Avoid selecting unusable watch binstubs

Author: ihabadham

react_on_rails/lib/generators/react_on_rails/generator_helper.rb

@@ -492,12 +492,13 @@ def detect_react_version
     nil
   end
 
-  # Prefer Shakapacker's optional watcher binstub when the application has it. Older
+  # Prefer Shakapacker's optional watcher binstub when the application has an executable copy. Older
   # supported Shakapacker installations can still run watch mode through the required
   # bin/shakapacker binstub, so React on Rails does not need to vendor a fallback watcher.
   def shakapacker_watch_command
     watch_binstub = File.join(destination_root, "bin/shakapacker-watch")
-    executable = File.exist?(watch_binstub) ? "bin/shakapacker-watch" : "bin/shakapacker"
+    watch_binstub_available = File.file?(watch_binstub) && File.executable?(watch_binstub)
+    executable = watch_binstub_available ? "bin/shakapacker-watch" : "bin/shakapacker"
 
     "#{executable} --watch"
   end

react_on_rails/spec/react_on_rails/generators/install_generator_spec.rb

@@ -520,15 +520,15 @@ def run_add_agent_files(options = {})
       run_generator_test_with_args(%w[], package_json: true) do
         simulate_preinstalled_shakapacker(source_path: "app/javascript", source_entry_path: "packs")
         simulate_existing_file("bin/shakapacker-watch", shakapacker_watch)
-        File.chmod(0o640, File.join(destination_root, "bin/shakapacker-watch"))
+        File.chmod(0o750, File.join(destination_root, "bin/shakapacker-watch"))
       end
     end
 
     it "preserves Shakapacker's binstub under --force" do
       assert_file "bin/shakapacker-watch" do |content|
         expect(content).to eq(shakapacker_watch)
       end
-      expect(File.stat(File.join(destination_root, "bin/shakapacker-watch")).mode & 0o777).to eq(0o640)
+      expect(File.stat(File.join(destination_root, "bin/shakapacker-watch")).mode & 0o777).to eq(0o750)
     end
 
     it "uses Shakapacker's binstub in generated Procfiles" do

react_on_rails/spec/react_on_rails/generators/rsc_generator_spec.rb

@@ -27,13 +27,25 @@
 
     it "uses Shakapacker's watch binstub when it is present" do
       simulate_existing_file("bin/shakapacker-watch", "#!/usr/bin/env sh\n")
+      File.chmod(0o755, File.join(destination_root, "bin/shakapacker-watch"))
 
       Dir.chdir(destination_root) { generator.send(:add_rsc_to_procfile) }
 
       assert_file "Procfile.dev" do |content|
         expect(content).to include("rsc-bundle: RSC_BUNDLE_ONLY=true bin/shakapacker-watch --watch")
       end
     end
+
+    it "uses the standard Shakapacker command when the watch binstub is not executable" do
+      simulate_existing_file("bin/shakapacker-watch", "#!/usr/bin/env sh\n")
+      File.chmod(0o644, File.join(destination_root, "bin/shakapacker-watch"))
+
+      Dir.chdir(destination_root) { generator.send(:add_rsc_to_procfile) }
+
+      assert_file "Procfile.dev" do |content|
+        expect(content).to include("rsc-bundle: RSC_BUNDLE_ONLY=true bin/shakapacker --watch")
+      end
+    end
   end
 
   # Unit tests for prerequisite validation