[cipd] Add AssertOnlyDefaultSubdir.

cipd/client/cipd/common/common.go

Index: cipd/client/cipd/common/common.go
diff --git a/cipd/client/cipd/common/common.go b/cipd/client/cipd/common/common.go
index 547a91f12a7f931e56a4c78b61d976a65ef1c3e0..af47970e525b360de2636343b0c745a8326ee8b6 100644
--- a/cipd/client/cipd/common/common.go
+++ b/cipd/client/cipd/common/common.go
@@ -6,6 +6,7 @@
 package common
 
 import (
+	"errors"
 	"fmt"
 	"path"
 	"regexp"
@@ -249,6 +250,21 @@ func (p PinSliceBySubdir) Validate() error {
 	return nil
 }
 
+// ErrSubdirsNotYetSupported is a transitionary error which indicates that the
+// invoked method does not yet work with sudbir values other than "".
+var ErrSubdirsNotYetSupported = errors.New(`subdirs other than "" not currently supported`)
+
+// AssertOnlyDefaultSubdir is a transitionary method which returns an error if
+// p contains subdirs other than "".
+func (p PinSliceBySubdir) AssertOnlyDefaultSubdir() error {
+	if _, ok := p[""]; ok && len(p) > 1 {
+		return ErrSubdirsNotYetSupported
+	} else if !ok && len(p) > 0 {
+		return ErrSubdirsNotYetSupported
+	}
+	return nil
+}
+
 // ToMap converts this to a PinMapBySubdir
 func (p PinSliceBySubdir) ToMap() PinMapBySubdir {
 	ret := make(PinMapBySubdir, len(p))