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PiperOrigin-RevId: 520171344

This change implements support for Zstd-based compression in LevelDB. Building
up from the Snappy compression (which has been supported since inception), this
change adds Zstd as an alternate compression algorithm.

We are implementing this to provide alternative options for users who might
have different performance and efficiency requirements. For instance, the
Zstandard website (https://facebook.github.io/zstd/) claims that the Zstd
algorithm can achieve around 30% higher compression ratios than Snappy, with
relatively smaller (~10%) slowdowns in de/compression speeds.

Benchmarking results:

$ blaze-bin/third_party/leveldb/db_bench
LevelDB:    version 1.23
Date:       Thu Feb  2 18:50:06 2023
CPU:        56 * Intel(R) Xeon(R) CPU E5-2690 v4 @ 2.60GHz
CPUCache:   35840 KB
Keys:       16 bytes each
Values:     100 bytes each (50 bytes after compression)
Entries:    1000000
RawSize:    110.6 MB (estimated)
FileSize:   62.9 MB (estimated)
------------------------------------------------
fillseq      :       2.613 micros/op;   42.3 MB/s
fillsync     :    3924.432 micros/op;    0.0 MB/s (1000 ops)
fillrandom   :       3.609 micros/op;   30.7 MB/s
overwrite    :       4.508 micros/op;   24.5 MB/s
readrandom   :       6.136 micros/op; (864322 of 1000000 found)
readrandom   :       5.446 micros/op; (864083 of 1000000 found)
readseq      :       0.180 micros/op;  613.3 MB/s
readreverse  :       0.321 micros/op;  344.7 MB/s
compact      :  827043.000 micros/op;
readrandom   :       4.603 micros/op; (864105 of 1000000 found)
readseq      :       0.169 micros/op;  656.3 MB/s
readreverse  :       0.315 micros/op;  350.8 MB/s
fill100K     :     854.009 micros/op;  111.7 MB/s (1000 ops)
crc32c       :       1.227 micros/op; 3184.0 MB/s (4K per op)
snappycomp   :       3.610 micros/op; 1081.9 MB/s (output: 55.2%)
snappyuncomp :       0.691 micros/op; 5656.3 MB/s
zstdcomp     :      15.731 micros/op;  248.3 MB/s (output: 44.1%)
zstduncomp   :       4.218 micros/op;  926.2 MB/s
PiperOrigin-RevId: 509957778

As allowed by the documentation for Env::Schedule(), ChromiumEnv may
execute functions on multiple threads and guarantees no sequencing.
EnvTest.RunMany assumed that functions ran in order, is the case for the
stock PosixEnv and WindowsEnv implementations. This change updates the
test to not assume sequential execution.

Fixes an unused variable warning.

There are a couple differences between ChromiumEnv and
PosixEnv/WindowsEnv which cause test failures that are fixed (or at
least patched over) in this change:

* NewSequentialFile() and NewRandomAccessFile() return Status::IOError
  rather than Status::NotFound when a file is not found, due to
  https://crbug.com/760362. This means a few tests need to expect a
  different error result.
* GetChildren() never returns the '.' or '..' entries.
* As allowed by the documentation for Env::Schedule(), ChromiumEnv may
  execute functions on multiple threads and guarantees no sequencing.
  EnvTest.RunMany assumed that functions ran in order. The test has been
  updated.

See https://github.com/google/leveldb/issues/1081

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This replaces a usage of fprintf to stderr

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Structs with user-declared constructors are no longer considered
aggregates.  Just remove the constructor declaration where applicable.

Bug: chromium:1284275

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* Align CONTRIBUTING.md with the google/new-project template.
* Explain the support story for the CMake config.

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This gives some flexibility to embedders.

Currently, embedders have to build a binary for each test file.

After this CL, embedders can still choose to have a binary for each test
file, by linking each test file with a googletest target that includes
main() (usually "gtest_main"). Embedders can also choose to build a
single binary for almost all test files, and link with a googletest
target that includes main(). The latter is more convenient for projects
that have very few test binaries, like Chromium.

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The benchmark in db/db_test.cc is extracted to its own file,
benchmarks/db_bench_log.cc.

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