Introduction

Ruby 3's Fiber scheduler allows writing non-blocking concurrent code without explicit async/await syntax. However, when code performs a blocking operation that the scheduler cannot intercept (like a native C extension call or unsupported I/O), the scheduler raises an error or the fiber hangs indefinitely. Not all gems are Fiber-scheduler-aware, and mixing blocking code with async code causes subtle failures.

Symptoms

  • Fiber scheduler blocked on non-fiber-aware code warning
  • Fiber hangs indefinitely on HTTP requests or database queries
  • ThreadError: Fiber scheduling not supported in certain contexts
  • Mixed blocking and non-blocking code causes deadlock
  • Async::IO gem reports unsupported operation

Debug with: ```ruby # Enable scheduler warnings $VERBOSE = true

# Set scheduler Fiber.set_scheduler(MyScheduler.new)

Fiber.schedule do # This may hang if the gem is not fiber-aware Net::HTTP.get(URI("https://example.com")) end ```

Common Causes

  • Using gems that perform blocking I/O without Fiber scheduler support
  • Native C extensions that do not release GVL (Global VM Lock)
  • Mixing Thread-based concurrency with Fiber scheduling
  • Database adapter not Fiber-aware (older ActiveRecord versions)
  • File I/O operations not supported by scheduler

Step-by-Step Fix

  1. 1.Use Fiber-aware HTTP client instead of Net::HTTP:
  2. 2.```ruby
  3. 3.# WRONG - Net::HTTP may not be fully Fiber-aware in all versions
  4. 4.Fiber.schedule do
  5. 5.Net::HTTP.get(URI("https://api.example.com/data"))
  6. 6.end

# CORRECT - Use async-compatible HTTP client require 'async' require 'async/http'

Async do endpoint = Async::HTTP::Endpoint.for("https://api.example.com") client = Async::HTTP::Client.new(endpoint) response = client.get("/data") puts response.read end ```

  1. 1.Configure Rails with Fiber-aware database adapter:
  2. 2.```ruby
  3. 3.# Gemfile - use Fiber-aware adapter
  4. 4.gem 'activerecord-fiber-adapter' # If available for your DB

# Or use connection pool with async # config/database.yml production: adapter: postgresql pool: <%= ENV.fetch("RAILS_MAX_THREADS") { 5 } %> checkout_timeout: 5

# In an Async task, ensure DB connections are managed Async do ActiveRecord::Base.connection_pool.with_connection do User.where(active: true).find_each do |user| # Process user in fiber end end end ```

  1. 1.Handle blocking operations with Thread fallback:
  2. 2.```ruby
  3. 3.# For operations that are not Fiber-aware, use Thread
  4. 4.Fiber.schedule do
  5. 5.# Non-blocking operations here
  6. 6.fetch_data_from_api

# Fallback to Thread for blocking operations result = Thread.new do # Blocking operation HeavyNativeLibrary.process_file(large_file_path) end.value

# Continue with non-blocking operations process_result(result) end ```

  1. 1.Use async gem for proper Fiber scheduling:
  2. 2.```ruby
  3. 3.# Gemfile
  4. 4.gem 'async'
  5. 5.gem 'async-http'
  6. 6.gem 'async-websocket'

# Application code require 'async'

class DataFetcher def fetch_all(urls) Async do |task| # Spawn a fiber for each URL urls.map do |url| task.async do fetch_single(url) end end.map(&:wait) # Wait for all to complete end end

private

def fetch_single(url) # async-http is Fiber-aware Async::HTTP::Internet.new.get(url) end end ```

  1. 1.Check if a gem is Fiber-scheduler-aware:
  2. 2.```ruby
  3. 3.# Check if a method supports Fiber scheduling
  4. 4.def fiber_aware?(klass, method_name)
  5. 5.method = klass.instance_method(method_name)
  6. 6.source = method.source_location
  7. 7.return false unless source

# Check if it uses Fiber.scheduler File.read(source[0]).include?('Fiber.scheduler') rescue false end

# Common Fiber-aware gems: # - async, async-http, async-websocket # - falcon (web server) # - database_cleaner (for test isolation) # - redis 4.8+ with hiredis-client ```

Prevention

  • Audit gems for Fiber scheduler compatibility before adopting Ruby 3 concurrency
  • Use the async gem ecosystem for HTTP, WebSocket, and DNS operations
  • Test concurrent code with Async::Test in your test suite
  • Avoid mixing Thread.new and Fiber.schedule in the same codebase
  • Monitor fiber execution time to detect blocking operations
  • Use Fiber.scheduler hooks to log and detect unsupported operations