[WIP] chore(bigframes): add specific build script for doctest to restrict execution

.kokoro/presubmit/presubmit-doctest-bigframes.cfg

@@ -9,4 +9,10 @@ env_vars: {
 env_vars: {
     key: "GOOGLE_CLOUD_PROJECT"
     value: "bigframes-testing"
+}
+
+# Override the build file to only run for bigframes.
+env_vars: {
+    key: "TRAMPOLINE_BUILD_FILE"
+    value: "github/google-cloud-python/packages/bigframes/scripts/run_doctest.sh"
 }

packages/bigframes/scripts/run_doctest.sh

@@ -0,0 +1,44 @@
+#!/bin/bash
+# `-e` enables the script to automatically fail when a command fails
+# `-o pipefail` sets the exit code to non-zero if any command fails,
+# or zero if all commands in the pipeline exit successfully.
+set -eo pipefail
+
+# Disable buffering, so that the logs stream through.
+export PYTHONUNBUFFERED=1
+
+# Assume we are running from the repo root or we need to find it.
+# If this script is in packages/bigframes/scripts/run_doctest.sh,
+# then repo root is 3 levels up.
+export PROJECT_ROOT=$(realpath "$(dirname "${BASH_SOURCE[0]}")/../../..")
+cd "$PROJECT_ROOT"
+
+# This is needed in order for `git diff` to succeed
+git config --global --add safe.directory "$(realpath .)"
+
+package_name="bigframes"
+package_path="packages/${package_name}"
+
+# Determine if we should skip based on git diff
+files_to_check="${package_path}"
+
+if [[ -n "${KOKORO_GITHUB_PULL_REQUEST_TARGET_BRANCH}" && -n "${KOKORO_GITHUB_PULL_REQUEST_COMMIT}" ]]; then
+    echo "checking changes with 'git diff ${KOKORO_GITHUB_PULL_REQUEST_TARGET_BRANCH}...${KOKORO_GITHUB_PULL_REQUEST_COMMIT} -- ${files_to_check}'"
+    set +e
+    package_modified=$(git diff "${KOKORO_GITHUB_PULL_REQUEST_TARGET_BRANCH}...${KOKORO_GITHUB_PULL_REQUEST_COMMIT}" -- "${files_to_check}" | wc -l)
+    set -e
+else
+    package_modified=0
+fi
+
+if [[ "${package_modified}" -gt 0 || "$KOKORO_BUILD_ARTIFACTS_SUBDIR" == *"continuous"* ]]; then
+    echo "------------------------------------------------------------"
+    echo "Running doctest for: ${package_name}"
+    echo "------------------------------------------------------------"
+
+    export GOOGLE_CLOUD_PROJECT="bigframes-testing"
+    cd "${package_path}"
+    python3 -m nox -s cleanup -s doctest
+else
+    echo "No changes in ${package_name} and not a continuous build, skipping."
+fi