Implement Remote Control for rclone #2111

This implements a remote control protocol activated with the --rc flag
and a new command `rclone rc` to use that interface.

Still to do
  * docs - need finishing
  * tests

fs/rc/registry.go

@@ -0,0 +1,73 @@
+// Define the registry
+
+package rc
+
+import (
+	"strings"
+	"sync"
+
+	"github.com/ncw/rclone/fs"
+)
+
+// Params is the input and output type for the Func
+type Params map[string]interface{}
+
+// Func defines a type for a remote control function
+type Func func(in Params) (out Params, err error)
+
+// Call defines info about a remote control function and is used in
+// the Add function to create new entry points.
+type Call struct {
+	Path  string // path to activate this RC
+	Fn    Func   `json:"-"` // function to call
+	Title string // help for the function
+	Help  string // multi-line markdown formatted help
+}
+
+// Registry holds the list of all the registered remote control functions
+type Registry struct {
+	mu   sync.RWMutex
+	call map[string]*Call
+}
+
+// NewRegistry makes a new registry for remote control functions
+func NewRegistry() *Registry {
+	return &Registry{
+		call: make(map[string]*Call),
+	}
+}
+
+// Add a call to the registry
+func (r *Registry) add(call Call) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	call.Path = strings.Trim(call.Path, "/")
+	call.Help = strings.TrimSpace(call.Help)
+	fs.Debugf(nil, "Adding path %q to remote control registry", call.Path)
+	r.call[call.Path] = &call
+}
+
+// get a Call from a path or nil
+func (r *Registry) get(path string) *Call {
+	r.mu.RLock()
+	defer r.mu.RUnlock()
+	return r.call[path]
+}
+
+// get a list of all calls
+func (r *Registry) list() (out []*Call) {
+	r.mu.RLock()
+	defer r.mu.RUnlock()
+	for _, call := range r.call {
+		out = append(out, call)
+	}
+	return out
+}
+
+// The global registry
+var registry = NewRegistry()
+
+// Add a function to the global registry
+func Add(call Call) {
+	registry.add(call)
+}