gdbstub: avoid busy loop while waiting for gdb

While waiting for a gdb response, or while sending an acknowledgement
there is not much to do, so do not mark the socket as non-blocking to
avoid a busy loop while paused at gdb. This only affects the user-mode
emulation (qemu-arm -g 1234 ./a.out).

Note that this issue was reported before at
https://lists.nongnu.org/archive/html/qemu-devel/2013-02/msg02277.html.

While at it, close the gdb client fd on EOF or error while reading.
Signed-off-by: NPeter Wu <peter@lekensteyn.nl>
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

gdbstub.c

@@ -333,7 +333,7 @@ static int get_char(GDBState *s)
         if (ret < 0) {
             if (errno == ECONNRESET)
                 s->fd = -1;
-            if (errno != EINTR && errno != EAGAIN)
+            if (errno != EINTR)
                 return -1;
         } else if (ret == 0) {
             close(s->fd);
@@ -394,7 +394,7 @@ static void put_buffer(GDBState *s, const uint8_t *buf, int len)
     while (len > 0) {
         ret = send(s->fd, buf, len, 0);
         if (ret < 0) {
-            if (errno != EINTR && errno != EAGAIN)
+            if (errno != EINTR)
                 return;
         } else {
             buf += ret;
@@ -1543,9 +1543,13 @@ gdb_handlesig(CPUState *cpu, int sig)
             for (i = 0; i < n; i++) {
                 gdb_read_byte(s, buf[i]);
             }
-        } else if (n == 0 || errno != EAGAIN) {
+        } else {
             /* XXX: Connection closed.  Should probably wait for another
                connection before continuing.  */
+            if (n == 0) {
+                close(s->fd);
+            }
+            s->fd = -1;
             return sig;
         }
     }
@@ -1600,8 +1604,6 @@ static void gdb_accept(void)
     gdb_has_xml = false;
 
     gdbserver_state = s;
-
-    fcntl(fd, F_SETFL, O_NONBLOCK);
 }
 
 static int gdbserver_open(int port)