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With lz4 1.8.3, this function can now decompress partial results into a smaller buffer. The release news don't say anything interesting, but the test case that was previously failing now works OK. Fixes #10259. A test is added. It shows that with *older* lz4, a partial decompression can occur with the returned size smaller then the requested number of bytes _and_ smaller then the size of the compressed data: (lz4-libs-1.8.2-1.fc29.x86_64) Compressed 4194304 → 16464 Decompressed → 4194304 Decompressed partial 12/4194304 → 4194304 Decompressed partial 1/1 → -2 (bad) Decompressed partial 2/2 → -2 (bad) Decompressed partial 3/3 → -2 (bad) Decompressed partial 4/4 → -2 (bad) Decompressed partial 5/5 → -2 (bad) Decompressed partial 6/6 → 6 (good) Decompressed partial 7/7 → 6 (good) Decompressed partial 8/8 → 6 (good) Decompressed partial 9/9 → 6 (good) Decompressed partial 10/10 → 6 (good) Decompressed partial 11/11 → 6 (good) Decompressed partial 12/12 → 6 (good) Decompressed partial 13/13 → 6 (good) Decompressed partial 14/14 → 6 (good) Decompressed partial 15/15 → 6 (good) Decompressed partial 16/16 → 6 (good) Decompressed partial 17/17 → 6 (good) Decompressed partial 18/18 → -16459 (bad) (lz4-libs-1.8.3-1.fc29.x86_64) Compressed 4194304 → 16464 Decompressed → 4194304 Decompressed partial 12/4194304 → 12 Decompressed partial 1/1 → 1 (good) Decompressed partial 2/2 → 2 (good) Decompressed partial 3/3 → 3 (good) Decompressed partial 4/4 → 4 (good) ... If we got such a short "successful" decompression in decompress_startswith() as implemented before this patch, we could be confused and return a false negative result. But it turns out that this only occurs with small output buffer sizes. We use greedy_realloc() to manager the buffer, so it is always at least 64 bytes. I couldn't hit a case where decompress_startswith() would actually return a bogus result. But since the lack of proof is not conclusive, the code for *older* lz4 is changed too, just to be safe. We cannot rule out that on a different architecture or with some unlucky compressed string we could hit this corner case. The fallback code is guarded by a version check. The check uses a function not the compile-time define, because there was no soversion bump in lz4 or new symbols, and we could be compiled against a newer lz4 and linked at runtime with an older one. (This happens routinely e.g. when somebody upgrades a subset of distro packages.) |
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