Fix pg_upgrade failure from servers older than 9.3

When upgrading from servers of versions 9.2 and older, and MultiXactIds
have been used in the old server beyond the first page (that is, 2048
multis or more in the default 8kB-page build), pg_upgrade would set the
next multixact offset to use beyond what has been allocated in the new
cluster.  This would cause a failure the first time the new cluster
needs to use this value, because the pg_multixact/offsets/ file wouldn't
exist or wouldn't be large enough.  To fix, ensure that the transient
server instances launched by pg_upgrade extend the file as necessary.

Per report from Jesse Denardo in
CANiVXAj4c88YqipsyFQPboqMudnjcNTdB3pqe8ReXqAFQ=HXyA@mail.gmail.com

1 files changed, 44 insertions, 0 deletions

diff --git a/src/backend/access/transam/slru.c b/src/backend/access/transam/slru.c
index 5a8f654fb73..5e53593a8f2 100644
--- a/src/backend/access/transam/slru.c
+++ b/src/backend/access/transam/slru.c
@@ -563,6 +563,50 @@ SimpleLruWritePage(SlruCtl ctl, int slotno)
 	SlruInternalWritePage(ctl, slotno, NULL);
 }
 
+/*
+ * Return whether the given page exists on disk.
+ *
+ * A false return means that either the file does not exist, or that it's not
+ * large enough to contain the given page.
+ */
+bool
+SimpleLruDoesPhysicalPageExist(SlruCtl ctl, int pageno)
+{
+	int			segno = pageno / SLRU_PAGES_PER_SEGMENT;
+	int			rpageno = pageno % SLRU_PAGES_PER_SEGMENT;
+	int			offset = rpageno * BLCKSZ;
+	char		path[MAXPGPATH];
+	int			fd;
+	bool		result;
+	off_t		endpos;
+
+	SlruFileName(ctl, path, segno);
+
+	fd = OpenTransientFile(path, O_RDWR | PG_BINARY, S_IRUSR | S_IWUSR);
+	if (fd < 0)
+	{
+		/* expected: file doesn't exist */
+		if (errno == ENOENT)
+			return false;
+
+		/* report error normally */
+		slru_errcause = SLRU_OPEN_FAILED;
+		slru_errno = errno;
+		SlruReportIOError(ctl, pageno, 0);
+	}
+
+	if ((endpos = lseek(fd, 0, SEEK_END)) < 0)
+	{
+		slru_errcause = SLRU_OPEN_FAILED;
+		slru_errno = errno;
+		SlruReportIOError(ctl, pageno, 0);
+	}
+
+	result = endpos >= (off_t) (offset + BLCKSZ);
+
+	CloseTransientFile(fd);
+	return result;
+}
 
 /*
  * Physical read of a (previously existing) page into a buffer slot