Add thread safety context

context/thread-safety.md

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+# Thread safety
+
+## Critical Context
+
+### Key Principles
+
+- **Data corruption the primary concern** - prevention is absolutely critical
+- Assume that code will be executed concurrently by multiple threads/processes
+- Assume that code may be suspended and resumed across fiber boundaries
+- **Fibers and threads are NOT the same thing**
+  - They do however share safety requirements
+- **Shared mutable state should be avoided**
+- **C extensions e.g. C/Rust etc. can block the fiber scheduler entirely**
+
+## Understanding Fibers vs Threads in Ruby
+
+### Fibers
+
+- **Cooperative multitasking** within a single thread
+- **Explicit yield points** (I/O operations, `Fiber.yield`)
+- **Separate stack** but shared heap memory within the same thread
+- **Faster context switching** than threads (lightweight ~4KB per fiber)
+- **No preemption** - code runs until it yields
+- **No true parallelism** - all fibers run sequentially within their thread
+
+### Threads
+
+- **Preemptive multitasking** with native OS threads
+- **Can be interrupted** at any point by the scheduler
+- **Separate stacks** but shared heap memory
+- **Heavier context switching** and memory usage
+- **Automatic execution** - OS scheduler manages thread execution
+- **Limited parallelism in MRI Ruby** due to Global Interpreter Lock (GIL/GVL):
+  - True parallelism only during I/O operations (GIL released)
+  - True parallelism for C extensions that release the GIL
+
+## Common patterns with potential issues
+
+1. **Memoization patterns** (`||=`)
+   - This is NOT atomic and is problematic with class variables and shared mutable data
+2. **Class and module variables** (`@@variable`, `class_attribute`)
+   - Should never be mutated if used
+   - Should be treated as generally problematic
+3. **Shared mutable state** (class instance variables accessed by multiple threads/fibers)
+   - AVOID
+4. **Lazy initialization**
+   - Especially on shared mutable sate
+5. **Hash and array mutations on shared objects**
+6. **C extensions that don't respect the fiber scheduler**
+7. **Thread-local storage**
+   - When using a fiber scheduler
+8. **Synchronization mechanisms** (Mutex, ReadWriteLock)
+   - Beware of deadlocks
+9. **Concurrent data structures**
+   - e.g. `Concurrent::Set` if available can reduce risk of thread safety issues
+   - This approach, like all approaches has trade offs
+   - Can only be used if the concurrent gem is available
+
+## Common unsafe patterns
+
+### 1. Memoization with `||=` on shared data
+
+```ruby
+class Foo
+  def self.bar
+    # Issue: Two threads can both see @data and can modify it without
+    # the other knowing
+    @data ||= fetch_next_fizzbuzz_from_fizzbuzz_api
+  end
+end
+```
+
+**Why is this problematic?**: Multiple threads can pass `nil?` checks simultaneously on shared mutable data. Best case this wastes resources, worst case the wrong data is used or data corruption occurs.
+
+**Potential fix with mutex** (WARNING: locks like mutex could cause deadlocks)
+
+```ruby
+class Foo
+  @mutex = Mutex.new
+
+  def self.bar
+    @mutex.synchronize do
+      return @data if defined?(@data)
+        @data = fetch_next_fizzbuzz_from_fizzbuzz_api
+      end
+  end
+end
+```
+
+**Concurrent::Map example** - Only available if the gem is available
+
+```ruby
+class Foo
+  @cache = Concurrent::Map.new
+
+  def self.bar(key:)
+    # Issue: Two threads can both see @cache and both can modify it without
+    # the other knowing
+    @cache.compute_if_absent(key) { expensive_operation(key) }
+  end
+end
+```
+
+**Safe if instances are not shared**
+
+```ruby
+class Foo
+  def expensive_operation
+    @result ||= calculate # Each instance has own @result
+  end
+end
+```
+
+
+### 2. Class Variables with shared state
+
+```ruby
+class GlobalConfig
+  @@settings = {} # Issue: Class variables are shared across inheritance
+  @settings = {} # Issue: Shared mutable state without synchronization
+end
+```
+
+**Why is this problematic?**: Class variables affect the entire inheritance hierarchy, shared mutable state causes race conditions.
+
+**Better alternatives:**
+
+- Use dependency injection for runtime configuration
+- Use `Concurrent::Hash.new` if shared state is required and the gem is available
+- Simply avoid if possible
+
+## Fiber safety
+
+**Key Difference**: When a fiber yields during I/O, another fiber may access the shared state.
+
+**Problematic - Thread.current in specific cases e.g. request scoped data when paired with servers like Falcon:**
+
+```ruby
+class SomeRequestSpecificData
+  def track_request(some_request_specific_data)
+    # Issue: Thread.current can leak between requests
+    Thread.current[:some_request_specific_data] = some_request_specific_data
+    external_api_call  # Fiber yields, other requests may be processed
+    log("Processing #{Thread.current[:some_request_specific_data]}")  # May have incorrect data!
+  end
+end
+```
+
+**Why is this problematic?**: When a fiber yields during I/O, a fiber based web server may process other requests causing data to become corrupted.
+
+**Better - Use Fiber storage for request data:**
+
+```ruby
+class SomeRequestSpecificData
+  def track_request(some_request_specific_data)
+    Fiber[:some_request_specific_data] = some_request_specific_data
+    external_api_call  # Fiber yields
+    log("Processing #{Fiber[:some_request_specific_data]}")
+  end
+end
+```
+
+**Acceptable - Thread.current for thread-level concerns:**
+
+```ruby
+def some_non_changing_config
+    Thread.current[:some_non_changing_config] || default_config
+end
+```
+
+## Essential Safe Patterns
+
+### 1. Mutex for Critical Sections
+
+```ruby
+class ResourcePool
+  def initialize
+    @resources = []
+    @mutex = Mutex.new
+  end
+
+  def checkout
+    @mutex.synchronize { @resources.pop || create_resource }
+  end
+
+  def checkin(resource)
+    @mutex.synchronize { @resources.push(resource) }
+  end
+end
+```
+
+### 2. Concurrent Data Structures (if available)
+
+Examples:
+
+```ruby
+@cache = Concurrent::Map.new
+@config = Concurrent::Hash.new
+@enabled = Concurrent::AtomicBoolean.new(false)
+```
+
+### 3. Request-Scoped storage when working with web servers
+
+```ruby
+# Use Fiber[:key] for request-scoped data
+Fiber[:user_context] = current_user
+
+# Access later
+def some_controller_method
+  Fiber[:user_context]
+end
+```
+
+## When `||=` can be safe
+
+### 1. Instance variables on unshared objects
+
+When each instance is only accessed by a single thread/fiber:
+
+```ruby
+class RequestHandler
+  def process_request
+    @parser ||= create_parser # Safe: each request has its own handler instance
+  end
+end
+```
+
+### 2. Synchronization primitives
+
+Synchronization objects are designed for concurrent access, so duplicate creation is wasteful but not harmful:
+
+```ruby
+class Task
+  def wait
+    @condition ||= Condition.new # Safe: condition variables handle concurrent access
+    @condition.wait
+  end
+end
+```
+
+**Why this is acceptable**:
+- Condition variables, mutexes, and semaphores intended for protecting critical sections
+- Synchronization still works correctly
+
+### 3. Immutable or effectively immutable objects
+
+```ruby
+class Calculator
+  def pi
+    @pi ||= Math::PI # Safe: immutable value
+  end
+
+  def default_config
+    @config ||= Config.new.freeze # Safe: frozen object
+  end
+end
+```
+
+## Performance Considerations
+
+### Synchronization Overhead
+
+| Mechanism | Use Case | Performance Impact |
+| :---- | :---- | :---- |
+| Mutex | General purpose locking | Medium overhead, can cause contention |
+| ReadWriteLock | Read-heavy workloads | Better for many readers, few writers |
+| Concurrent::* | Lock-free operations | Generally faster, but higher memory usage |
+| Atomic operations | Simple counters/flags | Fastest for simple operations |
+
+### Choosing the Right Approach
+
+1. **No shared state** > **Immutable shared state** > **Synchronized mutable state**
+2. **Lock-free (Concurrent::*)** > **Fine-grained locks** > **Coarse-grained locks**
+
+## Quick reference
+
+### Thread Safety
+
+| Unsafe Pattern | Safe Alternative |
+| :---- | :---- |
+| `@data ||= expensive_operation` | `@mutex.synchronize { @data ||= expensive_operation }` |
+| `@@class_var` | dependency injection |
+| `@shared_array << item` | `@concurrent_set.add(item)` if available or mutex |
+| Nested mutex acquisition | Consistent lock ordering |
+
+### Fiber Safety
+
+| Unsafe Pattern | Safe Alternative |
+| :---- | :---- |
+| `Thread.current[:some_request_based_id] = id` | `Fiber[:some_request_based_id] = id` |
+| `@cache ||= {}` (in shared objects) | `Fiber[:cache] ||= {}` |
+
+**Note**: `Thread.current` is ok for thread-level concerns like debugging
+
+## Summary
+
+1. **Prevent data corruption** - Primary production concern
+2. **Avoid shared mutable state**
+3. **Request isolation** - **Always use `Fiber[:key]` for request-scoped data**
+4. **Be aware of performance trade-offs** - Synchronization has costs
+
+### Key Takeaways
+
+- **Fibers != Threads**: Different models but **similar safety requirements**
+- **Avoid shared mutable state**: Use dependency injection or immutable objects
+- **Prevention over testing**: Write inherently thread-safe code
+- **Lock ordering matters**: Prevent deadlocks with consistent acquisition order
+- **Snapshot concurrent collections**: Before iteration to avoid inconsistencies
+
+**Critical**: Data corruption from race conditions is the primary concern. Prevention through proper design is a requirement.