Allow user opt in to build script default behavior (#585)

This checks the user's package.json for a heroku-run-build-script key. If it is set to true the new build step will be run instead of the old step, and a message will be printed at the beginning of the build to confirm that is the case.

bin/compile

@@ -62,9 +62,6 @@ trap 'handle_failure' ERR
 [ -f "$BUILD_DIR/yarn.lock" ] && YARN=true || YARN=false
 [ -f "$BUILD_DIR/package-lock.json" ] && NPM_LOCK=true || NPM_LOCK=false
 
-### Behavior flags
-[ ! "$NEW_BUILD_SCRIPT_BEHAVIOR" ] && NEW_BUILD_SCRIPT_BEHAVIOR=false
-
 ### Failures that should be caught immediately
 
 fail_dot_heroku "$BUILD_DIR"
@@ -74,6 +71,10 @@ fail_multiple_lockfiles "$BUILD_DIR"
 warn_prebuilt_modules "$BUILD_DIR"
 warn_missing_package_json "$BUILD_DIR"
 
+### Behavior flags
+[ ! "$NEW_BUILD_SCRIPT_BEHAVIOR" ] && NEW_BUILD_SCRIPT_BEHAVIOR=$(read_json "$BUILD_DIR/package.json" ".[\"heroku-run-build-script\"]")
+warn_build_script_behavior_opt_in $NEW_BUILD_SCRIPT_BEHAVIOR | output "$LOG_FILE"
+
 ### Compile
 
 create_env() {

lib/dependencies.sh

@@ -46,6 +46,17 @@ run_build_script() {
   fi
 }
 
+warn_build_script_behavior_opt_in() {
+  local opted_in="$1"
+  if [[ "$opted_in" = true ]]; then
+    header "Opting in to new default build script behavior"
+    echo "You have set \"heroku-run-build-script\" = true in your package.json"
+    echo ""
+    echo "- If a \"build\" script is defined in package.json it will be executed by default"
+    echo "- The \"heroku-postbuild\" script will be executed instead if present"
+  fi
+}
+
 log_build_scripts() {
   local build=$(read_json "$BUILD_DIR/package.json" ".scripts[\"build\"]")
   local heroku_prebuild=$(read_json "$BUILD_DIR/package.json" ".scripts[\"heroku-prebuild\"]")

test/fixtures/build-script-opt-in/README.md

+A fake README, to keep npm from polluting stderr.
\ No newline at end of file

test/fixtures/build-script-opt-in/package.json

+{
+  "name": "node-buildpack-test-app",
+  "version": "0.0.1",
+  "description": "node buildpack integration test app",
+  "repository" : {
+    "type" : "git",
+    "url" : "http://github.com/example/example.git"
+  },
+  "engines": {
+    "node": "10.x"
+  },
+  "scripts" : {
+    "build" : "echo build hook message"
+  },
+  "heroku-run-build-script": true
+}

test/run

@@ -26,6 +26,13 @@ testBuildScriptBehavior() {
   assertCapturedSuccess
 }
 
+testBuildScriptOptIn() {
+  compile "build-script-opt-in"
+  assertCaptured "Running build"
+  assertCaptured "Opting in to new default build script behavior"
+  assertCapturedSuccess
+}
+
 testPrePostBuildScripts() {
   compile "pre-post-build-scripts"
   assertCaptured "Running heroku-prebuild"