From 238e12123690debaf2cf8488fb2a4d4003a06506 Mon Sep 17 00:00:00 2001
From: Jeff Epler <jepler@gmail.com>
Date: Tue, 3 Dec 2019 14:50:37 -0600
Subject: protocols: Allow them to be (optionally) type-safe

Protocols are nice, but there is no way for C code to verify whether
a type's "protocol" structure actually implements some particular
protocol.  As a result, you can pass an object that implements the
"vfs" protocol to one that expects the "stream" protocol, and the
opposite of awesomeness ensues.

This patch adds an OPTIONAL (but enabled by default) protocol identifier
as the first member of any protocol structure.  This identifier is
simply a unique QSTR chosen by the protocol designer and used by each
protocol implementer.  When checking for protocol support, instead of
just checking whether the object's type has a non-NULL protocol field,
use `mp_proto_get` which implements the protocol check when possible.

The existing protocols are now named:
    protocol_framebuf
    protocol_i2c
    protocol_pin
    protocol_stream
    protocol_spi
    protocol_vfs
(most of these are unused in CP and are just inherited from MP; vfs and
stream are definitely used though)

I did not find any crashing examples, but here's one to give a flavor of what
is improved, using `micropython_coverage`.  Before the change,
the vfs "ioctl" protocol is invoked, and the result is not intelligible
as json (but it could have resulted in a hard fault, potentially):

    >>> import uos, ujson
    >>> u = uos.VfsPosix('/tmp')
    >>> ujson.load(u)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    ValueError: syntax error in JSON

After the change, the vfs object is correctly detected as not supporting
the stream protocol:
    >>> ujson.load(p)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    OSError: stream operation not supported
---
 extmod/machine_spi.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

(limited to 'extmod/machine_spi.c')

diff --git a/extmod/machine_spi.c b/extmod/machine_spi.c
index c5707a3d0..4ead321d9 100644
--- a/extmod/machine_spi.c
+++ b/extmod/machine_spi.c
@@ -67,7 +67,7 @@ mp_obj_t mp_machine_spi_make_new(const mp_obj_type_t *type, size_t n_args, const
 
 STATIC mp_obj_t machine_spi_init(size_t n_args, const mp_obj_t *args, mp_map_t *kw_args) {
     mp_obj_base_t *s = (mp_obj_base_t*)MP_OBJ_TO_PTR(args[0]);
-    mp_machine_spi_p_t *spi_p = (mp_machine_spi_p_t*)s->type->protocol;
+    mp_machine_spi_p_t *spi_p = (mp_machine_spi_p_t*)mp_proto_get(QSTR_protocol_spi, s);
     spi_p->init(s, n_args - 1, args + 1, kw_args);
     return mp_const_none;
 }
@@ -75,7 +75,7 @@ STATIC MP_DEFINE_CONST_FUN_OBJ_KW(machine_spi_init_obj, 1, machine_spi_init);
 
 STATIC mp_obj_t machine_spi_deinit(mp_obj_t self) {
     mp_obj_base_t *s = (mp_obj_base_t*)MP_OBJ_TO_PTR(self);
-    mp_machine_spi_p_t *spi_p = (mp_machine_spi_p_t*)s->type->protocol;
+    mp_machine_spi_p_t *spi_p = (mp_machine_spi_p_t*)mp_proto_get(QSTR_protocol_spi, s);
     if (spi_p->deinit != NULL) {
         spi_p->deinit(s);
     }
@@ -85,7 +85,7 @@ STATIC MP_DEFINE_CONST_FUN_OBJ_1(machine_spi_deinit_obj, machine_spi_deinit);
 
 STATIC void mp_machine_spi_transfer(mp_obj_t self, size_t len, const void *src, void *dest) {
     mp_obj_base_t *s = (mp_obj_base_t*)MP_OBJ_TO_PTR(self);
-    mp_machine_spi_p_t *spi_p = (mp_machine_spi_p_t*)s->type->protocol;
+    mp_machine_spi_p_t *spi_p = (mp_machine_spi_p_t*)mp_proto_get(QSTR_protocol_spi, s);
     spi_p->transfer(s, len, src, dest);
 }
 
@@ -268,6 +268,7 @@ STATIC void mp_machine_soft_spi_transfer(mp_obj_base_t *self_in, size_t len, con
 }
 
 const mp_machine_spi_p_t mp_machine_soft_spi_p = {
+    MP_PROTO_IMPLEMENT(MP_QSTR_protocol_spi)
     .init = mp_machine_soft_spi_init,
     .deinit = NULL,
     .transfer = mp_machine_soft_spi_transfer,
-- 
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