cri: add the sandboxfiles package

The files a pod shares with its containers (hostname, hosts, resolv.conf,
the shm tmpfs) are produced by the podsandbox controller from the pod
config. A sandbox implementation outside the CRI server, such as a
Sandbox API shim, has to produce the same files. Put the pure helpers
(file and mount path constants, hostname and resolv.conf content, the shm
mount data) in a leaf package with no CRI server dependencies so that
both can use them. podsandbox is left unchanged for now.

Signed-off-by: Maksym Pavlenko <pavlenko.maksym@gmail.com>

internal/cri/sandboxfiles/sandboxfiles.go

@@ -0,0 +1,88 @@
+/*
+   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 sandboxfiles builds the content of the files a pod sandbox shares
+// with every container of the pod: /etc/hostname, /etc/hosts, /etc/resolv.conf
+// and the /dev/shm tmpfs.
+//
+// The package is deliberately a leaf: it has no dependency on the CRI server
+// or on any sandbox implementation, so that both the in-process podsandbox
+// controller and a shim that implements the Sandbox API can produce the same
+// files. Where the files live and how they reach the containers is decided by
+// the caller.
+package sandboxfiles
+
+import (
+	"fmt"
+	"strings"
+)
+
+const (
+	// HostnameFile is the file name of the pod hostname file.
+	HostnameFile = "hostname"
+	// HostsFile is the file name of the pod hosts file.
+	HostsFile = "hosts"
+	// ResolvConfFile is the file name of the pod resolv.conf.
+	ResolvConfFile = "resolv.conf"
+	// ShmDir is the directory name of the pod /dev/shm tmpfs.
+	ShmDir = "shm"
+
+	// EtcHostname is the path of the hostname file inside a container.
+	EtcHostname = "/etc/hostname"
+	// EtcHosts is the path of the hosts file on the host and inside a container.
+	EtcHosts = "/etc/hosts"
+	// ResolvConfPath is the path of resolv.conf on the host and inside a container.
+	ResolvConfPath = "/etc/resolv.conf"
+	// DevShm is the path of the shared memory tmpfs on the host and inside a container.
+	DevShm = "/dev/shm"
+
+	// DefaultShmSize is the default size of the pod /dev/shm tmpfs.
+	DefaultShmSize = int64(1024 * 1024 * 64)
+)
+
+// HostnameContent returns the content of the pod hostname file.
+func HostnameContent(hostname string) []byte {
+	return []byte(hostname + "\n")
+}
+
+// ResolvConfContent renders resolv.conf content from CRI DNS options. When no
+// option is specified the result is empty.
+func ResolvConfContent(servers, searches, options []string) []byte {
+	var b strings.Builder
+
+	if len(searches) > 0 {
+		fmt.Fprintf(&b, "search %s\n", strings.Join(searches, " "))
+	}
+
+	if len(servers) > 0 {
+		fmt.Fprintf(&b, "nameserver %s\n", strings.Join(servers, "\nnameserver "))
+	}
+
+	if len(options) > 0 {
+		fmt.Fprintf(&b, "options %s\n", strings.Join(options, " "))
+	}
+
+	return []byte(b.String())
+}
+
+// ShmMountData returns the tmpfs mount data for a pod /dev/shm of the given
+// size. A non-positive size falls back to DefaultShmSize.
+func ShmMountData(size int64) string {
+	if size <= 0 {
+		size = DefaultShmSize
+	}
+	return fmt.Sprintf("mode=1777,size=%d", size)
+}

internal/cri/sandboxfiles/sandboxfiles_test.go

@@ -0,0 +1,64 @@
+/*
+   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 sandboxfiles
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestResolvConfContent(t *testing.T) {
+	for _, test := range []struct {
+		desc     string
+		servers  []string
+		searches []string
+		options  []string
+		expected string
+	}{
+		{
+			desc:     "empty dns options should return empty content",
+			expected: "",
+		},
+		{
+			desc:     "non-empty dns options should return correct content",
+			servers:  []string{"8.8.8.8", "8.8.4.4"},
+			searches: []string{"114.114.114.114"},
+			options:  []string{"timeout:1"},
+			expected: "search 114.114.114.114\nnameserver 8.8.8.8\nnameserver 8.8.4.4\noptions timeout:1\n",
+		},
+		{
+			desc:     "servers only",
+			servers:  []string{"1.1.1.1"},
+			expected: "nameserver 1.1.1.1\n",
+		},
+	} {
+		t.Run(test.desc, func(t *testing.T) {
+			assert.Equal(t, test.expected, string(ResolvConfContent(test.servers, test.searches, test.options)))
+		})
+	}
+}
+
+func TestHostnameContent(t *testing.T) {
+	assert.Equal(t, "pod-1\n", string(HostnameContent("pod-1")))
+}
+
+func TestShmMountData(t *testing.T) {
+	assert.Equal(t, "mode=1777,size=67108864", ShmMountData(0))
+	assert.Equal(t, "mode=1777,size=67108864", ShmMountData(-1))
+	assert.Equal(t, "mode=1777,size=1024", ShmMountData(1024))
+}