Implement asyncio-dangling-task to track asyncio.create_task calls (#2935)

This rule guards against `asyncio.create_task` usages of the form:

```py
asyncio.create_task(coordinator.ws_connect())  # Error
```

...which can lead to unexpected bugs due to the lack of a strong reference to the created task. See Will McGugan's blog post for reference: https://textual.textualize.io/blog/2023/02/11/the-heisenbug-lurking-in-your-async-code/.

Note that we can't detect issues like:

```py
def f():
    # Stored as `task`, but never used...
    task = asyncio.create_task(coordinator.ws_connect())
```

So that would be a false negative. But this catches the common case of failing to assign the task in any way.

Closes #2809.

README.md

@@ -1503,6 +1503,7 @@ For more, see [tryceratops](https://pypi.org/project/tryceratops/1.1.0/) on PyPI
 | RUF003 | ambiguous-unicode-character-comment | Comment contains ambiguous unicode character `{confusable}` (did you mean `{representant}`?) | 🛠 |
 | RUF004 | keyword-argument-before-star-argument | Keyword argument `{name}` must come after starred arguments |  |
 | RUF005 | unpack-instead-of-concatenating-to-collection-literal | Consider `{expr}` instead of concatenation | 🛠 |
+| RUF006 | [asyncio-dangling-task](https://beta.ruff.rs/docs/rules/asyncio-dangling-task/) | Store a reference to the return value of `asyncio.create_task` |  |
 | RUF100 | unused-noqa | Unused `noqa` directive | 🛠 |
 
 <!-- End auto-generated sections. -->

crates/ruff/resources/test/fixtures/ruff/RUF006.py

@@ -0,0 +1,52 @@
+import asyncio
+
+
+# Error
+def f():
+    asyncio.create_task(coordinator.ws_connect())  # Error
+
+
+# OK
+def f():
+    background_tasks = set()
+
+    for i in range(10):
+        task = asyncio.create_task(some_coro(param=i))
+
+        # Add task to the set. This creates a strong reference.
+        background_tasks.add(task)
+
+        # To prevent keeping references to finished tasks forever,
+        # make each task remove its own reference from the set after
+        # completion:
+        task.add_done_callback(background_tasks.discard)
+
+
+# OK
+def f():
+    ctx.task = asyncio.create_task(make_request())
+
+
+# OK
+def f():
+    tasks.append(asyncio.create_task(self._populate_collection(coll, coll_info)))
+
+
+# OK
+def f():
+    asyncio.wait([asyncio.create_task(client.close()) for client in clients.values()])
+
+
+# OK
+def f():
+    tasks = [asyncio.create_task(task) for task in tasks]
+
+
+# Ok (false negative)
+def f():
+    task = asyncio.create_task(coordinator.ws_connect())
+
+
+# Ok (potential false negative)
+def f():
+    do_nothing_with_the_task(asyncio.create_task(coordinator.ws_connect()))

crates/ruff/src/checkers/ast.rs

@@ -1795,6 +1795,13 @@ where
                 {
                     flake8_simplify::rules::use_capital_environment_variables(self, value);
                 }
+                if self.settings.rules.enabled(&Rule::AsyncioDanglingTask) {
+                    if let Some(diagnostic) = ruff::rules::asyncio_dangling_task(value, |expr| {
+                        self.resolve_call_path(expr)
+                    }) {
+                        self.diagnostics.push(diagnostic);
+                    }
+                }
             }
             _ => {}
         }

crates/ruff/src/codes.rs

@@ -596,6 +596,7 @@ pub fn code_to_rule(linter: Linter, code: &str) -> Option<Rule> {
         (Ruff, "003") => Rule::AmbiguousUnicodeCharacterComment,
         (Ruff, "004") => Rule::KeywordArgumentBeforeStarArgument,
         (Ruff, "005") => Rule::UnpackInsteadOfConcatenatingToCollectionLiteral,
+        (Ruff, "006") => Rule::AsyncioDanglingTask,
         (Ruff, "100") => Rule::UnusedNOQA,
 
         // flake8-django

crates/ruff/src/registry.rs

@@ -558,6 +558,7 @@ ruff_macros::register_rules!(
     rules::ruff::rules::AmbiguousUnicodeCharacterComment,
     rules::ruff::rules::KeywordArgumentBeforeStarArgument,
     rules::ruff::rules::UnpackInsteadOfConcatenatingToCollectionLiteral,
+    rules::ruff::rules::AsyncioDanglingTask,
     rules::ruff::rules::UnusedNOQA,
     // flake8-django
     rules::flake8_django::rules::NullableModelStringField,

crates/ruff/src/rules/ruff/mod.rs

@@ -18,6 +18,7 @@ mod tests {
 
     #[test_case(Rule::KeywordArgumentBeforeStarArgument, Path::new("RUF004.py"); "RUF004")]
     #[test_case(Rule::UnpackInsteadOfConcatenatingToCollectionLiteral, Path::new("RUF005.py"); "RUF005")]
+    #[test_case(Rule::AsyncioDanglingTask, Path::new("RUF006.py"); "RUF006")]
     fn rules(rule_code: Rule, path: &Path) -> Result<()> {
         let snapshot = format!("{}_{}", rule_code.noqa_code(), path.to_string_lossy());
         let diagnostics = test_path(

crates/ruff/src/rules/ruff/rules/asyncio_dangling_task.rs

@@ -0,0 +1,78 @@
+use rustpython_parser::ast::{Expr, ExprKind};
+
+use ruff_macros::{define_violation, derive_message_formats};
+
+use crate::ast::types::{CallPath, Range};
+use crate::registry::Diagnostic;
+use crate::violation::Violation;
+
+define_violation!(
+    /// ## What it does
+    /// Checks for `asyncio.create_task` calls that do not store a reference
+    /// to the returned result.
+    ///
+    /// ## Why is this bad?
+    /// Per the `asyncio` documentation, the event loop only retains a weak
+    /// reference to tasks. If the task returned by `asyncio.create_task` is
+    /// not stored in a variable, or a collection, or otherwise referenced, it
+    /// may be garbage collected at any time. This can lead to unexpected and
+    /// inconsistent behavior, as your tasks may or may not run to completion.
+    ///
+    /// ## Example
+    /// ```python
+    /// import asyncio
+    ///
+    /// for i in range(10):
+    ///    # This creates a weak reference to the task, which may be garbage
+    ///    # collected at any time.
+    ///    asyncio.create_task(some_coro(param=i))
+    /// ```
+    ///
+    /// Use instead:
+    /// ```python
+    /// import asyncio
+    ///
+    /// background_tasks = set()
+    ///
+    /// for i in range(10):
+    ///     task = asyncio.create_task(some_coro(param=i))
+    ///
+    ///     # Add task to the set. This creates a strong reference.
+    ///     background_tasks.add(task)
+    ///
+    ///     # To prevent keeping references to finished tasks forever,
+    ///     # make each task remove its own reference from the set after
+    ///     # completion:
+    ///     task.add_done_callback(background_tasks.discard)
+    /// ```
+    ///
+    /// ## References
+    /// * [_The Heisenbug lurking in your async code_](https://textual.textualize.io/blog/2023/02/11/the-heisenbug-lurking-in-your-async-code/)
+    /// * [`asyncio.create_task`](https://docs.python.org/3/library/asyncio-task.html#asyncio.create_task)
+    pub struct AsyncioDanglingTask;
+);
+
+impl Violation for AsyncioDanglingTask {
+    #[derive_message_formats]
+    fn message(&self) -> String {
+        format!("Store a reference to the return value of `asyncio.create_task`")
+    }
+}
+
+/// RUF006
+pub fn asyncio_dangling_task<'a, F>(expr: &'a Expr, resolve_call_path: F) -> Option<Diagnostic>
+where
+    F: FnOnce(&'a Expr) -> Option<CallPath<'a>>,
+{
+    if let ExprKind::Call { func, .. } = &expr.node {
+        if resolve_call_path(func).map_or(false, |call_path| {
+            call_path.as_slice() == ["asyncio", "create_task"]
+        }) {
+            return Some(Diagnostic::new(
+                AsyncioDanglingTask,
+                Range::from_located(expr),
+            ));
+        }
+    }
+    None
+}

crates/ruff/src/rules/ruff/rules/mod.rs

@@ -1,4 +1,5 @@
 mod ambiguous_unicode_character;
+mod asyncio_dangling_task;
 mod keyword_argument_before_star_argument;
 mod unpack_instead_of_concatenating_to_collection_literal;
 mod unused_noqa;
@@ -7,6 +8,7 @@ pub use ambiguous_unicode_character::{
     ambiguous_unicode_character, AmbiguousUnicodeCharacterComment,
     AmbiguousUnicodeCharacterDocstring, AmbiguousUnicodeCharacterString,
 };
+pub use asyncio_dangling_task::{asyncio_dangling_task, AsyncioDanglingTask};
 pub use keyword_argument_before_star_argument::{
     keyword_argument_before_star_argument, KeywordArgumentBeforeStarArgument,
 };

crates/ruff/src/rules/ruff/snapshots/ruff__rules__ruff__tests__RUF006_RUF006.py.snap

@@ -0,0 +1,15 @@
+---
+source: crates/ruff/src/rules/ruff/mod.rs
+expression: diagnostics
+---
+- kind:
+    AsyncioDanglingTask: ~
+  location:
+    row: 6
+    column: 4
+  end_location:
+    row: 6
+    column: 49
+  fix: ~
+  parent: ~
+

ruff.schema.json

@@ -1864,6 +1864,7 @@
         "RUF003",
         "RUF004",
         "RUF005",
+        "RUF006",
         "RUF1",
         "RUF10",
         "RUF100",