Add blobserver.RefTypes and blob.TypeAlphabet

To be implemented later, but adding now so googledrive's blob storage implementation
can depend on them.

Change-Id: Ief374e8592bd696c79aa2b80ded11e301063750b

pkg/blob/ref.go

@@ -564,3 +564,13 @@ type SizedByRef []SizedRef
 func (s SizedByRef) Len() int           { return len(s) }
 func (s SizedByRef) Less(i, j int) bool { return s[i].Less(s[j].Ref) }
 func (s SizedByRef) Swap(i, j int)      { s[i], s[j] = s[j], s[i] }
+
+// TypeAlphabet returns the valid characters in the given blobref type.
+// It returns the empty string if the typ is unknown.
+func TypeAlphabet(typ string) string {
+	switch typ {
+	case "sha1":
+		return hexDigit
+	}
+	return ""
+}

pkg/blobserver/enumerate.go

@@ -65,3 +65,13 @@ func EnumerateAllFrom(ctx *context.Context, src BlobEnumerator, after string, fn
 		}
 	}
 }
+
+// RefTypes returns a list of blobref types appearing on the provided enumerator.
+// A blobref type is a string like "sha1", or whatever is on the left side
+// of the hyphen in a blobref.
+// To get the alphabet valid for the right side of the hyphen, use blob.TypeAlphabet(type).
+func RefTypes(src BlobEnumerator) ([]string, error) {
+	// TODO(bradfitz): implement, with various short enumerate
+	// reads at the right places.
+	return []string{"sha1"}, nil
+}