fs: Add --disable flag to disable optional features - fixes #1551

Eg to disable server side copy use `--disable copy`, to see a list of
what you can disable, `--disable help`.

fs/fs.go

@@ -8,8 +8,10 @@ import (
 	"math"
 	"os"
 	"path/filepath"
+	"reflect"
 	"regexp"
 	"sort"
+	"strings"
 	"time"
 
 	"github.com/pkg/errors"
@@ -347,6 +349,46 @@ type Features struct {
 	ListR ListRFn
 }
 
+// Disable nil's out the named feature.  If it isn't found then it
+// will log a message.
+func (ft *Features) Disable(name string) *Features {
+	v := reflect.ValueOf(ft).Elem()
+	vType := v.Type()
+	for i := 0; i < v.NumField(); i++ {
+		vName := vType.Field(i).Name
+		field := v.Field(i)
+		if strings.EqualFold(name, vName) {
+			if !field.CanSet() {
+				Errorf(nil, "Can't set Feature %q", name)
+			} else {
+				zero := reflect.Zero(field.Type())
+				field.Set(zero)
+				Debugf(nil, "Reset feature %q", name)
+			}
+		}
+	}
+	return ft
+}
+
+// List returns a slice of all the possible feature names
+func (ft *Features) List() (out []string) {
+	v := reflect.ValueOf(ft).Elem()
+	vType := v.Type()
+	for i := 0; i < v.NumField(); i++ {
+		out = append(out, vType.Field(i).Name)
+	}
+	return out
+}
+
+// DisableList nil's out the comma separated list of named features.
+// If it isn't found then it will log a message.
+func (ft *Features) DisableList(list []string) *Features {
+	for _, feature := range list {
+		ft.Disable(strings.TrimSpace(feature))
+	}
+	return ft
+}
+
 // Fill fills in the function pointers in the Features struct from the
 // optional interfaces.  It returns the original updated Features
 // struct passed in.
@@ -387,7 +429,7 @@ func (ft *Features) Fill(f Fs) *Features {
 	if do, ok := f.(ListRer); ok {
 		ft.ListR = do.ListR
 	}
-	return ft
+	return ft.DisableList(Config.DisableFeatures)
 }
 
 // Mask the Features with the Fs passed in
@@ -438,7 +480,7 @@ func (ft *Features) Mask(f Fs) *Features {
 	if mask.ListR == nil {
 		ft.ListR = nil
 	}
-	return ft
+	return ft.DisableList(Config.DisableFeatures)
 }
 
 // Wrap makes a Copy of the features passed in, overriding the UnWrap