Replace pid and started file with State type

Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@docker.com> (github: crosbymichael)

state.go

@@ -0,0 +1,48 @@
+package libcontainer
+
+import (
+	"encoding/json"
+	"os"
+	"path/filepath"
+)
+
+// State represents a running container's state
+type State struct {
+	// Pid1 is the process id for the container's pid 1 in it's parent namespace
+	Pid1 int `json:"pid1,omitempty"`
+	// Pid1StartTime is the process start time for the container's pid 1
+	Pid1StartTime string `json:"pid1_start_time,omitempty"`
+}
+
+// WriteState writes the container's runtime state to a state.json file
+// in the specified path
+func WriteState(basePath string, state *State) error {
+	f, err := os.Create(filepath.Join(basePath, "state.json"))
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+
+	return json.NewEncoder(f).Encode(state)
+}
+
+// LoadState reads the state.json file for a running container
+func LoadState(basePath string) (*State, error) {
+	f, err := os.Open(filepath.Join(basePath, "state.json"))
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+
+	var state *State
+	if err := json.NewDecoder(f).Decode(&state); err != nil {
+		return nil, err
+	}
+
+	return state, nil
+}
+
+// DeleteState deletes the state.json file
+func DeleteState(basePath string) error {
+	return os.Remove(filepath.Join(basePath, "state.json"))
+}