How to Fix: Panic in fuel_remaining when all fuel has been consumed by wasm.

When all WebAssembly fuel is exhausted, calling fuel_remaining can panic instead of reporting 0. That behavior is surprising, breaks host-side accounting logic, and usually appears in projects that enable deterministic execution with Wasmtime fuel metering.

The Problem

This issue shows up when an application enables fuel consumption, runs a Wasm module until that fuel is fully consumed, and then asks the store how much fuel remains. Instead of safely returning Some(0) or an equivalent exhausted state, the implementation can hit an internal invariant and panic.

In practical terms, the failure pattern usually looks like this:

1. Create an Engine with fuel metering enabled.
2. Create a Store and add a fixed amount of fuel.
3. Run guest Wasm until execution traps because fuel is exhausted.
4. Call fuel_remaining on the store.
5. Observe a panic instead of a stable result.

If your host application uses fuel for resource governance, sandboxing, or billing-style metering, this is especially dangerous because a panic turns a normal exhaustion event into a host crash path.

Understanding the Root Cause

At a technical level, the bug comes from a mismatch between the execution state and the host-facing accounting API. When Wasmtime consumes fuel during execution, it may internally represent the remaining amount in a way optimized for JIT/runtime checks rather than direct user reporting. Once fuel drops fully to zero, the internal state can transition into an exhausted sentinel state used to trigger the out-of-fuel trap.

The problem happens when fuel_remaining assumes that state is always convertible back into a normal positive or zero-valued counter. If the implementation instead encounters a special exhausted representation, an unchecked conversion, subtraction, or invariant assertion can fail and cause a panic.

In other words, the root cause is not that fuel ran out. Running out of fuel is expected. The root cause is that the post-exhaustion code path in the API does not defensively normalize the exhausted state before reporting it back to the host.

A correct implementation should treat these states equivalently from the API caller’s perspective:

• Positive remaining fuel – return the amount.
• Exactly zero fuel – return 0.
• Exhausted internal sentinel – also return 0, not panic.

That is why the proper fix is usually in Wasmtime itself: ensure fuel_remaining saturates to zero or otherwise handles the exhausted internal representation safely.

Step-by-Step Solution

The safest solution is to update the runtime behavior so that fuel_remaining never panics after exhaustion. If you are fixing this in Wasmtime or patching a local fork, the implementation should normalize the exhausted state and return zero.

1. Reproduce the bug with a focused test

Start with a regression test that proves the failure and locks in the expected behavior.

use wasmtime::*;

#[test]
fn fuel_remaining_after_exhaustion_returns_zero() -> Result<()> {
    let mut config = Config::new();
    config.consume_fuel(true);

    let engine = Engine::new(&config)?;
    let mut store = Store::new(&engine, ());
    store.set_fuel(10)?;

    let module = Module::new(
        &engine,
        r#"(module
            (func (export \"run\")
                (loop br 0)
            )
        )"#,
    )?;

    let instance = Instance::new(&mut store, &module, &[])?;
    let run = instance.get_typed_func::<(), ()>(&mut store, "run")?;

    let result = run.call(&mut store, ());
    assert!(result.is_err());

    let remaining = store.get_fuel()?;
    assert_eq!(remaining, 0);

    Ok(())
}

If your local API uses add_fuel, fuel_remaining, or a slightly different method name depending on Wasmtime version, adapt the test accordingly. The key assertion is that post-exhaustion fuel queries must be safe and must report zero.

2. Patch the runtime to saturate exhausted fuel to zero

The exact source location depends on the Wasmtime version, but the logic should look like this conceptually:

pub fn fuel_remaining(&self) -> Option<u64> {
    let raw = self.internal_fuel_state()?;

    if raw.is_exhausted_sentinel() {
        return Some(0);
    }

    Some(raw.remaining().max(0) as u64)
}

In real code, the implementation might not expose methods like is_exhausted_sentinel. You may instead need to guard an internal subtraction, clamp a signed value, or avoid an assertion that assumes fuel is still available.

The important fix pattern is:

• Do not assume internal fuel state is always positive.
• Do not panic on exhausted state.
• Always normalize exhausted state to zero for public API consumption.

3. Keep the host application defensive

Even after the library-level fix, host applications should treat out-of-fuel as a normal execution result, not a crash-only event.

use wasmtime::*;

fn run_with_fuel() -> Result<()> {
    let mut config = Config::new();
    config.consume_fuel(true);

    let engine = Engine::new(&config)?;
    let mut store = Store::new(&engine, ());
    store.set_fuel(100)?;

    let module = Module::new(
        &engine,
        r#"(module
            (func (export \"run\")
                (loop br 0)
            )
        )"#,
    )?;

    let instance = Instance::new(&mut store, &module, &[])?;
    let func = instance.get_typed_func::<(), ()>(&mut store, "run")?;

    match func.call(&mut store, ()) {
        Ok(()) => {
            println!("guest completed normally");
        }
        Err(trap) => {
            println!("guest trapped: {trap}");
        }
    }

    let remaining = store.get_fuel().unwrap_or(0);
    println!("remaining fuel: {remaining}");

    Ok(())
}

This prevents your host from assuming that a trap means accounting APIs are unusable afterward.

4. Add a regression test for exact-zero accounting

It is worth testing not only the trap path but also the exact boundary where fuel reaches zero.

#[test]
fn exact_zero_fuel_is_reported_without_panic() -> Result<()> {
    let mut config = Config::new();
    config.consume_fuel(true);

    let engine = Engine::new(&config)?;
    let mut store = Store::new(&engine, ());
    store.set_fuel(1)?;

    let module = Module::new(
        &engine,
        r#"(module (func (export \"run\")))"#,
    )?;

    let instance = Instance::new(&mut store, &module, &[])?;
    let run = instance.get_typed_func::<(), ()>(&mut store, "run")?;
    let _ = run.call(&mut store, ());

    let remaining = store.get_fuel()?;
    assert!(remaining <= 1);

    Ok(())
}

This kind of test helps catch off-by-one mistakes in fuel metering and makes the API contract more stable.

5. Upgrade if the fix is already available upstream

If this issue has already been addressed in Wasmtime, the best production fix is to upgrade to the first release that includes the patch. Check the project’s Wasmtime repository and release notes for the version containing the regression fix.

If you cannot upgrade immediately:

• Patch your local dependency.
• Wrap post-trap fuel queries defensively.
• Add a regression test in your own codebase.

Common Edge Cases

1. Fuel API differences across Wasmtime versions

Some versions use methods like set_fuel and get_fuel, while others may expose different naming around remaining fuel. Make sure your fix and tests match the exact API surface of your dependency version.

2. Confusing trap handling with panic handling

An out-of-fuel trap is expected runtime behavior. A panic in the host or library is a bug. These should never be treated as equivalent in tests.

3. Negative or sentinel internal counters

Some metering implementations use signed intermediates or reserved internal values. If you patch the runtime, verify every conversion to u64 or subtraction involving remaining fuel.

4. Reusing the same store after exhaustion

After fuel is exhausted, some applications top the store back up and continue. That path should also be tested to ensure the exhausted state is fully reset and not left in a broken sentinel form.

store.set_fuel(50)?;
let remaining = store.get_fuel()?;
assert_eq!(remaining, 50);

5. Multi-call execution paths

If a host calls multiple exported functions in sequence, fuel may be consumed by earlier calls and exhausted by later ones. Tests should cover both single long-running calls and many short calls.

6. Embedders relying on exact accounting

If your platform uses fuel for quotas, billing, or request fairness, make sure zero is handled consistently in logs, telemetry, and retry behavior. A panic in this area can cascade into incorrect service-level behavior.

FAQ

Why should fuel_remaining return zero instead of an error after exhaustion?

Because fuel exhaustion is a normal runtime outcome, not an invalid state. The host still needs a stable way to inspect execution accounting after a trap. Returning 0 preserves a predictable API contract.

Is this a bug in my Wasm module?

Usually no. An infinite loop or high-cost guest code may legitimately consume all fuel, but that should only trigger an out-of-fuel trap. The panic occurs because the host runtime mishandles the exhausted accounting state.

Can I work around this without patching Wasmtime?

Yes, partially. You can treat out-of-fuel as terminal for that store instance, avoid calling the problematic fuel query afterward, or guard the query path in your application. But the real fix is in the runtime: normalize exhausted fuel to zero and prevent the panic.

If you are preparing a contribution, the most effective patch includes three things: a minimal reproducer, a runtime-side saturation fix, and a regression test asserting that post-exhaustion fuel inspection is safe.