Function check_circular_dependency 

pub async fn check_circular_dependency(
    pool: &SqlitePool,
    blocking_task_id: i64,
    blocked_task_id: i64,
) -> Result<bool>

Check if adding a dependency would create a circular dependency.

This function implements a depth-first search using SQLite’s recursive CTE to detect cycles in the dependency graph.

Algorithm

To check if we can add “blocked_task depends on blocking_task”:

1. Start from blocking_task (the new prerequisite)
2. Traverse what blocking_task depends on (its blocking tasks)
3. If we ever reach blocked_task, adding this dependency would create a cycle

Example

Existing: A depends on B (stored as: blocking=B, blocked=A) Trying to add: B depends on A (would be: blocking=A, blocked=B)

Check: Does A depend on B?

• Start from A (new blocking task)
• Find what A depends on: B
• We reached B (new blocked task) → Cycle detected!

Performance

• Time complexity: O(V + E) where V is tasks and E is dependencies
• Expected: <10ms for graphs with 10,000 tasks
• Depth limit: 100 levels to prevent infinite loops

Arguments

• pool - Database connection pool
• blocking_task_id - ID of the task that must be completed first
• blocked_task_id - ID of the task that depends on the blocking task

Returns

• Ok(true) if adding this dependency would create a cycle
• Ok(false) if the dependency is safe to add
• Err if database query fails