chore(migration): Migrate code from googleapis/python-bigquery-dataframes into packages/bigframes

[chalmerlowe]

See #15999.

This PR should be merged with a merge-commit, not a squash-commit, in order to preserve the git history.

[gemini-code-assist]

Code Review

This pull request introduces the foundational codebase for the bigframes package, including core execution nodes, BigQuery operation compilers, and extensive CI/CD configurations. The review feedback identifies a bug in the exponentiation logic for negative bases and highlights opportunities to improve behavioral parity with Pandas by returning null instead of zero for division and modulo by zero operations. Additionally, a suggested fix addresses a potential infinite loop in the repository root detection script used during environment setup.

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packages/bigframes/bigframes/core/compile/ibis_compiler/scalar_op_registry.py (1548)

The odd_exponent calculation for negative bases is incorrect for negative odd integers. In BigQuery (and many other SQL dialects), the MOD operator returns a negative value for negative inputs (e.g., MOD(-3, 2) returns -1). As a result, the condition == _ibis_num(1) will fail for negative odd exponents, leading to an incorrect sign in the final result when overflow_cond is true. Using the absolute value of the exponent or checking for a non-zero remainder would fix this.

    odd_exponent = (x_val < _ZERO) & (y_val.cast(ibis_dtypes.int64).abs() % _ibis_num(2) == _ibis_num(1))

packages/bigframes/.kokoro/trampoline_v2.sh (232-238)

The repo_root function can enter an infinite loop if it is executed in a directory that is not part of a git repository. This happens because dirname "/" returns /, so the while loop condition [[ ! -d "${dir}/.git" ]] will never become false if /.git does not exist. Adding a check for the root directory or a maximum depth would make the script more robust.

function repo_root() {
    local dir="$1"
    while [[ ! -d "${dir}/.git" && "$dir" != "/" ]]; do
	dir="$(dirname "$dir")"
    done
    if [[ ! -d "${dir}/.git" ]]; then
        echo "Error: Could not find .git directory in any parent of $1" >&2
        exit 1
    fi
    echo "${dir}"
}

packages/bigframes/bigframes/core/compile/ibis_compiler/scalar_op_registry.py (1634)

In floordiv_op, integer division by zero currently returns 0 (via _ZERO * x_numeric at line 1637). This is inconsistent with Pandas behavior for nullable integers (Int64), where division by zero should result in a null/NA value. Returning 0 can lead to silent errors in data processing. It is recommended to return null for the integer case.

    zero_result = _INF if (x.type().is_floating() or y.type().is_floating()) else ibis.null().cast(x.type())

packages/bigframes/bigframes/core/compile/ibis_compiler/scalar_op_registry.py (1741-1744)

In _int_mod, the modulo operation with a zero divisor returns 0 (via _ZERO * x). To maintain consistency with Pandas and avoid mathematically incorrect results, this should return null instead. Returning 0 hides the division-by-zero error and produces an incorrect value.

        .when(
            y == _ZERO,
            ibis.null().cast(x.type()),
        )  # Return NULL for division by zero to match pandas behavior