[ruby-cvs:78453] 4d7a6d04b2 (master): Avoid unnecessary tzset() call

KOSAKI Motohiro	2019-12-02 01:21:05 +0900 (Mon, 02 Dec 2019)

  New Revision: 4d7a6d04b2

  https://github.com/ruby/ruby/commit/4d7a6d04b2

  Log:
    Avoid unnecessary tzset() call
    
    Akatsuki reported ENV['TZ'] = 'UTC' improved 7x-8x faster on following code.
    t = Time.now; 100000.times { Time.new(2019) }; Time.now - t
    https://hackerslab.aktsk.jp/2019/12/01/141551
    
    commit 4bc1669127(reduce tzset) dramatically improved this situation. But still,
    TZ=UTC is faster than default.
    
    This patch removs unnecessary tzset() call completely.
    
    Performance check
      ----------------------
    test program: t = Time.now; 100000.times { Time.new(2019) }; Time.now - t
    before:         0.387sec
    before(w/ TZ):  0.197sec
    after:          0.162sec
    after(w/ TZ):   0.165sec
    
    OK. Now, Time creation 2x faster *and* TZ=UTC doesn't improve anything.
    We can forget this hack completely. :)
    
    Side note:
    This patch slightly changes Time.new(t) behavior implicitly. Before this patch, it might changes
    default timezone implicitly. But after this patch, it doesn't. You need to reset TZ
    (I mean ENV['TZ'] = nil) explicitly.
    But I don't think this is big impact. Don't try to change /etc/localtime on runtime.
    
    Side note2: following test might be useful for testing "ENV['TZ'] = nil".
      -----------------------------------------
    % cat <<'End' | sudo sh -s
    rm -f /etc/localtime-; cp -a /etc/localtime /etc/localtime-
    rm /etc/localtime; ln -s /usr/share/zoneinfo/Asia/Tokyo /etc/localtime
    ./ruby -e '
    p Time.new(2000).zone # JST
    File.unlink("/etc/localtime"); File.symlink("/usr/share/zoneinfo/America/Los_Angeles", "/etc/localtime")
    p Time.new(2000).zone # JST (ruby does not follow /etc/localtime modification automatically)
    ENV["TZ"] = nil
    p Time.new(2000).zone # PST (ruby detect /etc/localtime modification)
    '
    rm /etc/localtime; cp -a /etc/localtime- /etc/localtime; rm /etc/localtime-
    End

  Modified files:
    hash.c
    time.c