wheel builds: react to changes in pip's handling of build constraints

ci/build_wheel.sh

@@ -1,5 +1,5 @@
 #!/bin/bash
-# SPDX-FileCopyrightText: Copyright (c) 2024-2025, NVIDIA CORPORATION.
+# SPDX-FileCopyrightText: Copyright (c) 2024-2026, NVIDIA CORPORATION.
 # SPDX-License-Identifier: BSD-3-Clause
 
 set -euo pipefail
@@ -21,11 +21,28 @@ cd "${package_dir}"
 sccache --stop-server 2>/dev/null || true
 
 rapids-logger "Building '${package_name}' wheel"
+
+RAPIDS_PIP_WHEEL_ARGS=(
+  -w dist
+  -v
+  --no-deps
+  --disable-pip-version-check
+)
+
+# Only use --build-constraint when build isolation is enabled.
+#
+# Passing '--build-constraint' and '--no-build-isolation` together results in an error from 'pip',
+# but we want to keep environment variable PIP_CONSTRAINT set unconditionally.
+# PIP_NO_BUILD_ISOLATION=0 means "add --no-build-isolation" (ref: https://github.com/pypa/pip/issues/573
+if [[ "${PIP_NO_BUILD_ISOLATION:-}" != "0" ]]; then
+    RAPIDS_PIP_WHEEL_ARGS+=(--build-constraint="${PIP_CONSTRAINT}")
+fi
+# unset that environment variable... it doesn't affect builds as of pip 25.3, and
+# results in an error from 'pip wheel' when set and --build-constraint is also passed
+unset PIP_CONSTRAINT
+
 rapids-pip-retry wheel \
-    -w dist \
-    -v \
-    --no-deps \
-    --disable-pip-version-check \
+    "${RAPIDS_PIP_WHEEL_ARGS[@]}" \
     .
 
 sccache --show-adv-stats

ci/build_wheel_ucxx.sh

@@ -1,5 +1,5 @@
 #!/bin/bash
-# SPDX-FileCopyrightText: Copyright (c) 2024-2025, NVIDIA CORPORATION.
+# SPDX-FileCopyrightText: Copyright (c) 2024-2026, NVIDIA CORPORATION.
 # SPDX-License-Identifier: BSD-3-Clause
 
 set -euo pipefail
@@ -14,8 +14,8 @@ RAPIDS_PY_CUDA_SUFFIX="$(rapids-wheel-ctk-name-gen "${RAPIDS_CUDA_VERSION}")"
 # Downloads libucxx wheel from this current build,
 # then ensures 'ucxx' wheel builds always use the 'libucxx' just built in the same CI run.
 #
-# Using env variable PIP_CONSTRAINT (initialized by 'rapids-init-pip') is necessary to ensure the constraints
-# are used when creating the isolated build environment.
+# env variable 'PIP_CONSTRAINT' is set up by rapids-init-pip. It constrains all subsequent
+# 'pip install', 'pip download', etc. calls (except those used in 'pip wheel', handled separately in build scripts)
 LIBUCXX_WHEELHOUSE=$(RAPIDS_PY_WHEEL_NAME="libucxx_${RAPIDS_PY_CUDA_SUFFIX}" rapids-download-wheels-from-github cpp)
 echo "libucxx-${RAPIDS_PY_CUDA_SUFFIX} @ file://$(echo "${LIBUCXX_WHEELHOUSE}"/libucxx_*.whl)" >> "${PIP_CONSTRAINT}"