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.

process.c

@@ -359,6 +359,8 @@ static ID id_MACH_ABSOLUTE_TIME_BASED_CLOCK_MONOTONIC;
 #endif
 static ID id_hertz;
 
+static VALUE cached_pid = Qnil;
+
 /* execv and execl are async-signal-safe since SUSv4 (POSIX.1-2008, XPG7) */
 #if defined(__sun) && !defined(_XPG7) /* Solaris 10, 9, ... */
 #define execv(path, argv) (rb_async_bug_errno("unreachable: async-signal-unsafe execv() is called", 0))
@@ -497,7 +499,23 @@ parent_redirect_close(int fd)
 static VALUE
 get_pid(void)
 {
-    return PIDT2NUM(getpid());
+    if (UNLIKELY(NIL_P(cached_pid))) {
+        cached_pid = PIDT2NUM(getpid());
+    }
+    return cached_pid;
+}
+
+static void
+clear_pid_cache(void)
+{
+    cached_pid = Qnil;
+}
+
+static inline void
+rb_process_atfork(void)
+{
+  clear_pid_cache();
+  rb_thread_atfork(); /* calls mjit_resume() */
 }
 
 /*
@@ -4059,7 +4077,7 @@ rb_fork_ruby2(struct rb_process_status *status)
         disable_child_handler_fork_parent(&old); /* yes, bad name */
 
         if (pid >= 0) { /* fork succeed */
-            if (pid == 0) rb_thread_atfork();
+            if (pid == 0) rb_process_atfork();
             return pid;
         }
 
@@ -6832,7 +6850,7 @@ rb_daemon(int nochdir, int noclose)
     before_fork_ruby();
     err = daemon(nochdir, noclose);
     after_fork_ruby();
-    rb_thread_atfork();
+    rb_process_atfork();
 #else
     int n;
 

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]'