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.
`zstreamtest --newapi` (and `--opaqueapi`) create and destroy way too many threads
resulting in failure of tsan tests,
and potentially connected to the qemu flaky tests.
This is because, at each test, the nb of threads can be changed (random).
The `--no-big-tests` directive reduce this choice to 1/2 threads,
in order to limit memory usage, especially for qemu and 32-bits builds.
Unfortunately, swapping between 1 and 2 threads is enough to constantly create/destroy new mtctx.
This patch takes advantage of the following property :
via compress_generic, no internal mtctx is needed for nbThreads < 2.
As a consequence, when nbThreads == 2, the currently active mtctx is necessarily good.
This dramatically reduces the nb of thread creations when invoking `zstreamtest --newapi --no-big-tests`
(only when parent cctx itself is created, which is randomized to 1/256 tests).
Expected outcome :
- at a minimum : tsan tests shall now work continuously without exploding the thread counter
- at best : flaky qemu tests on `zstreamtest --newapi --no-big-tests` may stop being flaky, due to less stress from constant thread creation/destruction
Real world impact :
minimal, I don't expect users to constantly change `nbThreads` between each invocation.
If `nbThreads` remains stable, existing implementation re-uses existing mtctx.
Also : `zstreamtest --newapi` but without `--no-big-tests` doesn't benefit as much,
since this test can select a random `nbThreads` value between 1 and 4.
The current patch only reduces opportunity to free/create mtctx (for example : 2->1->2 doesn't need a new mtctx)
but doesn't completely eliminate it, since `nbThreads` can still change between 2/3/4.
A more complete solution could be to only use 2 out of 4 allocated threads, thus keeping the pool at a constant size.
This would require a larger change to `POOL_*` api though.
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
last such side-effect was modifying cctx->loadedDictEnd on setting forceWindow.
It is no a useless operation, so it's removed.
No side-effect left when setting a compression parameter.
ZSTD_updateTree() expected to be followed by a Bt match finder, which would update zc->nextToUpdate.
With the new optimal match finder, it's not necessarily the case : a match might be found during repcode or hash3, and stops there because it reaches sufficient_len, without even entering the binary tree.
Previous policy was to nonetheless update zc->nextToUpdate, but the current position would not be inserted, creating "holes" in the btree, aka positions that will no longer be searched.
Now, when current position is not inserted, zc->nextToUpdate is not update, expecting ZSTD_updateTree() to fill the tree later on.
Solution selected is that ZSTD_updateTree() takes care of properly setting zc->nextToUpdate,
so that it no longer depends on a future function to do this job.
It took time to get there, as the issue started with a memory sanitizer error.
The pb would have been easier to spot with a proper `assert()`.
So this patch add a few of them.
Additionnally, I discovered that `make test` does not enable `assert()` during CLI tests.
This patch enables them.
Unfortunately, these `assert()` triggered other (unrelated) bugs during CLI tests, mostly within zstdmt.
So this patch also fixes them.
- Changed packed structure for gcc memory access : memory sanitizer would complain that a read "might" reach out-of-bound position on the ground that the `union` is larger than the type accessed.
Now, to avoid this issue, each type is independent.
- ZSTD_CCtxParams_setParameter() : @return provides the value of parameter, clamped/fixed appropriately.
- ZSTDMT : changed constant name to ZSTDMT_JOBSIZE_MIN
- ZSTDMT : multithreading is automatically disabled when srcSize <= ZSTDMT_JOBSIZE_MIN, since only one thread will be used in this case (saves memory and runtime).
- ZSTDMT : nbThreads is automatically clamped on setting the value.
added some traces and assert
related to hunting a potential ubsan error in 32-bits more
(it ends up being a compiler-side issue : https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82802).
Modified one pointer arithmetic expression for a more conformant way.
It was multiple reasons stacked :
- Visual use a different code path, because ZSTD_NEWAPI is not defined
- fileio.c sends `0` as `pledgedSrcSize` to mean `ZSTD_CONTENTSIZE_UNKNOWN` (fixed)
- ZSTDMT_resetCCtx() interpreted `0` as "empty" instead of "unknown" (fixed)
In some complex scenario,
the buffer would be freed because it's too large,
another buffer would be allocated, but fail,
trigger an error,
and the general buffer pool would then be freed,
where the definition of the already freed buffer would be found
(beyond total index, but still), and freed again, resulting in double-free error.
