Fix: Race Condition in Connection Accept

[Techhackontime999]

Description

When two users simultaneously accept each other's connection requests, duplicate connections are created in the database.

Steps to Reproduce

1. User A sends connection request to User B
2. User B sends connection request to User A
3. Both users click "Accept" at nearly the same time
4. Two connection records are created instead of one

Expected Behavior

Only one connection record should exist regardless of timing. Use database-level locking or atomic operations to prevent duplicates.

Root Cause

The �ccept_request view doesn't check for an existing accepted connection before creating a new one, and there's no database constraint or lock preventing race conditions.

Files to Fix

• linkup/network/views.py - Add select_for_update() in accept view, check for existing accepted connection
• linkup/network/models.py - Add unique constraint on (user, friend) pairs
• Run python manage.py makemigrations and migrate

Acceptance Criteria

• No duplicate connections can be created
• Atomic operation prevents race condition
• Both users see each other as connected
• Database constraint enforced at schema level