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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I recently encountered a problem with this because Windows doesn't allow
files to be deleted when there's open handles to them.

Other files opened by leveldb are not affected because by and large they
are using CreateFileA, which does not allow inheritance when
lpSecurityAttributes is null (ref:
https://docs.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-createfilea)

However, fopen() _does_ allow inheritance, and it needs to be expressly
disabled.
https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/fopen-wfopen?view=msvc-160

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Xcode (drives macOS image) : 12.2 => 12.5
Clang                      : 10 => 12
GCC                        : 10 => 11
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Roll third-party dependencies.

key across multiple files.

As reported in Github issue #339, it is incorrect to split the
same user key across multiple compacted files since it causes
tombstones/newer-versions to be dropped, thereby exposing obsolete
data. There was a fix for #339, but it ended up not fully fixing
the problem. (It checked for boundary problems in the first level
being compacted, but not the second). This problem was revealed
by Github issue 887.

We now adjust boundaries to avoid splitting user keys in both the
first level and the second level.

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Using %zu for size_t instead of %ld.

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Using the partial path offers more flexibility to projects which
may checkout google/benchmark to a different location.

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Fixes https://github.com/google/leveldb/issues/872

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Add logging with debugging information when failing to load a version set.

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This is beneficial when iterators are reused and seeks are not random
but increasing. It is additionally beneficial with larger block sizes and keys with common prefixes.

Add a benchmark "seekordered" to db_bench that reuses iterators across
increasing seeks.  Add support to the benchmark to count comparisons made and to support common key prefix length. Change benchmark random seeds to be reproducible for entire benchmark suite executions but unique for threads in different benchmarks runs.  This changes a benchmark suite of readrandom,seekrandom from having a 100% found ratio as previously it had the same seed used for fillrandom.

./db_bench --benchmarks=fillrandom,compact,seekordered --block_size=262144 --comparisons=1 --key_prefix=100

without this change (though with benchmark changes):
seekrandom   :      55.309 micros/op; (631820 of 1000000 found)
Comparisons: 27001049
seekordered  :       1.732 micros/op; (631882 of 1000000 found)
Comparisons: 26998402

with this change:
seekrandom   :      55.866 micros/op; (631820 of 1000000 found)
Comparisons: 26952143
seekordered  :       1.686 micros/op; (631882 of 1000000 found)
Comparisons: 25549369

For ordered seeking, this is a reduction of 5% comparisons and a 3% speedup. For random seeking (with single use iterators) the comparisons and speed are less than 1% and likely noise.

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Fixup for adding the third_party/benchmark submodule.

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remove return value of GetApproximateSizes in index.md

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This CL makes the following substitutions.

* assert.h -> cassert
* math.h -> cmath
* stdarg.h -> cstdarg
* stddef.h -> cstddef
* stdint.h -> cstdint
* stdio.h -> cstdio
* stdlib.h -> cstdlib
* string.h -> cstring

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Clang 10 includes the optimizations described in
https://bugs.llvm.org/show_bug.cgi?id=41761. This means that the
platform-independent implementations of {Decode,Encode}Fixed{32,64}()
compile to one instruction on the most recent Clang and GCC.

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