Introduce bash scripting unittests (#724)

### Description of the change

This adds an environment for unit testing our bash scripts, using [BATS](https://github.com/bats-core/bats-core).
It implements first tests for `config_environment.sh`.

### Benefits

Writing unit tests for bash scripts documents the expected behavior and allows it being a quality gate in our CI.

### Possible drawbacks

Not everyone is familiar with this approach and unit testing framework. Me neither, it took me some hours to get into it.

### Applicable issues

- Related to #691 where an issue in `config_environment.sh` was detected. It doesn't fixes it yet. This will be a dedicated Pull Request.

### Additional information

I've verified that the changes for Renovate are indeed working.

You may wonder why there is only one `run $PROJECT_ROOT/scripts/init-containers/config/config_environment.sh` and many `run execute_test_script` calls.
Usually, testing a script itself would be executing `run $PROJECT_ROOT/scripts/init-containers/config/config_environment.sh`. You then can assert the exit code and other things. Since the `config_environment.sh` exports environment variables and we are not able to access them from outside a `run` execution, the function `execute_test_script` wraps our script execution between environment comparison. Doing so allows us capture environment variables that were added/removed during script execution.

Reviewed-on: https://gitea.com/gitea/helm-chart/pulls/724
Reviewed-by: pat-s <pat-s@noreply.gitea.com>
Co-authored-by: justusbunsi <sk.bunsenbrenner@gmail.com>
Co-committed-by: justusbunsi <sk.bunsenbrenner@gmail.com>

unittests/bash/test_helper/common-setup.bash

@@ -0,0 +1,7 @@
+#!/usr/bin/env bash
+
+function common_setup() {
+  load "$TEST_ROOT/test_helper/bats-support/load"
+  load "$TEST_ROOT/test_helper/bats-assert/load"
+  load "$TEST_ROOT/test_helper/bats-mock/stub"
+}

unittests/bash/tests/init-containers/config/config_environment.bats

@@ -0,0 +1,204 @@
+#!/usr/bin/env bats
+
+function setup() {
+  PROJECT_ROOT="$(git rev-parse --show-toplevel)"
+  TEST_ROOT="$PROJECT_ROOT/unittests/bash"
+  load "$TEST_ROOT/test_helper/common-setup"
+  common_setup
+
+  export GITEA_APP_INI="$BATS_TEST_TMPDIR/app.ini"
+  export TMP_EXISTING_ENVS_FILE="$BATS_TEST_TMPDIR/existing-envs"
+  export ENV_TO_INI_MOUNT_POINT="$BATS_TEST_TMPDIR/env-to-ini-mounts"
+
+  stub gitea \
+      "generate secret INTERNAL_TOKEN : echo 'mocked-internal-token'" \
+      "generate secret SECRET_KEY : echo 'mocked-secret-key'" \
+      "generate secret JWT_SECRET : echo 'mocked-jwt-secret'" \
+      "generate secret LFS_JWT_SECRET : echo 'mocked-lfs-jwt-secret'"
+}
+
+function teardown() {
+  unstub gitea
+  # This condition exists due to https://github.com/jasonkarns/bats-mock/pull/37 being still open
+  if [ $ENV_TO_INI_EXPECTED -eq 1 ]; then
+    unstub environment-to-ini
+  fi
+}
+
+# This function exists due to https://github.com/jasonkarns/bats-mock/pull/37 being still open
+function expect_environment_to_ini_call() {
+  export ENV_TO_INI_EXPECTED=1
+  stub environment-to-ini \
+    "-o $GITEA_APP_INI : echo 'Stubbed environment-to-ini was called!'"
+}
+
+function execute_test_script() {
+  currentEnvsBefore=$(env | sort)
+  source $PROJECT_ROOT/scripts/init-containers/config/config_environment.sh
+  local exitCode=$?
+  currentEnvsAfter=$(env | sort)
+
+  # diff as unified +/- output without context before/after
+  diff --unified=0 <(echo "$currentEnvsBefore") <(echo "$currentEnvsAfter")
+
+  exit $exitCode
+}
+
+function write_mounted_file() {
+  # either "inlines" or "additionals"
+  scope="${1}"
+  file="${2}"
+  content="${3}"
+
+  mkdir -p "$ENV_TO_INI_MOUNT_POINT/$scope/..data/"
+  echo "${content}" > "$ENV_TO_INI_MOUNT_POINT/$scope/..data/$file"
+  ln -sf "$ENV_TO_INI_MOUNT_POINT/$scope/..data/$file" "$ENV_TO_INI_MOUNT_POINT/$scope/$file"
+}
+
+@test "works as expected when nothing is configured" {
+  expect_environment_to_ini_call
+  run $PROJECT_ROOT/scripts/init-containers/config/config_environment.sh
+
+  assert_success
+  assert_line '...Initial secrets generated'
+  assert_line 'Reloading preset envs...'
+  assert_line '=== All configuration sources loaded ==='
+  assert_line 'Stubbed environment-to-ini was called!'
+}
+
+@test "exports initial secrets" {
+  expect_environment_to_ini_call
+  run execute_test_script
+
+  assert_success
+  assert_line '+GITEA__OAUTH2__JWT_SECRET=mocked-jwt-secret'
+  assert_line '+GITEA__SECURITY__INTERNAL_TOKEN=mocked-internal-token'
+  assert_line '+GITEA__SECURITY__SECRET_KEY=mocked-secret-key'
+  assert_line '+GITEA__SERVER__LFS_JWT_SECRET=mocked-lfs-jwt-secret'
+}
+
+@test "does NOT export initial secrets when app.ini already exists" {
+  expect_environment_to_ini_call
+  touch $GITEA_APP_INI
+
+  run execute_test_script
+
+  assert_success
+  assert_line --partial 'An app.ini file already exists.'
+  refute_line '+GITEA__OAUTH2__JWT_SECRET=mocked-jwt-secret'
+  refute_line '+GITEA__SECURITY__INTERNAL_TOKEN=mocked-internal-token'
+  refute_line '+GITEA__SECURITY__SECRET_KEY=mocked-secret-key'
+  refute_line '+GITEA__SERVER__LFS_JWT_SECRET=mocked-lfs-jwt-secret'
+}
+
+@test "ensures that preset environment variables take precedence over auto-generated ones" {
+  expect_environment_to_ini_call
+  export GITEA__OAUTH2__JWT_SECRET="pre-defined-jwt-secret"
+
+  run execute_test_script
+
+  assert_success
+  refute_line '+GITEA__OAUTH2__JWT_SECRET=mocked-jwt-secret'
+}
+
+@test "ensures that preset environment variables take precedence over mounted ones" {
+  expect_environment_to_ini_call
+  export GITEA__OAUTH2__JWT_SECRET="pre-defined-jwt-secret"
+  write_mounted_file "inlines" "oauth2" "$(cat << EOF
+JWT_SECRET=inline-jwt-secret
+EOF
+)"
+
+  run execute_test_script
+
+  assert_success
+  refute_line '+GITEA__OAUTH2__JWT_SECRET=mocked-jwt-secret'
+  refute_line '+GITEA__OAUTH2__JWT_SECRET=inline-jwt-secret'
+}
+
+@test "ensures that additionals take precedence over inlines" {
+  expect_environment_to_ini_call
+  write_mounted_file "inlines" "oauth2" "$(cat << EOF
+JWT_SECRET=inline-jwt-secret
+EOF
+)"
+  write_mounted_file "additionals" "oauth2" "$(cat << EOF
+JWT_SECRET=additional-jwt-secret
+EOF
+)"
+
+  run execute_test_script
+
+  assert_success
+  refute_line '+GITEA__OAUTH2__JWT_SECRET=mocked-jwt-secret'
+  refute_line '+GITEA__OAUTH2__JWT_SECRET=inline-jwt-secret'
+  assert_line '+GITEA__OAUTH2__JWT_SECRET=additional-jwt-secret'
+}
+
+@test "ensures that dotted/dashed sections are properly masked" {
+  expect_environment_to_ini_call
+  write_mounted_file "inlines" "repository.pull-request" "$(cat << EOF
+WORK_IN_PROGRESS_PREFIXES=WIP:,[WIP]
+EOF
+)"
+
+  run execute_test_script
+
+  assert_success
+  assert_line '+GITEA__REPOSITORY_0X2E_PULL_0X2D_REQUEST__WORK_IN_PROGRESS_PREFIXES=WIP:,[WIP]'
+}
+
+###############################################################
+##### THIS IS A BUG, BUT I WANT IT TO BE COVERED BY TESTS #####
+###############################################################
+@test "ensures uppercase section and setting names (🐞)" {
+  expect_environment_to_ini_call
+  export GITEA__oauth2__JwT_Secret="pre-defined-jwt-secret"
+  write_mounted_file "inlines" "repository.pull-request" "$(cat << EOF
+WORK_IN_progress_PREFIXES=WIP:,[WIP]
+EOF
+)"
+
+  run execute_test_script
+
+  assert_success
+  assert_line '+GITEA__REPOSITORY_0X2E_PULL_0X2D_REQUEST__WORK_IN_PROGRESS_PREFIXES=WIP:,[WIP]'
+  assert_line '+GITEA__OAUTH2__JWT_SECRET=pre-defined-jwt-secret'
+}
+
+@test "treats top-level configuration as section-less" {
+  expect_environment_to_ini_call
+  write_mounted_file "inlines" "_generals_" "$(cat << EOF
+APP_NAME=Hello top-level configuration
+RUN_MODE=dev
+EOF
+)"
+
+  run execute_test_script
+
+  assert_success
+  assert_line '+GITEA____APP_NAME=Hello top-level configuration'
+  assert_line '+GITEA____RUN_MODE=dev'
+}
+
+@test "fails on invalid setting" {
+  write_mounted_file "inlines" "_generals_" "$(cat << EOF
+some random invalid string
+EOF
+)"
+
+  run execute_test_script
+
+  assert_failure
+}
+
+@test "treats empty setting name as invalid setting" {
+  write_mounted_file "inlines" "_generals_" "$(cat << EOF
+=value
+EOF
+)"
+
+  run execute_test_script
+
+  assert_failure
+}