[PATCH 1/1] hurd: Avoid extra ctty RPCs in init_dtable ()

[Sergey Bugaev]

It is common to have (some of) stdin, stdout and stderr point to the
very same port. We were making the ctty RPCs that _hurd_port2fd () does
for each one of them separately:

1. term_getctty ()
2. mach_port_deallocate ()
3. term_open_ctty ()

Instead, let's detect this case and duplicate the ctty port we already
have. This means we do 1 RPC instead of 3 (and create a single protid
on the server side) if the file is our ctty, and no RPCs instead of 1
if it's not. A clear win!

Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>
---
 hurd/dtable.c | 46 +++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 39 insertions(+), 7 deletions(-)

diff --git a/hurd/dtable.c b/hurd/dtable.c
index 5dd04131..41f4d7af 100644
--- a/hurd/dtable.c
+++ b/hurd/dtable.c
@@ -60,18 +60,50 @@ init_dtable (void)
        _hurd_dtable[i] = NULL;
       else
        {
+         int copy;
          /* Allocate a new file descriptor structure.  */
          struct hurd_fd *new = malloc (sizeof (struct hurd_fd));
          if (new == NULL)
            __libc_fatal ("hurd: Can't allocate initial file descriptors\n");
 
-         /* Initialize the port cells.  */
-         _hurd_port_init (&new->port, MACH_PORT_NULL);
-         _hurd_port_init (&new->ctty, MACH_PORT_NULL);
-
-         /* Install the port in the descriptor.
-            This sets up all the ctty magic.  */
-         _hurd_port2fd (new, _hurd_init_dtable[i], 0);
+         /* See if this file descriptor is the same as a previous one we have
+            already installed.  In this case, we can just copy over the same
+            ctty port without making any more RPCs.  We only check the the
+            immediately preceding fd and fd 0 -- this should be enough to
+            handle the common cases while not requiring quadratic
+            complexity.  */
+         if (i > 0 && _hurd_init_dtable[i] == _hurd_init_dtable[i - 1])
+           copy = i - 1;
+         else if (i > 0 && _hurd_init_dtable[i] == _hurd_init_dtable[0])
+           copy = 0;
+         else
+           copy = -1;
+
+         if (copy < 0)
+           {
+             /* Initialize the port cells.  */
+             _hurd_port_init (&new->port, MACH_PORT_NULL);
+             _hurd_port_init (&new->ctty, MACH_PORT_NULL);
+
+             /* Install the port in the descriptor.
+                This sets up all the ctty magic.  */
+             _hurd_port2fd (new, _hurd_init_dtable[i], 0);
+           }
+         else
+           {
+             /* Copy over ctty from the already set up file descriptor that
+                contains the same port.  We can access the contents of the
+                cell without any locking since no one could have seen it
+                yet.  */
+             mach_port_t ctty = _hurd_dtable[copy]->ctty.port;
+
+             if (MACH_PORT_VALID (ctty))
+               __mach_port_mod_refs (__mach_task_self (), ctty,
+                                     MACH_PORT_RIGHT_SEND, +1);
+
+             _hurd_port_init (&new->port, _hurd_init_dtable[i]);
+             _hurd_port_init (&new->ctty, ctty);
+           }
 
          _hurd_dtable[i] = new;
        }
-- 
2.39.2