add __builtins__ to globals in py.run() if they're missing

Python code doesn't like to run without `__builtins__`, so adding them
if missing seems to be a good idea. Also that's what CPython >3.10 does.

See python/cpython#24564
Resolves #3370

newsfragments/3378.changed.md

@@ -0,0 +1 @@
+Add `__builtins__` to globals in `py.run()` if they're missing.

src/marker.rs

@@ -632,6 +632,29 @@ impl<'py> Python<'py> {
                 .unwrap_or_else(|| ffi::PyModule_GetDict(mptr));
             let locals = locals.map(AsPyPointer::as_ptr).unwrap_or(globals);
 
+            // If `globals` don't provide `__builtins__`, most of the code will fail if Python
+            // version is <3.10. That's probably not what user intended, so insert `__builtins__`
+            // for them.
+            //
+            // See also:
+            // - https://github.com/python/cpython/pull/24564 (the same fix in CPython 3.10)
+            // - https://github.com/PyO3/pyo3/issues/3370
+            let builtins_s = crate::intern!(self, "__builtins__").as_ptr();
+            let has_builtins = ffi::PyDict_Contains(globals, builtins_s);
+            if has_builtins == -1 {
+                return Err(PyErr::fetch(self));
+            }
+            if has_builtins == 0 {
+                // Inherit current builtins.
+                let builtins = ffi::PyEval_GetBuiltins();
+
+                // `PyDict_SetItem` doesn't take ownership of `builtins`, but `PyEval_GetBuiltins`
+                // seems to return a borrowed reference, so no leak here.
+                if ffi::PyDict_SetItem(globals, builtins_s, builtins) == -1 {
+                    return Err(PyErr::fetch(self));
+                }
+            }
+
             let code_obj = ffi::Py_CompileString(code.as_ptr(), "<string>\0".as_ptr() as _, start);
             if code_obj.is_null() {
                 return Err(PyErr::fetch(self));
@@ -1031,7 +1054,7 @@ impl<'unbound> Python<'unbound> {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use crate::types::{IntoPyDict, PyList};
+    use crate::types::{IntoPyDict, PyList, PyDict};
     use crate::Py;
     use std::sync::Arc;
 
@@ -1166,4 +1189,14 @@ mod tests {
             assert!(v.eq(py.Ellipsis()).unwrap());
         });
     }
+
+    #[test]
+    fn test_py_run_inserts_globals() {
+        Python::with_gil(|py| {
+            let namespace = PyDict::new(py);
+            py.run("class Foo: pass", Some(namespace), Some(namespace)).unwrap();
+            assert!(namespace.get_item("Foo").is_some());
+            assert!(namespace.get_item("__builtins__").is_some());
+        })
+    }
 }