Bug: Sentinel connection with AbortOnConnectFail=false fails to return multiplexer and leaks objects on startup failure

[stevestock]

Version: StackExchange.Redis 2.9.32

Description:

When calling ConnectionMultiplexer.ConnectAsync() with Sentinel configuration & AbortOnConnectFail = false, if sentinels are unavailable at startup:

1. Library creates a Sentinel ConnectionMultiplexer internally (via SentinelConnectAsync())
2. Defaults to ServerType.Standalone
3. Calls GetSentinelMasterConnection() which throws: RedisConnectionException: "Sentinel: The ConnectionMultiplexer is not a Sentinel connection. Detected as: Standalone"
4. Never returns the ConnectionMultiplexer - making it inaccessible to caller
5. Leaks the ConnectionMultiplexer - when LoggerFactory is attached, logs continue being written from the orphaned multiplexer showing ongoing retry attempts

Reproduction:

var options = new ConfigurationOptions
{
    EndPoints = { "sentinel1:26379", "sentinel2:26379", "sentinel3:26379" },
    ServiceName = "mymaster",
    AbortOnConnectFail = false,
    LoggerFactory = myLoggerFactory
};

// Start application BEFORE Redis/Sentinel servers are running
var multiplexer = await ConnectionMultiplexer.ConnectAsync(options); 
// Throws exception but multiplexer continues retrying in background (leaked)

Expected Behavior:

With AbortOnConnectFail = false, ConnectAsync() should return the ConnectionMultiplexer even when initial connection fails, allowing event handler registration and automatic reconnection.

Actual Behavior:

• Exception thrown, no multiplexer returned
• Multiplexer created internally but inaccessible (leaked)
• Manual retry loop required, likely creating additional leaked objects on each attempt

Why This Bug Matters

Redis Sentinel is the actively maintained, official solution for high availability in non-clustered deployments. This bug prevents proper usage of StackExchange.Redis with standard Redis HA deployments, particularly in orchestrated environments (Kubernetes, Docker Compose) where applications commonly start before dependencies.

Related Issues: #2173, #2649, #2808, #2893