socket.raw: don't use select()

The current implementation of select() in ljsyscall is highly
inefficient.  This commit leverages the return values of attempted
read() and write() calls to determine the status of the socket
instead.

src/apps/socket/raw.lua

@@ -52,26 +52,26 @@ function RawSocket:pull ()
    local l = self.output.tx
    if l == nil then return end
    local limit = engine.pull_npackets
-   while limit > 0 and self:can_receive() do
+   while limit > 0 and self:try_read() do
       limit = limit - 1
       link.transmit(l, self:receive())
    end
 end
 
-function RawSocket:can_receive ()
-   local t, err = S.select({readfds = {self.sock}}, 0)
-   while not t and (err.AGAIN or err.INTR) do
-      t, err = S.select({readfds = {self.sock}}, 0)
+function RawSocket:try_read ()
+   local rxp = self.rx_p
+   local bytes = S.read(self.sock, rxp.data, packet.max_payload)
+   if bytes then
+      rxp.length = bytes
+      return true
+   else
+      return false
    end
-   assert(t, err)
-   return t.count == 1
 end
 
 function RawSocket:receive ()
    local p = self.rx_p
-   local sz = assert(S.read(self.sock, p.data, packet.max_payload))
-   p.length = sz
-   counter.add(self.shm.rxbytes, sz)
+   counter.add(self.shm.rxbytes, p.length)
    counter.add(self.shm.rxpackets)
    if ethernet:is_mcast(p.data) then
       counter.add(self.shm.rxmcast)
@@ -85,34 +85,33 @@ end
 function RawSocket:push ()
    local l = self.input.rx
    if l == nil then return end
-   while not link.empty(l) and self:can_transmit() do
-      local p = link.receive(l)
-      self:transmit(p)
-      counter.add(self.shm.txbytes, p.length)
-      counter.add(self.shm.txpackets)
-      if ethernet:is_mcast(p.data) then
-         counter.add(self.shm.txmcast)
+   while not link.empty(l) do
+      local p = link.front(l)
+      if self:try_transmit(p) then
+         link.receive(l)
+         counter.add(self.shm.txbytes, p.length)
+         counter.add(self.shm.txpackets)
+         if ethernet:is_mcast(p.data) then
+            counter.add(self.shm.txmcast)
+         end
+         if ethernet:is_bcast(p.data) then
+            counter.add(self.shm.txbcast)
+         end
+         packet.free(p)
+      else
+         break
       end
-      if ethernet:is_bcast(p.data) then
-         counter.add(self.shm.txbcast)
-      end
-      packet.free(p)
    end
 end
 
-function RawSocket:can_transmit ()
-   local t, err = S.select({writefds = {self.sock}}, 0)
-   while not t and (err.AGAIN or err.INTR) do
-      t, err = S.select({writefds = {self.sock}}, 0)
-   end
-   assert(t, err)
-   return t.count == 1
-end
-
-function RawSocket:transmit (p)
+function RawSocket:try_transmit (p)
    local sz, err = S.write(self.sock, p.data, p.length)
+   if (not sz and err.AGAIN) then
+      return false
+   end
    assert(sz, err)
    assert(sz == p.length)
+   return true
 end
 
 function RawSocket:stop()