dropbox: test file name length before upload to fix upload loop

Before this change rclone would upload the whole of multipart files
before receiving a message from dropbox that the path was too long.

This change hard codes the 255 rune limit and checks that before
uploading any files.

Fixes #4805

backend/dropbox/dropbox_internal_test.go

@@ -0,0 +1,44 @@
+package dropbox
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestInternalCheckPathLength(t *testing.T) {
+	rep := func(n int, r rune) (out string) {
+		rs := make([]rune, n)
+		for i := range rs {
+			rs[i] = r
+		}
+		return string(rs)
+	}
+	for _, test := range []struct {
+		in string
+		ok bool
+	}{
+		{in: "", ok: true},
+		{in: rep(maxFileNameLength, 'a'), ok: true},
+		{in: rep(maxFileNameLength+1, 'a'), ok: false},
+		{in: rep(maxFileNameLength, '£'), ok: true},
+		{in: rep(maxFileNameLength+1, '£'), ok: false},
+		{in: rep(maxFileNameLength, '☺'), ok: true},
+		{in: rep(maxFileNameLength+1, '☺'), ok: false},
+		{in: rep(maxFileNameLength, '你'), ok: true},
+		{in: rep(maxFileNameLength+1, '你'), ok: false},
+		{in: "/ok/ok", ok: true},
+		{in: "/ok/" + rep(maxFileNameLength, 'a') + "/ok", ok: true},
+		{in: "/ok/" + rep(maxFileNameLength+1, 'a') + "/ok", ok: false},
+		{in: "/ok/" + rep(maxFileNameLength, '£') + "/ok", ok: true},
+		{in: "/ok/" + rep(maxFileNameLength+1, '£') + "/ok", ok: false},
+		{in: "/ok/" + rep(maxFileNameLength, '☺') + "/ok", ok: true},
+		{in: "/ok/" + rep(maxFileNameLength+1, '☺') + "/ok", ok: false},
+		{in: "/ok/" + rep(maxFileNameLength, '你') + "/ok", ok: true},
+		{in: "/ok/" + rep(maxFileNameLength+1, '你') + "/ok", ok: false},
+	} {
+
+		err := checkPathLength(test.in)
+		assert.Equal(t, test.ok, err == nil, test.in)
+	}
+}