Remove need for mutable reference to static

Mutable references to static data are inherently unsafe and typically unsound in Rust, because statics are implicitly shared between threads, and the borrow checker can only enforce the shared/exclusive reference limitations within a single thread here. This just moves the thread-exclusion logic into the individual elements of the `static`, where the `GILOnceCell` can correctly handle the runtime exclusion of multiple threads. (The GIL is no longer suitable for thread exclusion if we were doing a freethreaded Python build, but that's a problem we have all over Qiskit, and would need to change to `OnceLock` or the like.)
Qiskit · Jan 21, 2025 · 1ceac25 · 1ceac25
1 parent 400f22d
commit 1ceac25
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Showing 2 changed files with 60 additions and 37 deletions.
diff --git a/crates/circuit/src/imports.rs b/crates/circuit/src/imports.rs
@@ -255,42 +255,65 @@ static STDGATE_IMPORT_PATHS: [[&str; 2]; STANDARD_GATE_SIZE] = [
 ///
 /// NOTE: the order here is significant it must match the StandardGate variant's number must match
 /// index of it's entry in this table. This is all done statically for performance
-static mut STDGATE_PYTHON_GATES: GILOnceCell<[Option<PyObject>; STANDARD_GATE_SIZE]> =
-    GILOnceCell::new();
-
-#[inline]
-pub fn populate_std_gate_map(py: Python, rs_gate: StandardGate, py_gate: PyObject) {
-    let gate_map = unsafe {
-        match STDGATE_PYTHON_GATES.get_mut() {
-            Some(gate_map) => gate_map,
-            None => {
-                let array: [Option<PyObject>; STANDARD_GATE_SIZE] = std::array::from_fn(|_| None);
-                STDGATE_PYTHON_GATES.set(py, array).unwrap();
-                STDGATE_PYTHON_GATES.get_mut().unwrap()
-            }
-        }
-    };
-    let gate_cls = &gate_map[rs_gate as usize];
-    if gate_cls.is_none() {
-        gate_map[rs_gate as usize] = Some(py_gate.clone_ref(py));
-    }
-}
+static STDGATE_PYTHON_GATES: [GILOnceCell<PyObject>; STANDARD_GATE_SIZE] = [
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+    GILOnceCell::new(),
+];
 
 #[inline]
-pub fn get_std_gate_class(py: Python, rs_gate: StandardGate) -> PyResult<PyObject> {
-    let gate_map =
-        unsafe { STDGATE_PYTHON_GATES.get_or_init(py, || std::array::from_fn(|_| None)) };
-    let gate = &gate_map[rs_gate as usize];
-    let populate = gate.is_none();
-    let out_gate = match gate {
-        Some(gate) => gate.clone_ref(py),
-        None => {
-            let [py_mod, py_class] = STDGATE_IMPORT_PATHS[rs_gate as usize];
-            py.import_bound(py_mod)?.getattr(py_class)?.unbind()
-        }
-    };
-    if populate {
-        populate_std_gate_map(py, rs_gate, out_gate.clone_ref(py));
-    }
-    Ok(out_gate)
+pub fn get_std_gate_class(py: Python, rs_gate: StandardGate) -> PyResult<&'static Py<PyAny>> {
+    STDGATE_PYTHON_GATES[rs_gate as usize].get_or_try_init(py, || {
+        let [py_mod, py_class] = STDGATE_IMPORT_PATHS[rs_gate as usize];
+        Ok(py.import_bound(py_mod)?.getattr(py_class)?.unbind())
+    })
 }
diff --git a/crates/circuit/src/operations.rs b/crates/circuit/src/operations.rs
@@ -736,7 +736,7 @@ impl StandardGate {
     }
 
     #[getter]
-    pub fn get_gate_class(&self, py: Python) -> PyResult<Py<PyAny>> {
+    pub fn get_gate_class(&self, py: Python) -> PyResult<&'static Py<PyAny>> {
         get_std_gate_class(py, *self)
     }