Optimize EventReactor thread safety with shared_mutex for better concurrency

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Problem

The EventReactor class was using std::mutex for all operations, creating unnecessary lock contention in high-concurrency scenarios. Read operations like DispatchEvent() and handler lookups were blocking each other even though they could safely run concurrently.

Solution

Replaced the thread protection mechanism from std::mutex to std::shared_mutex with read-write lock semantics:

Changes Made

1. Header Updates (event_reactor.h):

   • Added #include <shared_mutex>
   • Changed std::mutex mutex_ to std::shared_mutex mutex_

2. Read Operations (use std::shared_lock for concurrent access):

   • DispatchEvent() - Multiple threads can now dispatch events concurrently
   • Run() method - Handler lookups can happen in parallel
   • ModifyHandler() - Only checks handler existence, no modification needed

3. Write Operations (use std::unique_lock for exclusive access):

   • RegisterHandler() - Exclusive access when adding handlers
   • RemoveHandler() - Exclusive access when removing handlers

Performance Benefits

• Concurrent Event Dispatching: Multiple threads can now process events simultaneously without blocking each other
• Reduced Lock Contention: Read operations no longer compete with each other for lock acquisition
• Better Scalability: Performance scales better with increased concurrent load

Example Impact

// Before: All operations serialized
std::unique_lock<std::mutex> lock(mutex_);  // Blocks all other operations

// After: Read operations can run concurrently
std::shared_lock<std::shared_mutex> lock(mutex_);  // Multiple readers allowed

Testing

• ✅ All existing unit tests pass
• ✅ TCP echo server example runs correctly
• ✅ No functional regressions detected
• ✅ Thread safety maintained

This optimization is particularly beneficial for high-throughput network applications where multiple events need to be processed concurrently.

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