This issue was solved with changeset r42492.
Usaku, thank you for reporting this issue.
Your contribution to Ruby is greatly appreciated.
May Ruby be with you.
merge revision(s) 42439,42453: [Backport #8752]
* test/coverage/test_coverage.rb (TestCoverage#test_big_code): use `1'
instead of `p' to get rid of a side effect.
Kernel#p without any argument seems to do nothing, but flushes stdout.
and, if stdout is redirected to file, fsync() will be called on
Windows. so, when running test-all on Windows with redirection, such
as CI environment, this test took a lot of time.
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