Fix value reported by get_maxrss on macos (#344)

* Fix value reported by get_maxrss on macos

The value of ru_maxrss is in bytes on macos, which contradicts
what the manpage says at:
https://developer.apple.com/library/archive/documentation/System/Conceptual/ManPages_iPhoneOS/man2/getrusage.2.html

Apparently, the manppage is wrong, but this information has never
been corrected. I can't seem to get the Apple bug reporting
software to work and so am unable to report this issue to Apple.

* Update harness/harness-common.rb

Co-authored-by: Takashi Kokubun <[email protected]>

---------

Co-authored-by: Takashi Kokubun <[email protected]>

Author: maximecb

harness/harness-common.rb

@@ -51,6 +51,10 @@ def get_rss
   end
 end
 
+def is_macos
+  RUBY_PLATFORM.match?(/darwin/)
+end
+
 def get_maxrss
   require 'fiddle'
   require 'rbconfig/sizeof'
@@ -70,7 +74,14 @@ def get_maxrss
   result = getrusage.call(rusage_self, buffer)
   raise unless result.zero?
   maxrss_kb = buffer[offset, Fiddle::SIZEOF_LONG].unpack1('q')
-  1024 * maxrss_kb
+
+  # On macos, this value is already in bytes
+  # (and the manpage is wrong)
+  if is_macos
+    maxrss_kb
+  else
+    1024 * maxrss_kb
+  end
 rescue LoadError
   warn "Failed to get max RSS: #{$!.message}"
   nil