Significantly speeds up call-process, making magit much faster. May not be safe. See http://lists.gnu.org/archive/html/bug-gnu-emacs/2017-04/msg00201.html for more info

GNU-Emacs-25.1-OS-X-enable-vfork.patch

Use vfork if possible on Darwin

src/conf_post.h: Remove defines forcing use of fork.
src/process.c (create_process): Use fork if pty_flag is set, otherwise
vfork.
---
 src/conf_post.h | 6 ------
 src/process.c   | 9 +++++++++
 2 files changed, 9 insertions(+), 6 deletions(-)

diff --git a/src/conf_post.h b/src/conf_post.h
index 30c948e39a..95ebd5511c 100644
--- a/src/conf_post.h
+++ b/src/conf_post.h
@@ -99,12 +99,6 @@ typedef bool bool_bf;
 #define realloc unexec_realloc
 #define free unexec_free
 #endif
-/* The following solves the problem that Emacs hangs when evaluating
-   (make-comint "test0" "/nodir/nofile" nil "") when /nodir/nofile
-   does not exist.  Also, setsid is not allowed in the vfork child's
-   context as of Darwin 9/Mac OS X 10.5.  */
-#undef HAVE_WORKING_VFORK
-#define vfork fork
 #endif  /* DARWIN_OS */
 
 /* If HYBRID_MALLOC is defined (e.g., on Cygwin), emacs will use
diff --git a/src/process.c b/src/process.c
index 2f2e5c1b25..47a85f1c95 100644
--- a/src/process.c
+++ b/src/process.c
@@ -2049,7 +2049,16 @@ create_process (Lisp_Object process, char **new_argv, Lisp_Object current_dir)
   int volatile forkerr_volatile = forkerr;
   struct Lisp_Process *p_volatile = p;
 
+#ifdef DARWIN_OS
+  /* Darwin doesn't let us run setsid after a vfork, so use fork when
+     necessary. */
+  if (pty_flag)
+    pid = fork ();
+  else
+    pid = vfork ();
+#else
   pid = vfork ();
+#endif
 
   current_dir = current_dir_volatile;
   lisp_pty_name = lisp_pty_name_volatile;
-- 
2.12.2


Use TIOCNOTTY instead of setsid on darwin

---
 src/callproc.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/src/callproc.c b/src/callproc.c
index 05048576ce..3f6df422f3 100644
--- a/src/callproc.c
+++ b/src/callproc.c
@@ -52,6 +52,8 @@ along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.  */
 #include "syswait.h"
 #include "blockinput.h"
 #include "frame.h"
+#include "systty.h"
+#include "keyboard.h"
 
 #ifdef MSDOS
 #include "msdos.h"
@@ -626,7 +628,18 @@ call_process (ptrdiff_t nargs, Lisp_Object *args, int filefd,
     {
       unblock_child_signal (&oldset);
 
+#ifdef DARWIN_OS
+      /* Darwin doesn't let us run setsid after a vfork, so use
+	 TIOCNOTTY when necessary. */
+      int j = emacs_open ('/dev/tty', O_RDWR, 0);
+      if (j >= 0)
+	{
+	  ioctl (j, TIOCNOTTY, 0);
+	  emacs_close (j);
+	}
+#else
       setsid ();
+#endif
 
       /* Emacs ignores SIGPIPE, but the child should not.  */
       signal (SIGPIPE, SIG_DFL);
-- 
2.12.2