Use try_lock in py_os_after_fork_child (#7178)

after_forkers_child.lock() can deadlock in the forked child
if another thread held the mutex at the time of fork.
Use try_lock and skip at-fork callbacks when the lock is
unavailable, matching the pattern used in after_fork_child
for thread_handles.

crates/vm/src/stdlib/nt.rs

@@ -6,7 +6,7 @@ pub use module::raw_set_handle_inheritable;
 #[pymodule(name = "nt", with(super::os::_os))]
 pub(crate) mod module {
     use crate::{
-        Py, PyObjectRef, PyResult, TryFromObject, VirtualMachine,
+        Py, PyResult, TryFromObject, VirtualMachine,
         builtins::{PyBaseExceptionRef, PyDictRef, PyListRef, PyStrRef, PyTupleRef},
         common::{crt_fd, suppress_iph, windows::ToWideString},
         convert::ToPyException,
@@ -1212,21 +1212,6 @@ pub(crate) mod module {
         }
     }
 
-    fn envobj_to_dict(env: ArgMapping, vm: &VirtualMachine) -> PyResult<PyDictRef> {
-        let obj = env.obj();
-        if let Some(dict) = obj.downcast_ref_if_exact::<crate::builtins::PyDict>(vm) {
-            return Ok(dict.to_owned());
-        }
-        let keys = vm.call_method(obj, "keys", ())?;
-        let dict = vm.ctx.new_dict();
-        for key in keys.get_iter(vm)?.into_iter::<PyObjectRef>(vm)? {
-            let key = key?;
-            let val = obj.get_item(&*key, vm)?;
-            dict.set_item(&*key, val, vm)?;
-        }
-        Ok(dict)
-    }
-
     #[cfg(target_env = "msvc")]
     #[pyfunction]
     fn execve(
@@ -1261,7 +1246,7 @@ pub(crate) mod module {
             .chain(once(core::ptr::null()))
             .collect();
 
-        let env = envobj_to_dict(env, vm)?;
+        let env = crate::stdlib::os::envobj_to_dict(env, vm)?;
         // Build environment strings as "KEY=VALUE\0" wide strings
         let mut env_strings: Vec<widestring::WideCString> = Vec::new();
         for (key, value) in env.into_iter() {

crates/vm/src/stdlib/os.rs

@@ -2,10 +2,10 @@
 
 use crate::{
     AsObject, Py, PyObjectRef, PyPayload, PyResult, TryFromObject, VirtualMachine,
-    builtins::{PyModule, PySet},
+    builtins::{PyDictRef, PyModule, PySet},
     common::crt_fd,
     convert::{IntoPyException, ToPyException, ToPyObject},
-    function::{ArgumentError, FromArgs, FuncArgs},
+    function::{ArgMapping, ArgumentError, FromArgs, FuncArgs},
 };
 use std::{fs, io, path::Path};
 
@@ -2038,6 +2038,32 @@ pub fn module_exec(vm: &VirtualMachine, module: &Py<PyModule>) -> PyResult<()> {
     Ok(())
 }
 
+/// Convert a mapping (e.g. os._Environ) to a plain dict for use by execve/posix_spawn.
+///
+/// For `os._Environ`, accesses the internal `_data` dict directly at the Rust level.
+/// This avoids Python-level method calls that can deadlock after fork() when
+/// parking_lot locks are held by threads that no longer exist.
+pub(crate) fn envobj_to_dict(env: ArgMapping, vm: &VirtualMachine) -> PyResult<PyDictRef> {
+    let obj = env.obj();
+    if let Some(dict) = obj.downcast_ref_if_exact::<crate::builtins::PyDict>(vm) {
+        return Ok(dict.to_owned());
+    }
+    if let Some(inst_dict) = obj.dict()
+        && let Ok(Some(data)) = inst_dict.get_item_opt("_data", vm)
+        && let Some(dict) = data.downcast_ref_if_exact::<crate::builtins::PyDict>(vm)
+    {
+        return Ok(dict.to_owned());
+    }
+    let keys = vm.call_method(obj, "keys", ())?;
+    let dict = vm.ctx.new_dict();
+    for key in keys.get_iter(vm)?.into_iter::<PyObjectRef>(vm)? {
+        let key = key?;
+        let val = obj.get_item(&*key, vm)?;
+        dict.set_item(&*key, val, vm)?;
+    }
+    Ok(dict)
+}
+
 #[cfg(not(windows))]
 use super::posix as platform;
 

crates/vm/src/stdlib/posix.rs

@@ -716,7 +716,15 @@ pub mod module {
         vm.signal_handlers
             .get_or_init(crate::signal::new_signal_handlers);
 
-        let after_forkers_child: Vec<PyObjectRef> = vm.state.after_forkers_child.lock().clone();
+        let after_forkers_child = match vm.state.after_forkers_child.try_lock() {
+            Some(guard) => guard.clone(),
+            None => {
+                // SAFETY: After fork in child process, only the current thread
+                // exists. The lock holder no longer exists.
+                unsafe { vm.state.after_forkers_child.force_unlock() };
+                vm.state.after_forkers_child.lock().clone()
+            }
+        };
         run_at_forkers(after_forkers_child, false, vm);
     }
 
@@ -1073,21 +1081,6 @@ pub mod module {
             .map_err(|err| err.into_pyexception(vm))
     }
 
-    fn envobj_to_dict(env: ArgMapping, vm: &VirtualMachine) -> PyResult<PyDictRef> {
-        let obj = env.obj();
-        if let Some(dict) = obj.downcast_ref_if_exact::<crate::builtins::PyDict>(vm) {
-            return Ok(dict.to_owned());
-        }
-        let keys = vm.call_method(obj, "keys", ())?;
-        let dict = vm.ctx.new_dict();
-        for key in keys.get_iter(vm)?.into_iter::<PyObjectRef>(vm)? {
-            let key = key?;
-            let val = obj.get_item(&*key, vm)?;
-            dict.set_item(&*key, val, vm)?;
-        }
-        Ok(dict)
-    }
-
     #[pyfunction]
     fn execve(
         path: OsPath,
@@ -1110,7 +1103,7 @@ pub mod module {
             return Err(vm.new_value_error("execve() arg 2 first element cannot be empty"));
         }
 
-        let env = envobj_to_dict(env, vm)?;
+        let env = crate::stdlib::os::envobj_to_dict(env, vm)?;
         let env = env
             .into_iter()
             .map(|(k, v)| -> PyResult<_> {