fix: resolve binary file upload errors in non-blocking I/O

[jos-felipe]

Summary

• Fixes critical binary file upload failures with "Empty reply from server" error
• Resolves non-blocking I/O handling issues for multi-packet HTTP requests
• Enhances file upload robustness and security validation

Problem Description

Binary file uploads were failing, particularly for files larger than 4KB that get sent in multiple TCP packets. The server was incorrectly treating temporary data unavailability as connection closure, terminating uploads prematurely.

Affected scenarios:

• Large binary files (images, executables, archives)
• Files sent in multiple TCP packets
• PNG/JPG image uploads
• Any file content with binary data

Root Cause Analysis

The issue occurred in the non-blocking I/O handling:

1. First packet: HTTP headers parsed successfully
2. Subsequent packets: Body data arrives later
3. Bug: recv() returns 0 (no data available yet)
4. Incorrect handling: Server treats this as "client closed connection"
5. Result: Connection terminated → upload fails

Technical Solution

Enhanced Connection State Management

• Added _connectionError flag to HttpRequest class
• Added hasConnectionError() method to distinguish between:
  • Actual connection closure (should terminate)
  • Temporary data unavailability (should wait)

Improved Non-blocking I/O Logic

• Smart handling of recv() == 0 based on current parsing state
• Keep connections open when expecting more body data
• Only close connections on real errors or at request boundaries

Additional Improvements

• Robust multipart parsing: Enhanced boundary extraction with quote handling
• Secure filename parsing: Path traversal protection and validation
• Directory validation: Check upload directory exists before file operations
• Consistent responses: Unified redirect to success page

Files Changed

• include/HttpRequest.hpp: Added connection error tracking
• src/http/HttpRequest.cpp: Enhanced I/O handling + upload improvements
• src/server/Server.cpp: Improved connection management logic

Testing Results

All test scenarios now pass with perfect binary integrity:

✅ Small binary files (< 4KB): Work correctly
✅ Large binary files (10KB+): Handle multi-packet uploads successfully
✅ PNG/image files: Upload with perfect integrity (MD5 verified)
✅ Text files: Maintain backward compatibility
✅ Security: Path traversal protection and directory validation

Test Plan

• Test small binary file upload (< 4KB)
• Test large binary file upload (10KB+)
• Test PNG/image file upload
• Test text file upload (regression test)
• Verify MD5 checksums match (no corruption)
• Test upload directory validation
• Test filename security validation
• Verify success page redirect works

Impact

• High: Fixes critical file upload functionality
• User Experience: Eliminates upload failures for binary files
• Compatibility: Maintains backward compatibility for existing functionality
• Security: Adds additional validation layers

This fix resolves a fundamental issue with binary file uploads while enhancing overall upload security and robustness.

[Adedayo-Sanni]

Accepted. Great work on verifying if the connection is complete!