During the previous attempt to generalize the UUID class, it was
suggested that we represent invalid UUIDs as length zero (previously, we
used an all-zero UUID for that). This meant that some valid build-ids
could not be represented (it's possible however unlikely that a checksum of
some file would be zero) and complicated adding support for variable
length build-ids (should a 16-byte empty UUID compare equal to a 20-byte
This patch resolves these issues by introducing a canonical
representation for an invalid UUID. The slight complication here is that
some clients (MachO) actually use the all-zero notation to mean "no UUID
has been set". To keep this use case working, I have introduced an
additional argument to the UUID constructor, which specifies whether an
all-zero vector should be considered a valid UUID. For the usages where
the UUID data comes from a MachO file, I set this argument to false.
I did not introduce a similar argument to the string parsing function
with the rationalle that if somebody went through the trouble of
spelling it out as a bunch of zeroes, he probably really means that.