py/objint: Fix int.to_bytes() buffer size checks.

Fixes and improvements to `int.to_bytes()` are:
- No longer overflows if byte size is 0 (closes #13041).
- Raises OverflowError in any case where number won't fit into byte length
  (now matches CPython, previously MicroPython would return a truncated
  bytes object).
- Document that `micropython int.to_bytes()` doesn't implement the optional
  signed kwarg, but will behave as if `signed=True` when the integer is
  negative (this is the current behaviour).  Add tests for this also.

Requires changes for small ints, MPZ large ints, and "long long" large
ints.

Adds a new set of unit tests for ints between 32 and 64 bits to increase
coverage of "long long" large ints, which are otherwise untested.

Tested on unix port (64 bit small ints, MPZ long ints) and Zephyr STM32WB
board (32 bit small ints, long long large ints).

This work was funded through GitHub Sponsors.

Signed-off-by: Angus Gratton <angus@redyak.com.au>

1 files changed, 2 insertions, 1 deletions

diff --git a/py/objint.h b/py/objint.h
index 5eed87705..28930e35a 100644
--- a/py/objint.h
+++ b/py/objint.h
@@ -55,7 +55,8 @@ char *mp_obj_int_formatted_impl(char **buf, size_t *buf_size, size_t *fmt_size,
     int base, const char *prefix, char base_char, char comma);
 mp_int_t mp_obj_int_hash(mp_obj_t self_in);
 mp_obj_t mp_obj_int_from_bytes_impl(bool big_endian, size_t len, const byte *buf);
-void mp_obj_int_to_bytes_impl(mp_obj_t self_in, bool big_endian, size_t len, byte *buf);
+// Returns true if 'self_in' fit into 'len' bytes of 'buf' without overflowing, 'buf' is truncated otherwise.
+bool mp_obj_int_to_bytes_impl(mp_obj_t self_in, bool big_endian, size_t len, byte *buf);
 int mp_obj_int_sign(mp_obj_t self_in);
 mp_obj_t mp_obj_int_unary_op(mp_unary_op_t op, mp_obj_t o_in);
 mp_obj_t mp_obj_int_binary_op(mp_binary_op_t op, mp_obj_t lhs_in, mp_obj_t rhs_in);