tests: Make neon_xlogflush() flush all WAL, if you omit the LSN arg

This makes it much more convenient to use in the common case that you
want to flush all the WAL. (Passing pg_current_wal_insert_lsn() as the
argument doesn't work for the same reasons as explained in the
comments: we need to be back off to the beginning of a page if the
previous record ended at page boundary.)

I plan to use this to fix the issue that Arseny Sher called out at
#7288 (comment)

pgxn/neon_test_utils/Makefile

@@ -7,7 +7,7 @@ OBJS = \
 	neontest.o
 
 EXTENSION = neon_test_utils
-DATA = neon_test_utils--1.1.sql
+DATA = neon_test_utils--1.2.sql
 PGFILEDESC = "neon_test_utils - helpers for neon testing and debugging"
 
 PG_CONFIG = pg_config

pgxn/neon_test_utils/neon_test_utils--1.1.sql → pgxn/neon_test_utils/neon_test_utils--1.2.sql

@@ -41,7 +41,7 @@ RETURNS bytea
 AS 'MODULE_PATHNAME', 'get_raw_page_at_lsn_ex'
 LANGUAGE C PARALLEL UNSAFE;
 
-CREATE FUNCTION neon_xlogflush(lsn pg_lsn)
+CREATE FUNCTION neon_xlogflush(lsn pg_lsn DEFAULT NULL)
 RETURNS VOID
 AS 'MODULE_PATHNAME', 'neon_xlogflush'
 LANGUAGE C PARALLEL UNSAFE;

pgxn/neon_test_utils/neon_test_utils.control

@@ -1,6 +1,6 @@
 # neon_test_utils extension
 comment = 'helpers for neon testing and debugging'
-default_version = '1.1'
+default_version = '1.2'
 module_pathname = '$libdir/neon_test_utils'
 relocatable = true
 trusted = true

pgxn/neon_test_utils/neontest.c

@@ -15,6 +15,7 @@
 #include "access/relation.h"
 #include "access/xact.h"
 #include "access/xlog.h"
+#include "access/xlog_internal.h"
 #include "catalog/namespace.h"
 #include "fmgr.h"
 #include "funcapi.h"
@@ -444,11 +445,46 @@ get_raw_page_at_lsn_ex(PG_FUNCTION_ARGS)
 
 /*
  * Directly calls XLogFlush(lsn) to flush WAL buffers.
+ *
+ * If 'lsn' is not specified (is NULL), flush all generated WAL.
  */
 Datum
 neon_xlogflush(PG_FUNCTION_ARGS)
 {
-	XLogRecPtr	lsn = PG_GETARG_LSN(0);
+	XLogRecPtr	lsn;
+
+	if (RecoveryInProgress())
+		ereport(ERROR,
+				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+				 errmsg("recovery is in progress"),
+				 errhint("cannot flush WAL during recovery.")));
+
+	if (!PG_ARGISNULL(0))
+		lsn = PG_GETARG_LSN(0);
+	else
+	{
+		lsn = GetXLogInsertRecPtr();
+
+		/*---
+		 * The LSN returned by GetXLogInsertRecPtr() is the position where the
+		 * next inserted record would begin. If the last record ended just at
+		 * the page boundary, the next record will begin after the page header
+		 * on the next page, and that's what GetXLogInsertRecPtr().returns,
+		 * but the page header has not been written yet. If we tried to flush
+		 * it, XLogFlush() would throw an error:
+		 *
+		 * ERROR : xlog flush request %X/%X is not satisfied --- flushed only to %X/%X
+		 *
+		 * To avoid that, if the insert position points to just after the page
+		 * header, back off to page boundary.
+		 */
+		if (lsn % XLOG_BLCKSZ == SizeOfXLogShortPHD &&
+			XLogSegmentOffset(lsn, wal_segment_size) > XLOG_BLCKSZ)
+			lsn -= SizeOfXLogShortPHD;
+		else if (lsn % XLOG_BLCKSZ == SizeOfXLogLongPHD &&
+				 XLogSegmentOffset(lsn, wal_segment_size) < XLOG_BLCKSZ)
+			lsn -= SizeOfXLogLongPHD;
+	}
 
 	XLogFlush(lsn);
 	PG_RETURN_VOID();