The deep fuzzer tests caught a subtle bug that was probably there for a long time.
The impact of the bug is not a crash, or any other clear error signal,
rather, it reduces performance, by cutting data into smaller blocks.
Eventually, the following test would fail because it produces too many 1-byte blocks,
requiring more space than buffer can provide :
`./zstreamtest_asan --mt -s3514 -t1678312 -i1678314`
The root scenario is as follows :
- Create context, initialize it using explicit parameters or a `cdict` to pin them down, set `pledgedSrcSize=1`
- The compression parameters will not be adapted, but `windowSize` and `blockSize` will be automatically set to `1`.
`windowSize` and `blockSize` are dynamic values, set within `ZSTD_resetCCtx_internal()`.
The automatic adaptation makes it possible to generate smaller contexts for smaller input sizes.
- Complete compression
- New compression with same context, using same parameters, but `pledgedSrcSize=ZSTD_CONTENTSIZE_UNKNOWN`
trigger "continue mode"
- Continue mode doesn't modify blockSize, because it used to depend on `windowLog` only,
but in fact, it also depends on `pledgedSrcSize`.
- The "old" blocksize (1) is still there,
next compression will use this value to cut input into blocks,
resulting in more blocks and worse performance than necessary performance.
Given the scenario, and its possible variants, I'm surprised it did not show up before.
But I suspect it did show up, it's just that it never triggered an error, because "worse performance" is not a trigger.
The above test is a special corner case, where performance is so impacted that it reaches an error case.
The fix works, but I'm not completely pleased.
I think the current code relies too much on implied relations between variables.
This will likely break again in the future when some related part of the code change.
Unfortunately, no time to make larger changes if we want to keep the release target for zstd v1.3.3.
So a longer term fix will have to be considered after the release.
To do : create a reliable test case which triggers this scenario for CI tests.
taking advantage of `btopt` improved speed to tune parameters.
Levels 16-19 are stronger than previous release, making the graph more favorable.
In theory, I should also update small-size tables,
but I got lazy on that one ...
zstd streaming API was adding a null-block at end of frame for small input.
Reason is : on small input, a single block is enough.
ZSTD_CStream would size its input buffer to expect a single block of this size,
automatically triggering a flush on reaching this size.
Unfortunately, that last byte was generally received before the "end" directive (at least in `fileio`).
The later "end" directive would force the creation of a 3-bytes last block to indicate end of frame.
The solution is to not flush automatically, which is btw the expected behavior.
It happens in this case because blocksize is defined with exactly the same size as input.
Just adding one-byte is enough to stop triggering the automatic flush.
I initially looked at another solution, solving the problem directly in the compression context.
But it felt awkward.
Now, the underlying compression API `ZSTD_compressContinue()` would take the decision the close a frame
on reaching its expected end (`pledgedSrcSize`).
This feels awkward, a responsability over-reach, beyond the definition of this API.
ZSTD_compressContinue() is clearly documented as a guaranteed flush,
with ZSTD_compressEnd() generating a guaranteed end.
I faced similar issue when trying to port a similar mechanism at the higher streaming layer.
Having ZSTD_CStream end a frame automatically on reaching `pledgedSrcSize` can surprise the caller,
since it did not explicitly requested an end of frame.
The only sensible action remaining after that is to end the frame with no additional input.
This adds additional logic in the ZSTD_CStream state to check this condition.
Plus some potential confusion on the meaning of ZSTD_endStream() with no additional input (ending confirmation ? new 0-size frame ?)
In the end, just enlarging input buffer by 1 byte feels the least intrusive change.
It's also a contract remaining inside the streaming layer, so the logic is contained in this part of the code.
The patch also introduces a new test checking that size of small frame is as expected, without additional 3-bytes null block.
This patch restores capability for each file to receive adapted compression parameters depending on its size.
The bug breaking this feature was relatively silly :
setting a parameter with a value "0" is supposed to be a no-op.
Unfortunately, it would pin down compression parameters as if they were manually set,
preventing later automatic adaptation.
Unfortunately, I'm currently short of a test case that could check this situation and trigger an error.
Compression parameters selection between tableID 0,1,2,3 is largely internal,
leaving no trace to outside world, not even in frame header.
- building cli from /tests preserves potential flags in MOREFLAGS (such as asan/usan)
- MT dictionary tests check for MT capability (MT is not enabled by default for zstd32)
windowLog is now enforced from provided compression parameters,
instead of being copied blindly from `cdict`
where it could be smaller.
also :
- fix a minor bug in zstreamtest --mt : advanced parameters must be set before init
- changed advanced parameter name to ZSTDMT_jobSize
While the final result is still, technically, a frame,
the resulting frame expands initial data instead of compressing it.
This is because the streaming API creates a tiny 1-byte buffer for input,
because it believes input is empty (0-bytes),
because in the past, 0 used to mean "unknown" instead.
This patch fixes the issue.
Todo : add a test which traps the issue.
The new macro might be a bit too restrictive.
Systems which do not support new test will simply default to <time.h>'s `clock_t clock()`,
suffering lesser benchmark accuracy.
Should it matter, the detection macro will have to be upgraded.
