Support >= 128 layers in overlayfs snapshots

Auto-detect longest common dir in lowerdir option and compact it if the
option size is hitting one page size. If does, Use chdir + CLONE to do
mount thing to avoid hitting one page argument buffer in linux kernel
mount.

Signed-off-by: Wei Fu <fhfuwei@163.com>

mount/mount_linux_test.go

@@ -0,0 +1,94 @@
+// +build linux
+
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package mount
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestLongestCommonPrefix(t *testing.T) {
+	tcases := []struct {
+		in       []string
+		expected string
+	}{
+		{[]string{}, ""},
+		{[]string{"foo"}, "foo"},
+		{[]string{"foo", "bar"}, ""},
+		{[]string{"foo", "foo"}, "foo"},
+		{[]string{"foo", "foobar"}, "foo"},
+		{[]string{"foo", "", "foobar"}, ""},
+	}
+
+	for i, tc := range tcases {
+		if got := longestCommonPrefix(tc.in); got != tc.expected {
+			t.Fatalf("[%d case] expected (%s), but got (%s)", i+1, tc.expected, got)
+		}
+	}
+}
+
+func TestCompactLowerdirOption(t *testing.T) {
+	tcases := []struct {
+		opts      []string
+		commondir string
+		newopts   []string
+	}{
+		// no lowerdir or only one
+		{
+			[]string{"workdir=a"},
+			"",
+			[]string{"workdir=a"},
+		},
+		{
+			[]string{"workdir=a", "lowerdir=b"},
+			"",
+			[]string{"workdir=a", "lowerdir=b"},
+		},
+
+		// >= 2 lowerdir
+		{
+			[]string{"lowerdir=/snapshots/1/fs:/snapshots/10/fs"},
+			"/snapshots/",
+			[]string{"lowerdir=1/fs:10/fs"},
+		},
+		{
+			[]string{"lowerdir=/snapshots/1/fs:/snapshots/10/fs:/snapshots/2/fs"},
+			"/snapshots/",
+			[]string{"lowerdir=1/fs:10/fs:2/fs"},
+		},
+
+		// if common dir is /
+		{
+			[]string{"lowerdir=/snapshots/1/fs:/other_snapshots/1/fs"},
+			"",
+			[]string{"lowerdir=/snapshots/1/fs:/other_snapshots/1/fs"},
+		},
+	}
+
+	for i, tc := range tcases {
+		dir, opts := compactLowerdirOption(tc.opts)
+		if dir != tc.commondir {
+			t.Fatalf("[%d case] expected common dir (%s), but got (%s)", i+1, tc.commondir, dir)
+		}
+
+		if !reflect.DeepEqual(opts, tc.newopts) {
+			t.Fatalf("[%d case] expected options (%v), but got (%v)", i+1, tc.newopts, opts)
+		}
+	}
+}