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, 2 insertions, 0 deletions

diff --git a/ports/unix/main.c b/ports/unix/main.c
index 9147feb32..fcf8b1558 100644
--- a/ports/unix/main.c
+++ b/ports/unix/main.c
@@ -64,7 +64,9 @@ long heap_size = 1024*1024 * (sizeof(mp_uint_t) / 4);
 
 STATIC void stderr_print_strn(void *env, const char *str, size_t len) {
     (void)env;
+    MP_THREAD_GIL_EXIT();
     ssize_t dummy = write(STDERR_FILENO, str, len);
+    MP_THREAD_GIL_ENTER();
     mp_uos_dupterm_tx_strn(str, len);
     (void)dummy;
 }