enhance: split config from server package

The github.com/containerd/containerd/services/server has a lot of
dependencies, like content, snapshots services implementation and
docker-metrics.

For the client side, it uses the config struct from server package
to start up the containerd in background. It will import a lot of
useless packages which might be conflict with existing vendor's package.

It makes integration easier with single config package.

Signed-off-by: Wei Fu <fuweid89@gmail.com>

services/server/config/config.go

@@ -0,0 +1,109 @@
+/*
+   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 config
+
+import (
+	"github.com/BurntSushi/toml"
+	"github.com/containerd/containerd/errdefs"
+	"github.com/pkg/errors"
+)
+
+// Config provides containerd configuration data for the server
+type Config struct {
+	// Root is the path to a directory where containerd will store persistent data
+	Root string `toml:"root"`
+	// State is the path to a directory where containerd will store transient data
+	State string `toml:"state"`
+	// GRPC configuration settings
+	GRPC GRPCConfig `toml:"grpc"`
+	// Debug and profiling settings
+	Debug Debug `toml:"debug"`
+	// Metrics and monitoring settings
+	Metrics MetricsConfig `toml:"metrics"`
+	// DisabledPlugins are IDs of plugins to disable. Disabled plugins won't be
+	// initialized and started.
+	DisabledPlugins []string `toml:"disabled_plugins"`
+	// Plugins provides plugin specific configuration for the initialization of a plugin
+	Plugins map[string]toml.Primitive `toml:"plugins"`
+	// OOMScore adjust the containerd's oom score
+	OOMScore int `toml:"oom_score"`
+	// Cgroup specifies cgroup information for the containerd daemon process
+	Cgroup CgroupConfig `toml:"cgroup"`
+	// ProxyPlugins configures plugins which are communicated to over GRPC
+	ProxyPlugins map[string]ProxyPlugin `toml:"proxy_plugins"`
+
+	md toml.MetaData
+}
+
+// GRPCConfig provides GRPC configuration for the socket
+type GRPCConfig struct {
+	Address        string `toml:"address"`
+	UID            int    `toml:"uid"`
+	GID            int    `toml:"gid"`
+	MaxRecvMsgSize int    `toml:"max_recv_message_size"`
+	MaxSendMsgSize int    `toml:"max_send_message_size"`
+}
+
+// Debug provides debug configuration
+type Debug struct {
+	Address string `toml:"address"`
+	UID     int    `toml:"uid"`
+	GID     int    `toml:"gid"`
+	Level   string `toml:"level"`
+}
+
+// MetricsConfig provides metrics configuration
+type MetricsConfig struct {
+	Address       string `toml:"address"`
+	GRPCHistogram bool   `toml:"grpc_histogram"`
+}
+
+// CgroupConfig provides cgroup configuration
+type CgroupConfig struct {
+	Path string `toml:"path"`
+}
+
+// ProxyPlugin provides a proxy plugin configuration
+type ProxyPlugin struct {
+	Type    string `toml:"type"`
+	Address string `toml:"address"`
+}
+
+// Decode unmarshals a plugin specific configuration by plugin id
+func (c *Config) Decode(id string, v interface{}) (interface{}, error) {
+	data, ok := c.Plugins[id]
+	if !ok {
+		return v, nil
+	}
+	if err := c.md.PrimitiveDecode(data, v); err != nil {
+		return nil, err
+	}
+	return v, nil
+}
+
+// LoadConfig loads the containerd server config from the provided path
+func LoadConfig(path string, v *Config) error {
+	if v == nil {
+		return errors.Wrapf(errdefs.ErrInvalidArgument, "argument v must not be nil")
+	}
+	md, err := toml.DecodeFile(path, v)
+	if err != nil {
+		return err
+	}
+	v.md = md
+	return nil
+}