unix: Release GIL during all system calls.

Addition of GIL EXIT/ENTER pairs are:

- modos: release the GIL during system calls.  CPython does this as well.

- moduselect: release the GIL during the poll() syscall.  This call can be
  blocking, so it is important to allow other threads to run at this time.

- modusocket: release the GIL during system calls.  Many of these calls can
  be blocking, so it is important to allow other threads to run.

- unix_mphal: release the GIL during the read and write syscalls in
  mp_hal_stdin_rx_chr and mp_hal_stdout_tx_strn.  If we don't do this
  threads are blocked when the REPL or the builtin input function are used.

- file, main, mpconfigport.h: release GIL during syscalls in built-in
  functions that could block.

1 files changed, 3 insertions, 0 deletions

diff --git a/ports/unix/moduselect.c b/ports/unix/moduselect.c
index d9b02ddc6..4a095ec29 100644
--- a/ports/unix/moduselect.c
+++ b/ports/unix/moduselect.c
@@ -39,6 +39,7 @@
 #include "py/objlist.h"
 #include "py/objtuple.h"
 #include "py/mphal.h"
+#include "py/mpthread.h"
 #include "fdfile.h"
 
 #define DEBUG 0
@@ -188,7 +189,9 @@ STATIC int poll_poll_internal(size_t n_args, const mp_obj_t *args) {
 
     self->flags = flags;
 
+    MP_THREAD_GIL_EXIT();
     int n_ready = poll(self->entries, self->len, timeout);
+    MP_THREAD_GIL_ENTER();
     RAISE_ERRNO(n_ready, errno);
     return n_ready;
 }