Cache `Process.pid`

[Feature #19443]

It's not uncommon for database client and similar network libraries
to protect themselves from Process.fork by regularly checking Process.pid

Until recently most libc would cache `getpid()` so this was a cheap
check to make.

However as of glibc version 2.25 the PID cache is removed and calls to
`getpid()` always invoke the actual system call which significantly degrades
the performance of existing applications.

The reason glibc removed the cache is that some libraries were bypassing
`fork(2)` by issuing system calls themselves, causing stale cache issues.

That isn't a concern for Ruby as bypassing MRI's primitive for forking
would render the VM unusable, so we can safely cache the PID.

1 files changed, 20 insertions, 0 deletions

diff --git a/test/ruby/test_process.rb b/test/ruby/test_process.rb
index beb181ccd4..3d0b11547e 100644
--- a/test/ruby/test_process.rb
+++ b/test/ruby/test_process.rb
@@ -2586,6 +2586,26 @@ EOS
     end
   end if Process.respond_to?(:_fork)
 
+  def test__fork_pid_cache
+    parent_pid = Process.pid
+    r, w = IO.pipe
+    pid = Process._fork
+    if pid == 0
+      begin
+        r.close
+        w << "ok: #{Process.pid}"
+        w.close
+      ensure
+        exit!
+      end
+    else
+      w.close
+      assert_equal("ok: #{pid}", r.read)
+      r.close
+      Process.waitpid(pid)
+    end
+  end if Process.respond_to?(:_fork)
+
   def test__fork_hook
     %w(fork Process.fork).each do |method|
       feature17795 = '[ruby-core:103400] [Feature #17795]'