fix return value for detecting yarn workspace plugin

CHANGELOG.md

@@ -3,6 +3,7 @@
 ## main
 - Prune devDependencies with Yarn 2 ([#891](https://github.com/heroku/heroku-buildpack-nodejs/pull/891))
 - Add 10.x, 12.x, and 14.x to inventory ([902](https://github.com/heroku/heroku-buildpack-nodejs/pull/902))
+- Fix yarn workspace plugin detection ([#905](https://github.com/heroku/heroku-buildpack-nodejs/pull/905))
 
 ## v183 (2020-02-03)
 - Update node version to 14 ([#885](https://github.com/heroku/heroku-buildpack-nodejs/pull/885))

lib/dependencies.sh

@@ -175,6 +175,7 @@ yarn_prune_devdependencies() {
     cd "$build_dir" || return
 
     if has_yarn_workspace_plugin_installed "$build_dir"; then
+      echo "Running 'yarn workspaces focus --all --production'"
       meta_set "workspace-plugin-present" "true"
 
       # The cache is removed beforehand because the command is running an install on devDeps, and

lib/yarn-2.sh

@@ -28,7 +28,15 @@ has_yarn_cache() {
 has_yarn_workspace_plugin_installed() {
   local build_dir="$1"
   local plugin_paths=$(get_yarn_plugin_paths "$build_dir")
-  [[ ! -n $plugin_paths ]] && echo "$plugin_paths" | grep plugin-workspace-tools
+  local path
+
+  if [[ -n $plugin_paths ]]; then
+    path=$(echo "$plugin_paths" | grep plugin-workspace-tools)
+  else
+    path=""
+  fi
+
+  [[ ! "$path" == "" ]]
 }
 
 get_yarn_plugin_paths() {

test/run

@@ -679,6 +679,7 @@ testYarn2() {
   assertCaptured "The buildpack won't cache the dependencies in .yarn/cache."
   assertCaptured "Running 'yarn install' with yarn.lock"
   assertCaptured "Version 3.9.7" # tsc output
+  assertCaptured "Running 'yarn workspaces focus --all --production'"
   assertCapturedSuccess
 }