Singleton LiteBox Instance: How to Enforce Single-Instance Constraints in Rust

The singleton LiteBox instance feature is an optional compile-time flag that restricts a process to creating exactly one LiteBox object by panicking on subsequent construction attempts.

In the microsoft/litebox repository, this feature provides a safety mechanism for applications that require a global, unique synchronization primitive provider. When enabled, the enforce_singleton_litebox_instance Cargo feature injects an atomic check into the LiteBox::new constructor, ensuring that only the first instantiation succeeds.

What Is the Singleton LiteBox Instance Feature?

The singleton LiteBox instance feature is a conditional compilation option defined in litebox/Cargo.toml. It addresses scenarios where multiple LiteBox instances could cause logical errors, such as when subsystems expect a single global source of truth for lock-tracing or descriptor tables.

By default, this feature is disabled, allowing developers to create multiple independent LiteBox objects. Each instance maintains its own Arc<LiteBoxX> internal state, making them safe for concurrent use in multi-instance configurations.

How the Singleton Enforcement Works

When the enforce_singleton_litebox_instance feature is active, the LiteBox::new method in litebox/src/litebox.rs executes a runtime atomic check before completing construction.

The AtomicBool Check in LiteBox::new

The implementation uses a static AtomicBool named LITEBOX_SINGLETON_INITIALIZED. The constructor performs a fetch_or operation with Ordering::SeqCst to atomically set the flag and retrieve the previous value:

#[cfg(feature = "enforce_singleton_litebox_instance")]
{
    static LITEBOX_SINGLETON_INITIALIZED: core::sync::atomic::AtomicBool =
        core::sync::atomic::AtomicBool::new(false);

    let previously_initialized =
        LITEBOX_SINGLETON_INITIALIZED.fetch_or(true, core::sync::atomic::Ordering::SeqCst);
    assert!(
        !previously_initialized,
        "In this configuration, there should be only one LiteBox instance ever made.  Failing to make second instance.",
    );
}

If previously_initialized is true, the assert! macro triggers a panic, preventing the second instantiation.

Feature Flag Configuration

The feature is declared in litebox/Cargo.toml as an empty feature flag, meaning it enables conditional compilation without additional dependencies:

[features]
enforce_singleton_litebox_instance = []

When to Use the Singleton LiteBox Instance

Enable the singleton LiteBox instance feature when your architecture assumes a single global synchronization provider. Common use cases include:

• Global lock-tracing systems that require a unified instrumentation point across the entire process.
• Descriptor table management where multiple instances could lead to resource leaks or inconsistent state.
• Shim layers that intercept system calls and expect a single LiteBox to coordinate primitive creation.

If your application requires isolated synchronization domains or testing with multiple configurations, leave the feature disabled to permit multiple instances.

Code Examples

Enabling the Feature

Add the feature to your dependency declaration in Cargo.toml:

[dependencies]
litebox = { path = "../litebox", features = ["enforce_singleton_litebox_instance"] }

Creating the First Instance

The first call to LiteBox::new succeeds and initializes the singleton flag:

use litebox::LiteBox;
use my_platform::MyPlatform;

static PLATFORM: MyPlatform = MyPlatform::new();

let box1 = LiteBox::new(&PLATFORM); // ✅ Succeeds

Attempting a Second Instance (Panic)

Any subsequent construction attempt triggers the assertion and panics:

let box2 = LiteBox::new(&PLATFORM); // ❌ Panics with:
// "In this configuration, there should be only one LiteBox instance ever made..."

Multiple Instances Without the Feature

With the default configuration, you can create independent instances:

[dependencies]
litebox = { path = "../litebox" } # No singleton feature

let box1 = LiteBox::new(&PLATFORM);
let box2 = LiteBox::new(&PLATFORM); // ✅ Both succeed

Summary

• The singleton LiteBox instance feature restricts a process to exactly one LiteBox object via the enforce_singleton_litebox_instance Cargo flag.
• When enabled, LiteBox::new in litebox/src/litebox.rs uses a static AtomicBool to detect and panic on duplicate construction attempts.
• The feature is disabled by default, allowing multiple independent instances for applications requiring isolated synchronization domains.
• Enable this feature only when subsystems require a global, unique LiteBox for coordination, such as in lock-tracing or descriptor table management.

Frequently Asked Questions

What happens if I try to create a second LiteBox with the singleton feature enabled?

The application will panic at runtime. The LiteBox::new constructor checks a static AtomicBool flag, and if it finds that an instance already exists, it triggers an assert! with the message: "In this configuration, there should be only one LiteBox instance ever made."

Is the singleton LiteBox instance feature enabled by default?

No. The enforce_singleton_litebox_instance feature is disabled by default in litebox/Cargo.toml. You must explicitly enable it in your dependency declaration to activate the singleton enforcement behavior.

Can I disable the singleton check after enabling it?

No. The singleton check is a compile-time configuration. Once you enable the enforce_singleton_litebox_instance feature and compile your project, the atomic check is baked into the LiteBox::new method. To allow multiple instances, you must recompile without the feature flag.

Why would I need only one LiteBox instance?

Certain architectures require a global synchronization provider to maintain consistency across subsystems. Common scenarios include lock-tracing instrumentation that must see all synchronization operations in one place, global descriptor tables that would leak resources if split across instances, and shim layers that intercept system calls and expect a single coordination point.

{
  "mcpServers": {
    "instagit": {
      "command": "npx",
      "args": ["-y", "instagit@latest"]
    }
  }
}