Use ctl->PagePrecedes for SLRU page comparison in SimpleLruDownloadSegment to address wraparround

Author: Konstantin Knizhnik

src/backend/access/transam/slru.c

@@ -618,6 +618,11 @@ SimpleLruWritePage(SlruCtl ctl, int slotno)
 }
 
 
+/*
+ * NEON: we do not want to include large pg_xact/multixact files in basebackup and prefer
+ * to download them on demand to reduce startup time.
+ * If SLRU segment is not found, we try to download it from page server
+ */
 static int
 SimpleLruDownloadSegment(SlruCtl ctl, int pageno, char const* path)
 {
@@ -628,8 +633,8 @@ SimpleLruDownloadSegment(SlruCtl ctl, int pageno, char const* path)
 
 	static SMgrRelationData dummy_smgr_rel = {0};
 
-	/* If page is beyond latest written page, then do not try to download segment from server */
-	if (pageno > ctl->shared->latest_page_number)
+	/* If page is greater than latest written page, then do not try to download segment from server */
+	if (ctl->PagePrecedes(ctl->shared->latest_page_number, pageno))
 		return -1;
 
 	if (!dummy_smgr_rel.smgr)

src/backend/storage/smgr/smgr.c

@@ -538,6 +538,15 @@ smgrwrite(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
 										buffer, skipFsync);
 }
 
+/*
+ * NEON: we do not want to include large pg_xact/multixact files in basebackup and prefer
+ * to download them on demand to reduce startup time.
+ * If SLRU segment is not found, we try to download it from page server
+ *
+ * This function returns number of blocks in segment. Usually it should be SLRU_PAGES_PER_SEGMENT but in case
+ * of partial segment, it can be smaller. Zero value means that segment doesn't exist.
+ * From Postgres point of view empty segment is the same as absent segment.
+ */
 int
 smgr_read_slru_segment(SMgrRelation reln, const char* path, int segno, void* buffer)
 {