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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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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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Fix typo of comment of FindLargestKey function

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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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Use function instead of original expression.