py/modio: Add uio.IOBase class to allow to define user streams.

A user class derived from IOBase and implementing readinto/write/ioctl can
now be used anywhere a native stream object is accepted.

The mapping from C to Python is:

    stream_p->read  --> readinto(buf)
    stream_p->write --> write(buf)
    stream_p->ioctl --> ioctl(request, arg)

Among other things it allows the user to:

- create an object which can be passed as the file argument to print:
  print(..., file=myobj), and then print will pass all the data to the
  object via the objects write method (same as CPython)
- pass a user object to uio.BufferedWriter to buffer the writes (same as
  CPython)
- use select.select on a user object
- register user objects with select.poll, in particular so user objects can
  be used with uasyncio
- create user files that can be returned from user filesystems, and import
  can import scripts from these user files

For example:

    class MyOut(io.IOBase):
        def write(self, buf):
            print('write', repr(buf))
            return len(buf)

    print('hello', file=MyOut())

The feature is enabled via MICROPY_PY_IO_IOBASE which is disabled by
default.

py/modio.c

@@ -30,6 +30,8 @@
 #include "py/runtime.h"
 #include "py/builtin.h"
 #include "py/stream.h"
+#include "py/binary.h"
+#include "py/objarray.h"
 #include "py/objstringio.h"
 #include "py/frozenmod.h"
 
@@ -38,6 +40,70 @@
 extern const mp_obj_type_t mp_type_fileio;
 extern const mp_obj_type_t mp_type_textio;
 
+#if MICROPY_PY_IO_IOBASE
+
+STATIC const mp_obj_type_t mp_type_iobase;
+
+STATIC mp_obj_base_t iobase_singleton = {&mp_type_iobase};
+
+STATIC mp_obj_t iobase_make_new(const mp_obj_type_t *type, size_t n_args, size_t n_kw, const mp_obj_t *args) {
+    (void)type;
+    (void)n_args;
+    (void)n_kw;
+    (void)args;
+    return MP_OBJ_FROM_PTR(&iobase_singleton);
+}
+
+STATIC mp_uint_t iobase_read_write(mp_obj_t obj, void *buf, mp_uint_t size, int *errcode, qstr qst) {
+    mp_obj_t dest[3];
+    mp_load_method(obj, qst, dest);
+    mp_obj_array_t ar = {{&mp_type_bytearray}, BYTEARRAY_TYPECODE, 0, size, buf};
+    dest[2] = MP_OBJ_FROM_PTR(&ar);
+    mp_obj_t ret = mp_call_method_n_kw(1, 0, dest);
+    if (ret == mp_const_none) {
+        *errcode = MP_EAGAIN;
+        return MP_STREAM_ERROR;
+    } else {
+        return mp_obj_get_int(ret);
+    }
+}
+STATIC mp_uint_t iobase_read(mp_obj_t obj, void *buf, mp_uint_t size, int *errcode) {
+    return iobase_read_write(obj, buf, size, errcode, MP_QSTR_readinto);
+}
+
+STATIC mp_uint_t iobase_write(mp_obj_t obj, const void *buf, mp_uint_t size, int *errcode) {
+    return iobase_read_write(obj, (void*)buf, size, errcode, MP_QSTR_write);
+}
+
+STATIC mp_uint_t iobase_ioctl(mp_obj_t obj, mp_uint_t request, uintptr_t arg, int *errcode) {
+    mp_obj_t dest[4];
+    mp_load_method(obj, MP_QSTR_ioctl, dest);
+    dest[2] = mp_obj_new_int_from_uint(request);
+    dest[3] = mp_obj_new_int_from_uint(arg);
+    mp_int_t ret = mp_obj_get_int(mp_call_method_n_kw(2, 0, dest));
+    if (ret >= 0) {
+        return ret;
+    } else {
+        *errcode = -ret;
+        return MP_STREAM_ERROR;
+    }
+}
+
+STATIC const mp_stream_p_t iobase_p = {
+    .read = iobase_read,
+    .write = iobase_write,
+    .ioctl = iobase_ioctl,
+};
+
+STATIC const mp_obj_type_t mp_type_iobase = {
+    { &mp_type_type },
+    .name = MP_QSTR_IOBase,
+    .make_new = iobase_make_new,
+    .protocol = &iobase_p,
+};
+
+#endif // MICROPY_PY_IO_IOBASE
+
 #if MICROPY_PY_IO_BUFFEREDWRITER
 typedef struct _mp_obj_bufwriter_t {
     mp_obj_base_t base;
@@ -187,6 +253,9 @@ STATIC const mp_rom_map_elem_t mp_module_io_globals_table[] = {
     // Note: mp_builtin_open_obj should be defined by port, it's not
     // part of the core.
     { MP_ROM_QSTR(MP_QSTR_open), MP_ROM_PTR(&mp_builtin_open_obj) },
+    #if MICROPY_PY_IO_IOBASE
+    { MP_ROM_QSTR(MP_QSTR_IOBase), MP_ROM_PTR(&mp_type_iobase) },
+    #endif
     #if MICROPY_PY_IO_RESOURCE_STREAM
     { MP_ROM_QSTR(MP_QSTR_resource_stream), MP_ROM_PTR(&resource_stream_obj) },
     #endif

py/mpconfig.h

@@ -998,6 +998,11 @@ typedef double mp_float_t;
 #define MICROPY_PY_IO (1)
 #endif
 
+// Whether to provide "io.IOBase" class to support user streams
+#ifndef MICROPY_PY_IO_IOBASE
+#define MICROPY_PY_IO_IOBASE (0)
+#endif
+
 // Whether to provide "uio.resource_stream()" function with
 // the semantics of CPython's pkg_resources.resource_stream()
 // (allows to access binary resources in frozen source packages).