Re: UUID v7

I've done a test of the v10 patch, and ran into an interesting behavior when passing in a timestamp to the function (which, as a side note, is actually very useful to have as a feature, to support creating time-based range partitions on UUIDv7 fields):

postgres=# SELECT uuid_extract_time(uuidv7());
     uuid_extract_time    
---------------------------
 2024-01-18 18:49:00.01-08
(1 row)

postgres=# SELECT uuid_extract_time(uuidv7('2024-04-01'));
   uuid_extract_time    
------------------------
 2024-04-01 00:00:00-07
(1 row)

postgres=# SELECT uuid_extract_time(uuidv7());
   uuid_extract_time    
------------------------
 2024-04-01 00:00:00-07
(1 row)

Note how calling the uuidv7 function again after having called it with a fixed future timestamp, returns the future timestamp, even though it should return the current time.

I believe this is caused by incorrectly re-using the cached previous_timestamp. In the second call here (with a fixed future timestamp), we end up setting ts and tms to 2024-04-01, with increment_counter = false, which leads us to set previous_timestamp to the passed in timestamp (else branch of the second if in uuidv7). When we then call the function again without an argument, we end up getting a new timestamp from gettimeofday, but because we try to detect backwards leaps, we set increment_counter to true, and thus end up reusing the previous (future) timestamp here:

/* protection from leap backward */
tms = previous_timestamp;

Not sure how to fix this, but clearly something is amiss here.

Thanks,

Lukas