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Buck doesn't fetch cxx_library from cache before linking go_binary

See original GitHub issue

When Buck’s cache is configured to a directory outside buck-out, buck clean will not clean the cache (assuming this is by design), but buck-out/gen and buck-out/bin are cleaned. So during next buck build, Buck will fetch artifacts from cache and populate buck-out/gen and buck-out/bin whenever possible. This works in most cases except when there is a cxx_library to be linked with a go_binrary. Steps to reproduce the problem

git clone git@github.com:linzhp/buck_go_examples.git
cd buck_go_examples
buck build sqliteTest:version
buck clean
echo " " >> sqliteTest/main.go
buck build sqliteTest:version

Then there will be an error from the linker:

/usr/local/Cellar/go/1.11.2/libexec/pkg/tool/darwin_amd64/link: running /usr/bin/clang++ failed: exit status 1
ld: file not found: buck-out/gen/vendor/github.com/mattn/go-sqlite3/cxx_lib#default,static-pic/libcxx_lib.a
clang: error: linker command failed with exit code 1 (use -v to see invocation)


Building: finished in 1.2 sec (100%) 7/7 jobs, 7 updated
  Total time: 1.8 sec
Command failed with exit code 2.
stderr: /usr/local/Cellar/go/1.11.2/libexec/pkg/tool/darwin_amd64/link: running /usr/bin/clang++ failed: exit status 1
ld: file not found: buck-out/gen/vendor/github.com/mattn/go-sqlite3/cxx_lib#default,static-pic/libcxx_lib.a
clang: error: linker command failed with exit code 1 (use -v to see invocation)


    When running <go link>.
    When building rule //sqliteTest:version.

The issue is, during the second build, Buck fetched all go_library dependencies from the cache during the compilation, but it didn’t fetch from the cache during the linking.

@mkaczanowski