modstruct: Improve compliance with python3

While checking whether we can enable -Wimplicit-fallthrough, I encountered
a diagnostic in mp_binary_set_val_array_from_int which led to discovering
the following bug:
```
>>> struct.pack("xb", 3)
b'\x03\x03'
```
That is, the next value (3) was used as the value of a padding byte, while
standard Python always fills "x" bytes with zeros.  I initially thought
this had to do with the unintentional fallthrough, but it doesn't.
Instead, this code would relate to an array.array with a typecode of
padding ('x'), which is ALSO not desktop Python compliant:
```
>>> array.array('x', (1, 2, 3))
array('x', [1, 0, 0])
```
Possibly this is dead code that used to be shared between struct-setting
and array-setting, but it no longer is.

I also discovered that the argument list length for struct.pack
and struct.pack_into were not checked, and that the length of binary data
passed to array.array was not checked to be a multiple of the element
size.

I have corrected all of these to conform more closely to standard Python
and revised some tests where necessary.  Some tests for micropython-specific
behavior that does not conform to standard Python and is not present
in CircuitPython was deleted outright.

py/binary.c

@@ -126,7 +126,6 @@ mp_obj_t mp_binary_get_val_array(char typecode, void *p, mp_uint_t index) {
             break;
         case BYTEARRAY_TYPECODE:
         case 'B':
-        case 'x':   // value will be discarded
             val = ((unsigned char*)p)[index];
             break;
         case 'h':
@@ -330,8 +329,12 @@ void mp_binary_set_val(char struct_type, char val_type, mp_obj_t val_in, byte **
         }
     }
 
+    if (val_type == 'x') {
+        memset(p, 0, 1);
+    } else {
         mp_binary_set_int(MIN((size_t)size, sizeof(val)), struct_type == '>', p, val);
     }
+}
 
 void mp_binary_set_val_array(char typecode, void *p, mp_uint_t index, mp_obj_t val_in) {
     switch (typecode) {
@@ -379,8 +382,6 @@ void mp_binary_set_val_array_from_int(char typecode, void *p, mp_uint_t index, m
         case 'B':
             ((unsigned char*)p)[index] = val;
             break;
-        case 'x':
-            ((unsigned char*)p)[index] = 0;
         case 'h':
             ((short*)p)[index] = val;
             break;

py/modstruct.c

@@ -183,16 +183,21 @@ MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(struct_unpack_from_obj, 2, 3, struct_unpack_
 
 // This function assumes there is enough room in p to store all the values
 STATIC void struct_pack_into_internal(mp_obj_t fmt_in, byte *p, size_t n_args, const mp_obj_t *args) {
+    size_t size;
+    size_t count = calc_size_items(mp_obj_str_get_str(fmt_in), &size);
+    if (count != n_args) {
+#if MICROPY_ERROR_REPORTING == MICROPY_ERROR_REPORTING_TERSE
+        mp_raise_ValueError(NULL);
+#else
+        mp_raise_ValueError_varg(translate("pack expected %d items for packing (got %d)"), count, n_args);
+#endif
+    }
     const char *fmt = mp_obj_str_get_str(fmt_in);
     char fmt_type = get_fmt_type(&fmt);
 
     size_t i;
     for (i = 0; i < n_args;) {
         mp_uint_t cnt = 1;
-        if (*fmt == '\0') {
-            // more arguments given than used by format string; CPython raises struct.error here
-            break;
-        }
         if (unichar_isdigit(*fmt)) {
             cnt = get_fmt_num(&fmt);
         }
@@ -208,8 +213,7 @@ STATIC void struct_pack_into_internal(mp_obj_t fmt_in, byte *p, size_t n_args, c
             memset(p + to_copy, 0, cnt - to_copy);
             p += cnt;
         } else {
-            // If we run out of args then we just finish; CPython would raise struct.error
-            while (cnt-- && i < n_args) {
+            while (cnt--) {
                 mp_binary_set_val(fmt_type, *fmt, args[i], &p);
                 // Pad bytes don't have a corresponding argument.
                 if (*fmt != 'x') {
@@ -222,7 +226,6 @@ STATIC void struct_pack_into_internal(mp_obj_t fmt_in, byte *p, size_t n_args, c
 }
 
 STATIC mp_obj_t struct_pack(size_t n_args, const mp_obj_t *args) {
-    // TODO: "The arguments must match the values required by the format exactly."
     mp_int_t size = MP_OBJ_SMALL_INT_VALUE(struct_calcsize(args[0]));
     vstr_t vstr;
     vstr_init_len(&vstr, size);

py/objarray.c

@@ -97,6 +97,9 @@ STATIC void array_print(const mp_print_t *print, mp_obj_t o_in, mp_print_kind_t
 
 #if MICROPY_PY_BUILTINS_BYTEARRAY || MICROPY_PY_ARRAY
 STATIC mp_obj_array_t *array_new(char typecode, size_t n) {
+    if (typecode == 'x') {
+        mp_raise_ValueError(translate("bad typecode"));
+    }
     int typecode_size = mp_binary_get_size('@', typecode, NULL);
     mp_obj_array_t *o = m_new_obj(mp_obj_array_t);
     #if MICROPY_PY_BUILTINS_BYTEARRAY && MICROPY_PY_ARRAY
@@ -126,8 +129,10 @@ STATIC mp_obj_t array_construct(char typecode, mp_obj_t initializer) {
                 || (MICROPY_PY_BUILTINS_BYTEARRAY && MP_OBJ_IS_TYPE(initializer, &mp_type_bytearray)))))
         && mp_get_buffer(initializer, &bufinfo, MP_BUFFER_READ)) {
         // construct array from raw bytes
-        // we round-down the len to make it a multiple of sz (CPython raises error)
         size_t sz = mp_binary_get_size('@', typecode, NULL);
+        if (bufinfo.len % sz) {
+            mp_raise_ValueError(translate("bytes length not a multiple of item size"));
+        }
         size_t len = bufinfo.len / sz;
         mp_obj_array_t *o = array_new(typecode, len);
         memcpy(o->items, bufinfo.buf, len * sz);

tests/basics/struct1.py

@@ -39,6 +39,28 @@ print(v == (10, 100, 200, 300))
 # network byte order
 print(struct.pack('!i', 123))
 
+# too short / too long arguments
+buf = bytearray(b'>>>123<<<')
+try:
+    struct.pack_into('bb', buf, 0, 3)
+except:
+    print('struct.error')
+
+try:
+    struct.pack_into('bb', buf, 0, 3, 1, 4)
+except:
+    print('struct.error')
+
+try:
+    struct.pack('bb', 3)
+except:
+    print('struct.error')
+
+try:
+    struct.pack('bb', 3, 1, 4)
+except:
+    print('struct.error')
+
 # check that we get an error if the buffer is too small
 try:
     struct.unpack('I', b'\x00\x00\x00')
@@ -96,3 +118,6 @@ try:
     print(struct.unpack_from('<b', buf, -11))
 except:
     print('struct.error')
+
+# check padding bytes
+print(struct.pack("xb", 3))

tests/basics/struct_micropython.py

@@ -1,41 +0,0 @@
-# test MicroPython-specific features of struct
-
-try:
-    import ustruct as struct
-except:
-    try:
-        import struct
-    except ImportError:
-        print("SKIP")
-        raise SystemExit
-
-try:
-    struct.pack('O', None)
-except ValueError:
-    print("SKIP")
-    raise SystemExit
-
-class A():
-    pass
-
-# pack and unpack objects
-o = A()
-s = struct.pack("<O", o)
-o2 = struct.unpack("<O", s)
-print(o is o2[0])
-
-# pack can accept less arguments than required for the format spec
-print(struct.pack('<2I', 1))
-
-# pack and unpack pointer to a string
-# This requires uctypes to get the address of the string and instead of
-# putting this in a dedicated test that can be skipped we simply pass
-# if the import fails.
-try:
-    import uctypes
-    o = uctypes.addressof('abc')
-    s = struct.pack("<S", o)
-    o2 = struct.unpack("<S", s)
-    assert o2[0] == 'abc'
-except ImportError:
-    pass

tests/basics/struct_micropython.py.exp

@@ -1,2 +0,0 @@
-True
-b'\x01\x00\x00\x00\x00\x00\x00\x00'

tests/misc/non_compliant.py

@@ -105,12 +105,6 @@ try:
 except NotImplementedError:
     print('NotImplementedError')
 
-# struct pack with too many args, not checked by uPy
-print(struct.pack('bb', 1, 2, 3))
-
-# struct pack with too few args, not checked by uPy
-print(struct.pack('bb', 1))
-
 # array slice assignment with unsupported RHS
 try:
     bytearray(4)[0:1] = [1, 2]

tests/misc/non_compliant.py.exp

@@ -14,8 +14,6 @@ NotImplementedError
 NotImplementedError
 NotImplementedError
 NotImplementedError
-b'\x01\x02'
-b'\x01\x00'
 NotImplementedError
 AttributeError
 TypeError