extmod/modussl_mbedtls: Support non-blocking handshake.

For this, add wrap_socket(do_handshake=False) param. CPython doesn't have
such a param at a module's global function, and at SSLContext.wrap_socket()
it has do_handshake_on_connect param, but that uselessly long.

Beyond that, make write() handle not just MBEDTLS_ERR_SSL_WANT_WRITE, but
also MBEDTLS_ERR_SSL_WANT_READ, as during handshake, write call may be
actually preempted by need to read next handshake message from peer.
Likewise, for read(). And even after the initial negotiation, situations
like that may happen e.g. with renegotiation. Both
MBEDTLS_ERR_SSL_WANT_READ and MBEDTLS_ERR_SSL_WANT_WRITE are however mapped
to the same None return code. The idea is that if the same read()/write()
method is called repeatedly, the progress will be made step by step anyway.
The caveat is if user wants to add the underlying socket to uselect.poll().
To be reliable, in this case, the socket should be polled for both POLL_IN
and POLL_OUT, as we don't know the actual expected direction. But that's
actually problematic. Consider for example that write() ends with
MBEDTLS_ERR_SSL_WANT_READ, but gets converted to None. We put the
underlying socket on pull using POLL_IN|POLL_OUT but that probably returns
immediately with POLL_OUT, as underlyings socket is writable. We call the
same ussl write() again, which again results in MBEDTLS_ERR_SSL_WANT_READ,
etc. We thus go into busy-loop.

So, the handling in this patch is temporary and needs fixing. But exact way
to fix it is not clear. One way is to provide explicit function for
handshake (CPython has do_handshake()), and let *that* return distinct
codes like WANT_READ/WANT_WRITE. But as mentioned above, past the initial
handshake, such situation may happen again with at least renegotiation. So
apparently, the only robust solution is to return "out of bound" special
sentinels like WANT_READ/WANT_WRITE from read()/write() directly. CPython
throws exceptions for these, but those are expensive to adopt that way for
efficiency-conscious implementation like MicroPython.

extmod/modussl_mbedtls.c

@@ -4,6 +4,7 @@
  * The MIT License (MIT)
  *
  * Copyright (c) 2016 Linaro Ltd.
+ * Copyright (c) 2019 Paul Sokolovsky
  *
  * Permission is hereby granted, free of charge, to any person obtaining a copy
  * of this software and associated documentation files (the "Software"), to deal
@@ -60,6 +61,7 @@ struct ssl_args {
     mp_arg_val_t cert;
     mp_arg_val_t server_side;
     mp_arg_val_t server_hostname;
+    mp_arg_val_t do_handshake;
 };
 
 STATIC const mp_obj_type_t ussl_socket_type;
@@ -184,10 +186,12 @@ STATIC mp_obj_ssl_socket_t *socket_new(mp_obj_t sock, struct ssl_args *args) {
         assert(ret == 0);
     }
 
-    while ((ret = mbedtls_ssl_handshake(&o->ssl)) != 0) {
-        if (ret != MBEDTLS_ERR_SSL_WANT_READ && ret != MBEDTLS_ERR_SSL_WANT_WRITE) {
-            printf("mbedtls_ssl_handshake error: -%x\n", -ret);
-            goto cleanup;
+    if (args->do_handshake.u_bool) {
+        while ((ret = mbedtls_ssl_handshake(&o->ssl)) != 0) {
+            if (ret != MBEDTLS_ERR_SSL_WANT_READ && ret != MBEDTLS_ERR_SSL_WANT_WRITE) {
+                printf("mbedtls_ssl_handshake error: -%x\n", -ret);
+                goto cleanup;
+            }
         }
     }
 
@@ -238,6 +242,11 @@ STATIC mp_uint_t socket_read(mp_obj_t o_in, void *buf, mp_uint_t size, int *errc
     }
     if (ret == MBEDTLS_ERR_SSL_WANT_READ) {
         ret = MP_EWOULDBLOCK;
+    } else if (ret == MBEDTLS_ERR_SSL_WANT_WRITE) {
+        // If handshake is not finished, read attempt may end up in protocol
+        // wanting to write next handshake message. The same may happen with
+        // renegotation.
+        ret = MP_EWOULDBLOCK;
     }
     *errcode = ret;
     return MP_STREAM_ERROR;
@@ -252,6 +261,11 @@ STATIC mp_uint_t socket_write(mp_obj_t o_in, const void *buf, mp_uint_t size, in
     }
     if (ret == MBEDTLS_ERR_SSL_WANT_WRITE) {
         ret = MP_EWOULDBLOCK;
+    } else if (ret == MBEDTLS_ERR_SSL_WANT_READ) {
+        // If handshake is not finished, write attempt may end up in protocol
+        // wanting to read next handshake message. The same may happen with
+        // renegotation.
+        ret = MP_EWOULDBLOCK;
     }
     *errcode = ret;
     return MP_STREAM_ERROR;
@@ -321,6 +335,7 @@ STATIC mp_obj_t mod_ssl_wrap_socket(size_t n_args, const mp_obj_t *pos_args, mp_
         { MP_QSTR_cert, MP_ARG_KW_ONLY | MP_ARG_OBJ, {.u_obj = MP_OBJ_NULL} },
         { MP_QSTR_server_side, MP_ARG_KW_ONLY | MP_ARG_BOOL, {.u_bool = false} },
         { MP_QSTR_server_hostname, MP_ARG_KW_ONLY | MP_ARG_OBJ, {.u_obj = mp_const_none} },
+        { MP_QSTR_do_handshake, MP_ARG_KW_ONLY | MP_ARG_BOOL, {.u_bool = true} },
     };
 
     // TODO: Check that sock implements stream protocol