AviKndr opened a new pull request, #39047:
URL: https://github.com/apache/beam/pull/39047
### What
`VarIntCoder` raises `OverflowError` when handed a Python int in the
unsigned 64-bit range `[2**63, 2**64)` (a uint64), even though such a value has
a well-defined VarInt encoding.
### Why
In the Cython build, the stream methods take a **signed** `int64_t`:
```cython
cpdef write_var_int64(self, libc.stdint.int64_t signed_v):
cdef libc.stdint.uint64_t v = signed_v # body already works on unsigned
```
The body re-casts to `uint64_t` and encodes the bit pattern correctly, but
Cython converts the incoming Python int to the signed `int64_t` parameter *at
the call boundary* — before the body runs — so any uint64 value is rejected
with `OverflowError`. The same applies to `get_varint_size` used by
`estimate_size`.
### Fix
A uint64 value `v` and the signed int64 `v - 2**64` (its two's-complement
twin) produce **identical** VarInt bytes. `VarIntCoderImpl` now folds uint64
values to that signed twin before they cross into Cython, in both
`encode_to_stream` and `estimate_size`:
```python
def _as_signed_int64(value):
if (1 << 63) <= value < (1 << 64):
return int(value) - (1 << 64)
return value
```
- **Wire-compatible** — byte-identical to Java's signed `VarIntCoder`.
- **No regression** — normal signed ints pass through untouched; the
pure-Python `slow_stream` path produces the same bytes.
- **Still bounded** — values `>= 2**64` are left unchanged and still
overflow in the Cython path, preserving the coder's 64-bit contract.
Note: decoding remains **signed** (`read_var_int64` returns `int64_t`),
matching Java — so the encoding of `2**64 - 1` decodes back to `-1`. This
change removes the crash and guarantees correct wire bytes; it does not make
the coder round-trip uint64 back to unsigned (that would be a separate coder).
### Tests
Adds `test_varint_coder_uint64` to the shared `coders_test_common.py` suite
(runs both compiled and uncompiled): no-overflow encoding, wire equivalence to
the signed twin, size estimation, signed decode semantics, and the
still-raising out-of-range case (guarded on the compiled implementation).
------------------------
Thank you for your contribution! Follow [this
checklist](https://github.com/apache/beam/blob/master/CONTRIBUTING.md) to help
us incorporate your contribution quickly and easily:
- [ ] Mention the appropriate issue in your description (for example:
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- [ ] Update `CHANGES.md` with noteworthy changes.
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