py/stream: Support both "exact size" and "one underlying call" operations.

Both read and write operations support variants where either a) a single
call is made to the undelying stream implementation and returned buffer
length may be less than requested, or b) calls are repeated until requested
amount of data is collected, shorter amount is returned only in case of
EOF or error.

These operations are available from the level of C support functions to be
used by other C modules to implementations of Python methods to be used in
user-facing objects.

The rationale of these changes is to allow to write concise and robust
code to work with *blocking* streams of types prone to short reads, like
serial interfaces and sockets. Particular object types may select "exact"
vs "once" types of methods depending on their needs. E.g., for sockets,
revc() and send() methods continue to be "once", while read() and write()
thus converted to "exactly" versions.

These changes don't affect non-blocking handling, e.g. trying "exact"
method on the non-blocking socket will return as much data as available
without blocking. No data available is continued to be signaled as None
return value to read() and write().

From the point of view of CPython compatibility, this model is a cross
between its io.RawIOBase and io.BufferedIOBase abstract classes. For
blocking streams, it works as io.BufferedIOBase model (guaranteeing
lack of short reads/writes), while for non-blocking - as io.RawIOBase,
returning None in case of lack of data (instead of raising expensive
exception, as required by io.BufferedIOBase). Such a cross-behavior
should be optimal for MicroPython needs.

1 files changed, 10 insertions, 4 deletions

diff --git a/py/modio.c b/py/modio.c
index 423315081..2fbe6bc1e 100644
--- a/py/modio.c
+++ b/py/modio.c
@@ -78,10 +78,13 @@ STATIC mp_uint_t bufwriter_write(mp_obj_t self_in, const void *buf, mp_uint_t si
         memcpy(self->buf + self->len, buf, rem);
         buf = (byte*)buf + rem;
         size -= rem;
-        mp_uint_t out_sz = mp_stream_writeall(self->stream, self->buf, self->alloc, errcode);
-        if (out_sz == MP_STREAM_ERROR) {
+        mp_uint_t out_sz = mp_stream_write_exactly(self->stream, self->buf, self->alloc, errcode);
+        if (*errcode != 0) {
             return MP_STREAM_ERROR;
         }
+        // TODO: try to recover from a case of non-blocking stream, e.g. move
+        // remaining chunk to the beginning of buffer.
+        assert(out_sz == self->alloc);
         self->len = 0;
     }
 
@@ -93,9 +96,12 @@ STATIC mp_obj_t bufwriter_flush(mp_obj_t self_in) {
 
     if (self->len != 0) {
         int err;
-        mp_uint_t out_sz = mp_stream_writeall(self->stream, self->buf, self->len, &err);
+        mp_uint_t out_sz = mp_stream_write_exactly(self->stream, self->buf, self->len, &err);
+        // TODO: try to recover from a case of non-blocking stream, e.g. move
+        // remaining chunk to the beginning of buffer.
+        assert(out_sz == self->len);
         self->len = 0;
-        if (out_sz == MP_STREAM_ERROR) {
+        if (err != 0) {
             nlr_raise(mp_obj_new_exception_arg1(&mp_type_OSError, MP_OBJ_NEW_SMALL_INT(err)));
         }
     }