Clippy lint · Suspicious · checked with Clippy 0.1.99

clippy::await_holding_lock: Checks for calls to await while holding a non-async-aware MutexGuard

What it meansChecks for calls to await while holding a non-async-aware MutexGuard. It is in the suspicious group and warns by default.

Does it matter?

The Mutex types found in std::sync and parking_lot are not designed to operate in an async context across await points. There are two potential solutions. One is to use an async-aware Mutex type. Many asynchronous foundation crates provide such a Mutex type. The other solution is to ensure the mutex is unlocked before calling await, either by introducing a scope or an explicit call to Drop::drop.

Code that is most likely wrong or useless. Warns by default.

How to fix it

Clippy does not fix this one automatically. Change the code by hand:

Flagged

async fn foo(x: &Mutex<u32>) {
  let mut guard = x.lock().unwrap();
  *guard += 1;
  baz().await;
}

async fn bar(x: &Mutex<u32>) {
  let mut guard = x.lock().unwrap();
  *guard += 1;
  drop(guard); // explicit drop
  baz().await;
}

Write this instead

async fn foo(x: &Mutex<u32>) {
  {
    let mut guard = x.lock().unwrap();
    *guard += 1;
  }
  baz().await;
}

async fn bar(x: &Mutex<u32>) {
  {
    let mut guard = x.lock().unwrap();
    *guard += 1;
  } // guard dropped here at end of scope
  baz().await;
}

Turn it off, or on

Allow it where it does not fit your code, as narrowly as you can:

On one item

#[allow(clippy::await_holding_lock)]
fn my_function() { /* ... */ }

Whole crate (Cargo.toml)

[lints.clippy]
await_holding_lock = "allow"

One run

cargo clippy -- -A clippy::await-holding-lock

The [lints] table needs Rust 1.74 or newer. In a workspace, put it under [workspace.lints.clippy] and add [lints] workspace = true to each member.

Suspiciousgroup

warndefault level

nofixed by cargo clippy --fix

1.45.0added in Rust

Clippy's own documentation

What it does

Checks for calls to await while holding a non-async-aware MutexGuard.

Why is this bad?

The Mutex types found in std::sync and parking_lot are not designed to operate in an async context across await points.

There are two potential solutions. One is to use an async-aware Mutex type. Many asynchronous foundation crates provide such a Mutex type. The other solution is to ensure the mutex is unlocked before calling await, either by introducing a scope or an explicit call to Drop::drop.

Known problems

Will report false positive for explicitly dropped guards (#6446). A workaround for this is to wrap the .lock() call in a block instead of explicitly dropping the guard.

Example

async fn foo(x: &Mutex<u32>) {
  let mut guard = x.lock().unwrap();
  *guard += 1;
  baz().await;
}

async fn bar(x: &Mutex<u32>) {
  let mut guard = x.lock().unwrap();
  *guard += 1;
  drop(guard); // explicit drop
  baz().await;
}

Use instead:

async fn foo(x: &Mutex<u32>) {
  {
    let mut guard = x.lock().unwrap();
    *guard += 1;
  }
  baz().await;
}

async fn bar(x: &Mutex<u32>) {
  {
    let mut guard = x.lock().unwrap();
    *guard += 1;
  } // guard dropped here at end of scope
  baz().await;
}

From the Clippy lint list (0.1.99), part of the Rust project, MIT or Apache-2.0. Official entry.

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