Re-investigating this on a z15 (machine 8561), and the earlier explanation
("hardware compression produces larger output for small data") does
not really seem to explain the test failure.
The actual cause is in pristine-tar:
- `pristine-tar --recompress` recompresses with the `gzip` binary (not zlib).
- If gzip uses s390x hardware deflate (DFLTCC), pristine-gz/zgz (software) can
not reproduce that output, so pristine-tar stores the whole file in the delta:
software gzip output -> delta 289 bytes
DFLTCC gzip output -> "pristine-gz cannot reproduce build ...; storing
entire file in delta!" -> delta 18107 bytes
- So `--recompress` is ineffective, and test_recompress_reduces_delta_size
correctly detects that ("ASSERT:compressed < regular").
Reproduced on z15, same pristine-tar (1.50+nmu2ubuntu1, test unskipped,
autopkgtest mode), only gzip varied:
- gzip 1.14-1~exp2ubuntu3: 5/5 OK
- gzip 1.13-1ubuntu4 (DFLTCC): 5/5 FAILED
- gzip 1.13-1ubuntu4 with DFLTCC=0: passes
- gzip 1.13-1ubuntu4 (DFLTCC) + fix below: 5/5 OK
Ubuntu s390x autopkgtest history confirms that, since every failure of
this test (questing and resolute) ran with gzip 1.12/1.13 built WITH
DFLTCC.
Why it currently passes everywhere (masked, not fixed):
- Debian's gzip has never been built with DFLTCC.
- Ubuntu's gzip 1.14 merge (March 2026) dropped --enable-dfltcc, so current
resolute/stonking gzip no longer uses DFLTCC.
- Since 1.50+nmu2ubuntu1 the test is skipped on s390x in Ubuntu anyway.
Proposed fix (instead of skipping the test), in pristine-tar `sub
recompress`:
# Hardware deflate (e.g. s390x DFLTCC) is not reproducible by zgz
local $ENV{DFLTCC} = '0';
@Eduard, could you please double-check and confirm this analysis?
Once confirmed, the Debian MR #13 could be replaced with the above
proposed fix.
And once the above has landed, the Ubuntu skip can be dropped.
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https://bugs.launchpad.net/bugs/2138882
Title:
[autopkgtest][resolute] s390x Test Failures on test_recompress.sh
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