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With the -R option, test_buffer.py needs an additional warm-up for
the unicode_latin1 cache. Patch attached.

I know the title says spurious, but is there a way to reproduce this at all? Can I run a certain suite N times or something? How did you find the problem?

I'm attaching a test case. You're right, in test_buffer it's
hard to reproduce but I've encountered the "leak" several times
in the past months.
Today I realized that the "leak" always occurred with format code
'c'. There's this passage in test_buffer:
 x = randrange(*fmtdict[mode][char])
 if char == 'c':
 x = bytes(chr(x), 'latin1')
After some head scratching I looked into regrtest.py and found
the warm_char_cache() function. The whole thing makes sense: In
each repetition of the refleak mode new characters can be added
to the unicode_latin1 cache.
So, you should be able to reproduce the issue:
$ ./python -m test -uall -R 2:2 test_pseudo_leak
[1/1] test_pseudo_leak
beginning 4 repetitions
1234
....
test_pseudo_leak leaked [8, 8] references, sum=16
1 test failed:
 test_pseudo_leak
[151225 refs]

Should we go all the way and also explicitly fill the small int cache?
Integers 0-255 should be initialized already, but I'm not sure
about [-5..-1] and 256. I haven't been able to create a test case
for integers though.

New changeset dc18d73e67a5 by Stefan Krah in branch 'default':
Closes #15632: regrtest.py: fix spurious refleaks due to various caches
http://hg.python.org/cpython/rev/dc18d73e67a5