Description

The refactor introduced in commit `8c74d0718dc11ed5` removed the `ThreadGroup.max_threads` attribute and replaced it with `thread_pool_size`. Several OpenStack services, notably Heat, still rely on the old attribute and fail when upgrading to oslo.service >= 4.4.0.

This regression leads to cascading failures across dependent services — growing in impact much like a Fibonacci sequence: each failing component amplifies issues in others, resulting in an increasingly complex upgrade path.

A patch is now proposed to restore `max_threads` as a read-only, deprecated property that maps to `thread_pool_size` and emits a `DeprecationWarning`. Applications are expected to migrate to the new attribute.

Impact

• Breaks compatibility for services still using `ThreadGroup.max_threads`
• Prevents smooth upgrades to oslo.service 4.4.0 and later
• Causes service startup failures (e.g., Heat)
• Introduces cumulative dependency issues (Fibonacci-like propagation)

Proposed Fix

• Reintroduce `max_threads` as an alias to `thread_pool_size`
• Emit `DeprecationWarning` to encourage migration
• Document the change in release notes
• Plan removal of the deprecated attribute in a future cycle

Steps to Reproduce

1. Upgrade to oslo.service >= 4.4.0
2. Start a service still referencing `ThreadGroup.max_threads`
3. Observe `AttributeError` on startup

Expected Behavior

Existing services continue functioning using `max_threads` until they migrate to `thread_pool_size`.

Environment

• oslo.service 4.4.0+
• Services depending on legacy `ThreadGroup.max_threads`