bpo-38061: subprocess uses closefrom() on FreeBSD

Doc/whatsnew/3.9.rst

@@ -471,6 +471,10 @@ Optimizations
   until the main thread handles signals.
   (Contributed by Victor Stinner in :issue:`40010`.)
 
+* Optimize the :mod:`subprocess` module on FreeBSD using ``closefrom()``.
+  (Contributed by Ed Maste, Conrad Meyer, Kyle Evans, Kubilay Kocak and Victor
+  Stinner in :issue:`38061`.)
+
 
 Build and C API Changes
 =======================

Misc/NEWS.d/next/Library/2020-04-24-01-55-00.bpo-38061.XmULB3.rst

@@ -0,0 +1,11 @@
+Optimize the :mod:`subprocess` module on FreeBSD using ``closefrom()``.
+A single ``close(fd)`` syscall is cheap, but when ``sysconf(_SC_OPEN_MAX)`` is
+high, the loop calling ``close(fd)`` on each file descriptor can take several
+milliseconds.
+
+The workaround on FreeBSD to improve performance was to load and mount the
+fdescfs kernel module, but this is not enabled by default.
+
+Initial patch by Ed Maste (emaste), Conrad Meyer (cem), Kyle Evans (kevans) and
+Kubilay Kocak (koobs):
+https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=242274

Modules/_posixsubprocess.c

@@ -264,9 +264,15 @@ _close_fds_by_brute_force(long start_fd, PyObject *py_fds_to_keep)
         start_fd = keep_fd + 1;
     }
     if (start_fd <= end_fd) {
+#if defined(__FreeBSD__)
+        /* Any errors encountered while closing file descriptors are ignored */
+        closefrom(start_fd);
+#else
         for (fd_num = start_fd; fd_num < end_fd; ++fd_num) {
-            close(fd_num);
+            /* Ignore errors */
+            (void)close(fd_num);
         }
+#endif
     }
 }