unix/modusocket: Finish socket.settimeout() implementation.

1. Return correct error code for non-blocking vs timed out socket
(POSIX returns EAGAIN for both, we want ETIMEDOUT in case of timed
out socket). To achieve this, blocking/non-blocking flag is added
to the mp_obj_socket_t, to avoid issuing fcntl() syscall each time
EAGAIN occurs. (mp_obj_socket_t used to be 8 bytes, having some room
in a standard 16-byte alloc block.)

2. Handle socket.settimeout(0) properly - in Python, that means
non-blocking mode, but SO_RCVTIMEO/SO_SNDTIMEO of 0 is infinite
timeout.

3. Overall, make sure that socket.settimeout() call switches blocking
state as expected.

ports/unix/modusocket.c

@@ -37,6 +37,7 @@
 #include <arpa/inet.h>
 #include <netdb.h>
 #include <errno.h>
+#include <math.h>
 
 #include "py/objtuple.h"
 #include "py/objstr.h"
@@ -65,6 +66,7 @@
 typedef struct _mp_obj_socket_t {
     mp_obj_base_t base;
     int fd;
+    bool blocking;
 } mp_obj_socket_t;
 
 const mp_obj_type_t mp_type_socket;
@@ -78,6 +80,7 @@ STATIC mp_obj_socket_t *socket_new(int fd) {
     mp_obj_socket_t *o = m_new_obj(mp_obj_socket_t);
     o->base.type = &mp_type_socket;
     o->fd = fd;
+    o->blocking = true;
     return o;
 }
 
@@ -92,12 +95,14 @@ STATIC mp_uint_t socket_read(mp_obj_t o_in, void *buf, mp_uint_t size, int *errc
     mp_obj_socket_t *o = MP_OBJ_TO_PTR(o_in);
     mp_int_t r = read(o->fd, buf, size);
     if (r == -1) {
-        *errcode = errno;
-
-        if (*errcode == EAGAIN) {
-            *errcode = MP_ETIMEDOUT;
+        int err = errno;
+        // On blocking socket, we get EAGAIN in case SO_RCVTIMEO/SO_SNDTIMEO
+        // timed out, and need to convert that to ETIMEDOUT.
+        if (err == EAGAIN && o->blocking) {
+            err = MP_ETIMEDOUT;
         }
 
+        *errcode = err;
         return MP_STREAM_ERROR;
     }
     return r;
@@ -107,12 +112,14 @@ STATIC mp_uint_t socket_write(mp_obj_t o_in, const void *buf, mp_uint_t size, in
     mp_obj_socket_t *o = MP_OBJ_TO_PTR(o_in);
     mp_int_t r = write(o->fd, buf, size);
     if (r == -1) {
-        *errcode = errno;
-
-        if (*errcode == EAGAIN) {
-            *errcode = MP_ETIMEDOUT;
+        int err = errno;
+        // On blocking socket, we get EAGAIN in case SO_RCVTIMEO/SO_SNDTIMEO
+        // timed out, and need to convert that to ETIMEDOUT.
+        if (err == EAGAIN && o->blocking) {
+            err = MP_ETIMEDOUT;
         }
 
+        *errcode = err;
         return MP_STREAM_ERROR;
     }
     return r;
@@ -320,6 +327,7 @@ STATIC mp_obj_t socket_setblocking(mp_obj_t self_in, mp_obj_t flag_in) {
     }
     flags = fcntl(self->fd, F_SETFL, flags);
     RAISE_ERRNO(flags, errno);
+    self->blocking = val;
     return mp_const_none;
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_2(socket_setblocking_obj, socket_setblocking);
@@ -327,21 +335,37 @@ STATIC MP_DEFINE_CONST_FUN_OBJ_2(socket_setblocking_obj, socket_setblocking);
 STATIC mp_obj_t socket_settimeout(mp_obj_t self_in, mp_obj_t timeout_in) {
     mp_obj_socket_t *self = MP_OBJ_TO_PTR(self_in);
     struct timeval tv = {0,};
+    bool new_blocking = true;
+
     if (timeout_in == mp_const_none) {
         setsockopt(self->fd, SOL_SOCKET, SO_RCVTIMEO, NULL, 0);
         setsockopt(self->fd, SOL_SOCKET, SO_SNDTIMEO, NULL, 0);
     } else {
-        tv.tv_sec = mp_obj_get_int(timeout_in);
-
         #if MICROPY_PY_BUILTINS_FLOAT
-        tv.tv_usec = (mp_obj_get_float(timeout_in) - tv.tv_sec) * 1000000;
+        mp_float_t val = mp_obj_get_float(timeout_in);
+        double ipart;
+        tv.tv_usec = round(modf(val, &ipart) * 1000000);
+        tv.tv_sec = ipart;
+        #else
+        tv.tv_sec = mp_obj_get_int(timeout_in);
         #endif
 
-        setsockopt(self->fd, SOL_SOCKET, SO_RCVTIMEO,
-                   &tv, sizeof(struct timeval));
-        setsockopt(self->fd, SOL_SOCKET, SO_SNDTIMEO,
-                   &tv, sizeof(struct timeval));
+        // For SO_RCVTIMEO/SO_SNDTIMEO, zero timeout means infinity, but
+        // for Python API it means non-blocking.
+        if (tv.tv_sec == 0 && tv.tv_usec == 0) {
+            new_blocking = false;
+        } else {
+            setsockopt(self->fd, SOL_SOCKET, SO_RCVTIMEO,
+                       &tv, sizeof(struct timeval));
+            setsockopt(self->fd, SOL_SOCKET, SO_SNDTIMEO,
+                       &tv, sizeof(struct timeval));
+        }
     }
+
+    if (self->blocking != new_blocking) {
+        socket_setblocking(self_in, mp_obj_new_bool(new_blocking));
+    }
+
     return mp_const_none;
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_2(socket_settimeout_obj, socket_settimeout);