py/mpz: Do sign extension in mpz_as_bytes for negative values.

Signed-off-by: Damien George <damien@micropython.org>

py/mpz.c

@@ -1609,7 +1609,6 @@ bool mpz_as_uint_checked(const mpz_t *i, mp_uint_t *value) {
     return true;
 }
 
-// writes at most len bytes to buf (so buf should be zeroed before calling)
 void mpz_as_bytes(const mpz_t *z, bool big_endian, size_t len, byte *buf) {
     byte *b = buf;
     if (big_endian) {
@@ -1641,6 +1640,15 @@ void mpz_as_bytes(const mpz_t *z, bool big_endian, size_t len, byte *buf) {
             }
         }
     }
+
+    // fill remainder of buf with zero/sign extension of the integer
+    if (big_endian) {
+        len = b - buf;
+    } else {
+        len = buf + len - b;
+        buf = b;
+    }
+    memset(buf, z->neg ? 0xff : 0x00, len);
 }
 
 #if MICROPY_PY_BUILTINS_FLOAT

py/objint_mpz.c

@@ -116,7 +116,6 @@ 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) {
     assert(mp_obj_is_type(self_in, &mp_type_int));
     mp_obj_int_t *self = MP_OBJ_TO_PTR(self_in);
-    memset(buf, 0, len);
     mpz_as_bytes(&self->mpz, big_endian, len, buf);
 }
 

tests/basics/struct1_intbig.py

@@ -36,3 +36,11 @@ print(struct.unpack("<Q", b"\xff\xff\xff\xff\xff\xff\xff\xff"))
 # check small int overflow
 print(struct.unpack("<i", b'\xff\xff\xff\x7f'))
 print(struct.unpack("<q", b'\xff\xff\xff\xff\xff\xff\xff\x7f'))
+
+# test with negative big integers that are actually small in magnitude
+bigzero = (1 << 70) - (1 << 70)
+for endian in "<>":
+    for type_ in "bhiq":
+        fmt = endian + type_
+        b = struct.pack(fmt, -2 + bigzero)
+        print(fmt, b, struct.unpack(fmt, b))