fix(ios): add jitter to reconnection backoff to prevent thundering herd

[bianbiandashen]

Summary

Add jitter (randomization) to exponential backoff in iOS reconnection logic to prevent the "thundering herd" problem.

Problem

When many iOS clients disconnect simultaneously (e.g., server restart, network outage, or app background/foreground transition), they all use identical exponential backoff intervals:

Client A: reconnect at 1s, 2s, 4s, 8s, 16s...
Client B: reconnect at 1s, 2s, 4s, 8s, 16s...
Client C: reconnect at 1s, 2s, 4s, 8s, 16s...

This causes all clients to hit the server at exactly the same moments, creating load spikes that can:

• Overwhelm the server during recovery
• Cause cascading failures
• Extend the overall recovery time

Solution

Add jitter (randomization) to spread reconnection attempts:

static func calculateBackoff(
    attempt: Int,
    baseDelay: TimeInterval = 1.0,
    maxDelay: TimeInterval = 60.0,
    jitterFactor: Double = 0.3  // NEW: 30% randomization
) -> TimeInterval {
    let exponentialDelay = baseDelay * pow(2.0, Double(attempt - 1))
    let cappedDelay = min(exponentialDelay, maxDelay)
    let jitter = cappedDelay * jitterFactor * Double.random(in: 0...1)
    return cappedDelay + jitter
}

With 30% jitter on a 16s base delay:

• Without jitter: all clients retry at exactly 16s
• With jitter: clients retry between 16s and 20.8s

This spreads the load and allows the server to recover gracefully.

Additional Fix: Max Reconnect Attempts

Also added a maximum reconnect limit (10 attempts) to BufferWebSocketClient to prevent infinite retry loops when the server is permanently unavailable.

Files Changed

• ios/VibeTunnel/Services/ReconnectionManager.swift

  • Added jitterFactor parameter to calculateBackoff()
  • Updated performReconnection() to use jitter

• ios/VibeTunnel/Services/BufferWebSocketClient.swift

  • Added jitter to scheduleReconnect()
  • Added maxReconnectAttempts constant (10)
  • Added max attempts check to prevent infinite retries

Test Plan

• Verify jitter produces varied delays (not deterministic)
• Verify max attempts stops reconnection after 10 failures
• Verify reconnection still works correctly with jitter