Generator doesn't configure precompile_hook in shakapacker.yml when Shakapacker is pre-installed

[ihabadham]

Environment

1. Ruby version: 3.3.7
2. Rails version: 8.0.4
3. Shakapacker version: 9.4.0
4. React on Rails version: 16.2.0.rc.0

Summary

When running rails generate react_on_rails:install on a fresh Rails app, the generator creates bin/shakapacker-precompile-hook but doesn't configure it in shakapacker.yml when Shakapacker is installed first (which is the normal flow).

Expected behavior

After running the generator, config/shakapacker.yml should have:

precompile_hook: 'bin/shakapacker-precompile-hook'

This is the intended configuration per PR #1916 commit message:

"This change leverages Shakapacker's new precompile_hook feature to automatically generate packs before webpack compilation, replacing the need to manually modify assets:precompile tasks or bin/dev scripts."

Actual behavior

The generator:

1. ✅ Creates bin/shakapacker-precompile-hook script
2. ❌ Leaves precompile_hook: ~ (commented out) in shakapacker.yml

$ ls -la bin/shakapacker-precompile-hook
-rwxr-xr-x 1 user user 1023 Jan 4 18:06 bin/shakapacker-precompile-hook

$ grep precompile_hook config/shakapacker.yml
  # Example: precompile_hook: 'bin/shakapacker-precompile-hook'
  # precompile_hook: ~

Root Cause

In base_generator.rb, when Shakapacker is installed first:

def copy_packer_config
  if File.exist?(".shakapacker_just_installed")
    puts "Skipping Shakapacker config copy..."
    File.delete(".shakapacker_just_installed")
    configure_rspack_in_shakapacker if options.rspack?  # rspack gets configured
    return                                               # but precompile_hook doesn't!
  end
  # ... copies template with precompile_hook configured
end

The generator has configure_rspack_in_shakapacker to modify existing shakapacker.yml for rspack, but no equivalent method for precompile_hook.

Steps to Reproduce

rails new test_app --minimal
cd test_app
# Add react_on_rails to Gemfile
bundle install
rails generate react_on_rails:install
grep precompile_hook config/shakapacker.yml
# Shows: # precompile_hook: ~

Impact

The app still works because bin/dev and adjust_precompile_task have fallback logic. But:

• The intended/optimized configuration isn't applied
• Pack generation happens later in the pipeline instead of before webpack
• Users don't get the benefit of the precompile_hook optimization

Suggested Fix

Add a configure_precompile_hook_in_shakapacker method similar to configure_rspack_in_shakapacker that adds the precompile_hook line to existing shakapacker.yml files.

[ihabadham]

Correction to Impact Section

After further investigation, the impact is more severe than originally described. The adjust_precompile_task fallback does NOT apply for standard Shakapacker flow.

Looking at configuration.rb:347:

return if skip_react_on_rails_precompile || build_production_command.blank?

It returns early when build_production_command is blank (which is the default), so the fallback logic never runs.

Actual Impact

Workflow	Without precompile_hook	With precompile_hook
bin/dev (any mode)	✅ Works (PackGenerator fallback)	✅ Works
Direct rails assets:precompile	❌ Packs NOT generated	✅ Works

This is critical for CI/CD deployments:

• Heroku deployments running assets:precompile directly will have missing component bundles
• CI/CD pipelines using rails assets:precompile without bin/dev will fail
• Only users who exclusively use bin/dev are unaffected

Evidence from reproduction

Without precompile_hook configured:

public/packs/js/
├── application-*.js
└── runtime-*.js
# NO generated/HelloWorld-*.js ❌

With precompile_hook configured:

public/packs/js/
├── application-*.js
├── runtime-*.js
└── generated/
    └── HelloWorld-*.js  ✅

Related Issue

Also found a secondary bug during investigation: #2279 (detection regex mismatch)