }
+
+/*
+ * 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 0ff5ba840ee..dbe9406ceb8 100644
--- a/src/backend/access/gist/gistvacuum.c
+++ b/src/backend/access/gist/gistvacuum.c
@@ -268,7 +268,7 @@ gistbulkdelete(PG_FUNCTION_ARGS)
 					pfree(rdata);
 				}
 				else
-					PageSetLSN(page, XLogRecPtrForTemp);
+					PageSetLSN(page, GetXLogRecPtrForTemp());
 
 				END_CRIT_SECTION();
 			}
diff --git a/src/include/access/gist_private.h b/src/include/access/gist_private.h
index 34cc5d5a8bf..f2dcbfb39b2 100644
--- a/src/include/access/gist_private.h
+++ b/src/include/access/gist_private.h
@@ -87,7 +87,6 @@ typedef struct GISTScanOpaqueData
 typedef GISTScanOpaqueData *GISTScanOpaque;
 
 /* XLog stuff */
-extern const XLogRecPtr XLogRecPtrForTemp;
 
 #define XLOG_GIST_PAGE_UPDATE		0x00
 #define XLOG_GIST_NEW_ROOT			0x20
@@ -326,6 +325,8 @@ extern void gistMakeUnionKey(GISTSTATE *giststate, int attno,
 				 GISTENTRY *entry2, bool isnull2,
 				 Datum *dst, bool *dstisnull);
 
+extern XLogRecPtr GetXLogRecPtrForTemp(void);
+
 /* gistvacuum.c */
 extern Datum gistbulkdelete(PG_FUNCTION_ARGS);
 extern Datum gistvacuumcleanup(PG_FUNCTION_ARGS);