After a research we realized that only the uuidd provides the guarantee
of uniqueness of UUIDs across different CPUs and different UIDs.
Without it, there is a probability that two different processes on two
different CPUs with a different UID will call uuid_generate_time_safe()
in the same time and generate duplicated UUIDs.
There is a LIBUUID_CLOCK_FILE that should prevent it. But this approach
can work properly for a single UID only. And creating it world-writable
file would cause more problems that it will fix.
That is why I propose that uuid_generate_time_safe() should always
return -1 in case that it cannot interact with uuidd.
To realize this, uuid_generate_time_generic() should return -1 instead
of __uuid_generate_time() return code.
It should not break uuidd, as it calls __uuid_generate_time() directly.
In the last month, we had a discussion about one clock file per UID in
the home directory and one for uuidd as a solution of LIBUUID_CLOCK_FILE
being handled in a first-UID-wins manner. It could improve the
situation, but only for UUIDs generated by the same UID.
Both changes are partially mutually exclusive: If
uuid_generate_time_safe() will always return -1, there will be no chance
to inform the program, that the alternative synchronization was
successfully used and that the UUIDs are safe in the local context. That
it why I propose a small change of API:
Add a new return code of uuid_generate_time_safe().
== RETURN VALUE
The newly created UUID is returned in the memory location pointed to by
_out_. *uuid_generate_time_safe*() returns zero if the UUID has been
generated in a safe manner, 1 if the UUID has been generated in a manner
that is safe for a single UID, -1 otherwise.
--
Best Regards / S pozdravem,
Stanislav Brabec
software developer
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