drivers/can: correctly unblock only one read at the time

The previous version of the code was posting the semaphore every time
the value was less or equal to the zero. In case there is read already
waiting on the semaphore the value will be -1. The first frame will post
the value to 0 and thus unblocking the waiting read. The read will take
frames out of FIFO. During that time if next frame is received it will
be placed to the FIFO but because rx_sem has value 0 at that time it
will be posted to value 1. Once read finishes the execution it will
check if there is something still in the FIFO and posts semaphore back
(expecting that the value is 0) thus increasing value to 2. Thus we end
up with read being unblocked once more triggering 'RX FIFO sem posted
but FIFO is empty.' error branch and returning zero (signaling EOF?).

The intuitive fix is to check the value of the semaphore before posting
it in the read's tail. This could have race condition case when
interrupt is delivered between nxsem_get_value and nxsem_post and
can_receive is called.

This fix instead uses the same condition to detect if semaphore must be
posted as read does. Thus we check if previously the FIFO was empty. We
post the semaphore on in case FIFO was initially empty, otherwise we
expect semaphore to be fully managed by can_read.

Signed-off-by: Karel Kočí <cynerd@email.cz>

drivers/can/can.c

@@ -1170,6 +1170,7 @@ int can_receive(FAR struct can_dev_s *dev, FAR struct can_hdr_s *hdr,
   int                      ret = -ENOMEM;
   int                      i;
   int                      sval;
+  bool                     was_empty;
 
   caninfo("ID: %" PRId32 " DLC: %d\n", (uint32_t)hdr->ch_id, hdr->ch_dlc);
 
@@ -1234,6 +1235,7 @@ int can_receive(FAR struct can_dev_s *dev, FAR struct can_hdr_s *hdr,
     {
       FAR struct can_reader_s *reader = (FAR struct can_reader_s *)node;
       fifo = &reader->fifo;
+      was_empty = fifo->rx_head == fifo->rx_tail;
 
       nexttail = fifo->rx_tail + 1;
       if (nexttail >= CONFIG_CAN_RXFIFOSIZE)
@@ -1270,22 +1272,12 @@ int can_receive(FAR struct can_dev_s *dev, FAR struct can_hdr_s *hdr,
 
           fifo->rx_tail = nexttail;
 
-          if (nxsem_get_value(&fifo->rx_sem, &sval) < 0)
-            {
-#ifdef CONFIG_CAN_ERRORS
-              /* Report unspecified error */
-
-              fifo->rx_error |= CAN_ERROR5_UNSPEC;
-#endif
-              continue;
-            }
-
           /* Unlock the binary semaphore, waking up can_read if it is
            * blocked. If can_read were not blocked, we would not be
            * executing this because interrupts would be disabled.
            */
 
-          if (sval <= 0)
+          if (was_empty)
             {
               nxsem_post(&fifo->rx_sem);
             }