From 3f8702a60cc1125060f6dca7f77a73f918d5c1be Mon Sep 17 00:00:00 2001
From: Jackson Owens <jackson@cockroachlabs.com>
Date: Tue, 7 Sep 2021 11:37:39 -0400
Subject: [PATCH] vfs/atomicfs: add Marker NextIter method

Add a NextIter method that allows clients to query an atomic marker for
its next iteration sequence number. In some usages, client code needs to
create a new file and then move the marker to the new file's path.
Creating the new file must not overrwrite the existing marked file.
Incorporating the marker's iteration number into the filename provides a
simple scheme client code may follow.
---
 vfs/atomicfs/marker.go       |  9 ++++++++-
 vfs/atomicfs/marker_test.go  | 10 +++++++++-
 vfs/atomicfs/testdata/marker | 16 ++++++++++++++++
 3 files changed, 33 insertions(+), 2 deletions(-)

diff --git a/vfs/atomicfs/marker.go b/vfs/atomicfs/marker.go
index 99bf60243c..22bbd554a0 100644
--- a/vfs/atomicfs/marker.go
+++ b/vfs/atomicfs/marker.go
@@ -2,7 +2,7 @@
 // of this source code is governed by a BSD-style license that can be found in
 // the LICENSE file.
 
-package vfs
+package atomicfs
 
 import (
 	"fmt"
@@ -195,6 +195,13 @@ func (a *Marker) Move(filename string) error {
 	return nil
 }
 
+// NextIter returns the next iteration number that the marker will use.
+// Clients may use this number for formulating filenames that are
+// unused.
+func (a *Marker) NextIter() uint64 {
+	return a.iter + 1
+}
+
 // RemoveObsolete removes any obsolete files discovered while locating
 // the marker or files unable to be removed during Move.
 func (a *Marker) RemoveObsolete() error {
diff --git a/vfs/atomicfs/marker_test.go b/vfs/atomicfs/marker_test.go
index 37d03f02bb..5668d25394 100644
--- a/vfs/atomicfs/marker_test.go
+++ b/vfs/atomicfs/marker_test.go
@@ -2,7 +2,7 @@
 // of this source code is governed by a BSD-style license that can be found in
 // the LICENSE file.
 
-package vfs
+package atomicfs
 
 import (
 	"bytes"
@@ -117,6 +117,14 @@ func TestMarker(t *testing.T) {
 			}
 			return ""
 
+		case "next-iter":
+			var dir, marker string
+			td.ScanArgs(t, "dir", &dir)
+			td.ScanArgs(t, "marker", &marker)
+			m := markers[memFS.PathJoin(dir, marker)]
+			require.NotNil(t, m)
+			return fmt.Sprintf("%d", m.NextIter())
+
 		case "remove-obsolete":
 			var dir, marker string
 			td.ScanArgs(t, "dir", &dir)
diff --git a/vfs/atomicfs/testdata/marker b/vfs/atomicfs/testdata/marker
index ffb7a7a71a..2450bd24bf 100644
--- a/vfs/atomicfs/testdata/marker
+++ b/vfs/atomicfs/testdata/marker
@@ -10,6 +10,14 @@ mkdir-all data
 locate dir=data marker=foo
 ----
 
+next-iter  dir=data marker=foo
+----
+1
+
+next-iter  dir=data marker=foo
+----
+1
+
 # The directory should still be empty.
 list data
 ----
@@ -23,11 +31,19 @@ list data
 ----
 marker.foo.000001.MANIFEST-000010
 
+next-iter  dir=data marker=foo
+----
+2
+
 # Moving the marker should move the existing marker file.
 move dir=data marker=foo
 MANIFEST-000016
 ----
 
+next-iter  dir=data marker=foo
+----
+3
+
 list data
 ----
 marker.foo.000002.MANIFEST-000016