cipd: Client can resolve tags to instance IDs.

go/src/infra/tools/cipd/common/common.go

Index: go/src/infra/tools/cipd/common/common.go
diff --git a/go/src/infra/tools/cipd/common/common.go b/go/src/infra/tools/cipd/common/common.go
index 338cf05d1240237639ac42a41d78d03007607dd6..0f4c3caf1fee28f6f8da23d2e7f9415152d88e4d 100644
--- a/go/src/infra/tools/cipd/common/common.go
+++ b/go/src/infra/tools/cipd/common/common.go
@@ -78,3 +78,15 @@ func ValidateInstanceTag(t string) error {
 	}
 	return nil
 }
+
+// ValidateInstanceVersion return error if a string doesn't look like
+// an instance ID or an instance tag.
+func ValidateInstanceVersion(v string) error {
+	if err := ValidateInstanceID(v); err == nil {

[nodir, 2015/05/21 17:07:48]

if ValidateInstanceID(v) == nil && ValidateInstanceTag(v) == nil {
  return nil
}

[Vadim Sh., 2015/05/21 18:00:37]

:) Done.

+		return nil
+	}
+	if err := ValidateInstanceTag(v); err == nil {
+		return nil
+	}
+	return fmt.Errorf("Bad version (not an instance ID or a tag): %s", v)
+}