ci: make python setup resilient

scripts/ensure-ci-python.sh

@@ -11,10 +11,52 @@ else
   PYTHON_BIN="python${PYTHON_MAJOR}"
 fi
 
-if command -v "${PYTHON_BIN}" >/dev/null 2>&1; then
-  command -v "${PYTHON_BIN}"
+python_matches() {
+  local candidate="$1"
+  [[ -x "${candidate}" ]] || return 1
+  "${candidate}" - "${PYTHON_MAJOR}" "${PYTHON_MINOR:-}" <<'PY'
+import sys
+
+expected_major = int(sys.argv[1])
+expected_minor = sys.argv[2]
+if sys.version_info.major != expected_major:
+    raise SystemExit(1)
+if expected_minor and sys.version_info.minor != int(expected_minor):
+    raise SystemExit(1)
+PY
+}
+
+python_has_venv() {
+  local candidate="$1"
+  "${candidate}" -m venv --help >/dev/null 2>&1
+}
+
+print_if_usable() {
+  local candidate="$1"
+  if python_matches "${candidate}" && python_has_venv "${candidate}"; then
+    printf '%s\n' "${candidate}"
     exit 0
   fi
+}
+
+if command -v "${PYTHON_BIN}" >/dev/null 2>&1; then
+  print_if_usable "$(command -v "${PYTHON_BIN}")"
+fi
+
+TOOLCACHE_DIR="${RUNNER_TOOL_CACHE:-/opt/hostedtoolcache}"
+HOME_DIR="${HOME:-/root}"
+
+shopt -s nullglob
+# shellcheck disable=SC2140
+for candidate in \
+  "${TOOLCACHE_DIR}"/Python/"${PYTHON_MAJOR}.${PYTHON_MINOR}"*/x64/bin/python \
+  "${TOOLCACHE_DIR}"/Python/"${PYTHON_MAJOR}.${PYTHON_MINOR}"*/x64/bin/python3 \
+  "${TOOLCACHE_DIR}"/uv-python/cpython-"${PYTHON_MAJOR}.${PYTHON_MINOR}"*/bin/python \
+  "${HOME_DIR}"/.local/share/uv/python/cpython-"${PYTHON_MAJOR}.${PYTHON_MINOR}"*/bin/python \
+  "${HOME_DIR}"/.cache/uv/python/cpython-"${PYTHON_MAJOR}.${PYTHON_MINOR}"*/bin/python; do
+  print_if_usable "${candidate}"
+done
+shopt -u nullglob
 
 APT_RUNNER=()
 if [[ "$(id -u)" -ne 0 ]]; then
@@ -23,12 +65,42 @@ fi
 
 export DEBIAN_FRONTEND=noninteractive
 
-"${APT_RUNNER[@]}" apt-get update >&2
-if ! "${APT_RUNNER[@]}" apt-get install -y "${PYTHON_BIN}" "${PYTHON_BIN}-venv" >&2; then
-  "${APT_RUNNER[@]}" apt-get install -y software-properties-common >&2
-  "${APT_RUNNER[@]}" add-apt-repository -y ppa:deadsnakes/ppa >&2
-  "${APT_RUNNER[@]}" apt-get update >&2
-  "${APT_RUNNER[@]}" apt-get install -y "${PYTHON_BIN}" "${PYTHON_BIN}-venv" >&2
+if "${APT_RUNNER[@]}" apt-get update >&2 \
+  && "${APT_RUNNER[@]}" apt-get install -y "${PYTHON_BIN}" "${PYTHON_BIN}-venv" >&2; then
+  print_if_usable "$(command -v "${PYTHON_BIN}")"
 fi
 
-command -v "${PYTHON_BIN}"
+install_uv_standalone() {
+  if command -v uv >/dev/null 2>&1; then
+    command -v uv
+    return 0
+  fi
+
+  local uv_dir
+  uv_dir="$(mktemp -d)"
+  local install_url="https://astral.sh/uv/${UV_VERSION:-0.7.2}/install.sh"
+  if ! curl -LsSf "${install_url}" | env UV_INSTALL_DIR="${uv_dir}/bin" sh >&2; then
+    curl -LsSf "https://astral.sh/uv/install.sh" | env UV_INSTALL_DIR="${uv_dir}/bin" sh >&2
+  fi
+  printf '%s\n' "${uv_dir}/bin/uv"
+}
+
+UV_BIN="$(install_uv_standalone)"
+UV_PYTHON_INSTALL_DIR="${TOOLCACHE_DIR:-${HOME_DIR}/.cache}/uv-python" \
+  "${UV_BIN}" python install "${TARGET_VERSION}" --no-progress >&2
+
+UV_PYTHON_CANDIDATE="$(
+  UV_PYTHON_INSTALL_DIR="${TOOLCACHE_DIR:-${HOME_DIR}/.cache}/uv-python" \
+    "${UV_BIN}" python find "${TARGET_VERSION}" --managed-python
+)"
+print_if_usable "${UV_PYTHON_CANDIDATE}"
+
+if "${APT_RUNNER[@]}" apt-get install -y software-properties-common >&2 \
+  && "${APT_RUNNER[@]}" add-apt-repository -y ppa:deadsnakes/ppa >&2 \
+  && "${APT_RUNNER[@]}" apt-get update >&2 \
+  && "${APT_RUNNER[@]}" apt-get install -y "${PYTHON_BIN}" "${PYTHON_BIN}-venv" >&2; then
+  print_if_usable "$(command -v "${PYTHON_BIN}")"
+fi
+
+printf 'Unable to install a usable Python %s with venv support\n' "${TARGET_VERSION}" >&2
+exit 1