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, 6 insertions, 1 deletions

diff --git a/ports/unix/mpconfigport.h b/ports/unix/mpconfigport.h
index f435c3ff3..2f01aff0f 100644
--- a/ports/unix/mpconfigport.h
+++ b/ports/unix/mpconfigport.h
@@ -285,7 +285,12 @@ void mp_unix_mark_exec(void);
 #if MICROPY_PY_OS_DUPTERM
 #define MP_PLAT_PRINT_STRN(str, len) mp_hal_stdout_tx_strn_cooked(str, len)
 #else
-#define MP_PLAT_PRINT_STRN(str, len) do { ssize_t ret = write(1, str, len); (void)ret; } while (0)
+#define MP_PLAT_PRINT_STRN(str, len) do { \
+    MP_THREAD_GIL_EXIT(); \
+    ssize_t ret = write(1, str, len); \
+    MP_THREAD_GIL_ENTER(); \
+    (void)ret; \
+} while (0)
 #endif
 
 #ifdef __linux__