Reduce the number of env vars exposed to subprocess (#1011)

The following env vars are no longer exposed to subprocesses run by the
buildpack (such as the `bin/pre_compile` and `bin/post_compile` hooks):

* `BPLOG_PREFIX`
* `CACHED_PYTHON_STACK`
* `DEFAULT_PYTHON_STACK`
* `DEFAULT_PYTHON_VERSION`
* `LATEST_27`
* `LATEST_34`
* `LATEST_35`
* `LATEST_36`
* `LATEST_37`
* `LATEST_38`
* `PIP_UPDATE`
* `PY27`
* `PY34`
* `PY35`
* `PY36`
* `PY37`
* `PYPY_27`
* `PYPY_36`
* `RECOMMENDED_PYTHON_VERSION`
* `WARNINGS_LOG`

There were previously no tests at all for the pre/post-compile hooks,
so I've added some now.

Fixes #1010.

test/fixtures/hooks/bin/post_compile

@@ -0,0 +1,4 @@
+set -euo pipefail
+
+echo "post_compile ran!"
+echo "post_compile env: $(printenv | cut -d '=' -f 1 | sort | xargs)."

test/fixtures/hooks/bin/pre_compile

@@ -0,0 +1,4 @@
+set -euo pipefail
+
+echo "pre_compile ran!"
+echo "pre_compile env: $(printenv | cut -d '=' -f 1 | sort | xargs)."

test/run-features

@@ -86,6 +86,49 @@ testDontWarnOldDjango() {
   assertCapturedSuccess
 }
 
+testHooks() {
+  # Test that the hooks are called correctly, and that the environment contains
+  # the app's config vars but no unexpected env vars from the buildpack.
+  local env_dir="$(mktmpdir)"
+  echo 'test' > "${env_dir}/SOME_APP_CONFIG_VAR"
+  local expected_env_vars=(
+    _
+    BIN_DIR
+    BUILD_DIR
+    BUILDPACK_LOG_FILE
+    CACHE_DIR
+    C_INCLUDE_PATH
+    CPLUS_INCLUDE_PATH
+    ENV_DIR
+    EXPORT_PATH
+    HOME
+    LANG
+    LD_LIBRARY_PATH
+    LIBRARY_PATH
+    OLDPWD
+    PATH
+    PKG_CONFIG_PATH
+    PROFILE_PATH
+    PWD
+    PYTHONUNBUFFERED
+    SHLVL
+    SOME_APP_CONFIG_VAR
+    STACK
+    VENDOR_URL
+  )
+  if [[ "${STACK}" == "cedar-14" || "${STACK}" == "heroku-16" ]]; then
+    # Remove "OLDPWD" from expected_env_vars since for bash <4.4 it's not exported to subshells:
+    # https://github.com/heroku/heroku-buildpack-python/pull/1011#issuecomment-665117835
+    read -ra expected_env_vars <<< "${expected_env_vars[@]/OLDPWD/}"
+  fi
+  compile 'hooks' '' "${env_dir}"
+  assertCaptured "pre_compile ran!"
+  assertCaptured "pre_compile env: ${expected_env_vars[*]}."
+  assertCaptured "post_compile ran!"
+  assertCaptured "post_compile env: ${expected_env_vars[*]}."
+  assertCapturedSuccess
+}
+
 pushd $(dirname 0) >/dev/null
 popd >/dev/null