Implement fairness mechanism to prevent high-priority frame starvation in ConnectionActor

[coderabbitai]

Currently, high-priority frames can starve low-priority ones under constant load due to the biased select! loop. Consider implementing a configurable fairness mechanism or occasional yield to the low-priority branch to ensure balanced processing.

Context:
This improvement was identified during review of the ConnectionActor implementation.

Reference:

• PR: Implement connection actor with prioritised write loop #129
• Comment: Implement connection actor with prioritised write loop #129 (comment)

Requested by: @leynos

[leynos]

@coderabbitai is this issue still relevant?

[coderabbitai]

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🧩 Analysis chain

🏁 Script executed:

#!/bin/bash
# Get the details of GitHub issue #132
gh issue view 132 --json title,body,state,createdAt,updatedAt,labels,assignees,comments

Length of output: 1324

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🏁 Script executed:

#!/bin/bash
# Search for ConnectionActor implementation files
fd -e rs ConnectionActor

# Also search for files containing ConnectionActor
rg "ConnectionActor" --files-with-matches

# Look for select! macros in the codebase that might be related to the issue
rg "select!" -A 5 -B 2

Length of output: 24037

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🏁 Script executed:

#!/bin/bash
# Examine the connection.rs file more closely to see if fairness mechanisms are implemented
cat src/connection.rs

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@leynos No, this issue is no longer relevant. The fairness mechanism to prevent high-priority frame starvation has already been fully implemented in the ConnectionActor.

Current Implementation

The src/connection.rs file now includes a comprehensive fairness system:

1. Configurable Fairness Settings

pub struct FairnessConfig {
    pub max_high_before_low: usize,  // Default: 8
    pub time_slice: Option<Duration>, // Default: None
}

2. Fairness Logic

• After processing 8 consecutive high-priority frames (configurable), the system yields to check the low-priority queue
• Optional time-based yielding is also supported
• The after_high() method opportunistically drains one low-priority frame when yield conditions are met

3. Implementation Details

• Counter tracking: high_counter and high_start fields
• should_yield_to_low_priority() method evaluates both thresholds
• try_recv() is used for non-blocking low-priority frame processing during yields
• Counters reset after processing any low-priority frame

4. Public API

• set_fairness() method allows runtime configuration of fairness parameters

Evidence of Completion

The roadmap document confirms this feature was completed:

- [x] **Fairness counter** to yield to the low-priority queue after bursts of 
  high-priority frames ([Design §3.2.1][design-fairness]).

Recommendation: This issue can be closed as the requested fairness mechanism has been implemented and addresses the exact starvation problem described in the original issue.