Prevent Runtime use over forks

scripts/gen_bridge_client.py

@@ -171,6 +171,7 @@ def generate_rust_service_call(service_descriptor: ServiceDescriptor) -> str:
     py: Python<'p>,
     call: RpcCall,
   ) -> PyResult<Bound<'p, PyAny>> {
+    self.runtime.assert_same_process("use client")?;
     use temporal_client::${descriptor_name};
     let mut retry_client = self.retry_client.clone();
     self.runtime.future_into_py(py, async move {

temporalio/bridge/src/client.rs

@@ -92,6 +92,7 @@ pub fn connect_client<'a>(
     config: ClientConfig,
 ) -> PyResult<Bound<'a, PyAny>> {
     let opts: ClientOptions = config.try_into()?;
+    runtime_ref.runtime.assert_same_process("create client")?;
     let runtime = runtime_ref.runtime.clone();
     runtime_ref.runtime.future_into_py(py, async move {
         Ok(ClientRef {

temporalio/bridge/src/client_rpc_generated.rs

@@ -15,6 +15,7 @@ impl ClientRef {
         py: Python<'p>,
         call: RpcCall,
     ) -> PyResult<Bound<'p, PyAny>> {
+        self.runtime.assert_same_process("use client")?;
         use temporal_client::WorkflowService;
         let mut retry_client = self.retry_client.clone();
         self.runtime.future_into_py(py, async move {
@@ -566,6 +567,7 @@ impl ClientRef {
         py: Python<'p>,
         call: RpcCall,
     ) -> PyResult<Bound<'p, PyAny>> {
+        self.runtime.assert_same_process("use client")?;
         use temporal_client::OperatorService;
         let mut retry_client = self.retry_client.clone();
         self.runtime.future_into_py(py, async move {
@@ -628,6 +630,7 @@ impl ClientRef {
     }
 
     fn call_cloud_service<'p>(&self, py: Python<'p>, call: RpcCall) -> PyResult<Bound<'p, PyAny>> {
+        self.runtime.assert_same_process("use client")?;
         use temporal_client::CloudService;
         let mut retry_client = self.retry_client.clone();
         self.runtime.future_into_py(py, async move {
@@ -842,6 +845,7 @@ impl ClientRef {
     }
 
     fn call_test_service<'p>(&self, py: Python<'p>, call: RpcCall) -> PyResult<Bound<'p, PyAny>> {
+        self.runtime.assert_same_process("use client")?;
         use temporal_client::TestService;
         let mut retry_client = self.retry_client.clone();
         self.runtime.future_into_py(py, async move {
@@ -881,6 +885,7 @@ impl ClientRef {
     }
 
     fn call_health_service<'p>(&self, py: Python<'p>, call: RpcCall) -> PyResult<Bound<'p, PyAny>> {
+        self.runtime.assert_same_process("use client")?;
         use temporal_client::HealthService;
         let mut retry_client = self.retry_client.clone();
         self.runtime.future_into_py(py, async move {

temporalio/bridge/src/runtime.rs

@@ -1,5 +1,5 @@
 use futures::channel::mpsc::Receiver;
-use pyo3::exceptions::{PyRuntimeError, PyValueError};
+use pyo3::exceptions::{PyAssertionError, PyRuntimeError, PyValueError};
 use pyo3::prelude::*;
 use pythonize::pythonize;
 use std::collections::HashMap;
@@ -33,6 +33,7 @@ pub struct RuntimeRef {
 
 #[derive(Clone)]
 pub(crate) struct Runtime {
+    pub(crate) pid: u32,
     pub(crate) core: Arc<CoreRuntime>,
     metrics_call_buffer: Option<Arc<MetricsCallBuffer<BufferedMetricRef>>>,
     log_forwarder_handle: Option<Arc<JoinHandle<()>>>,
@@ -173,6 +174,7 @@ pub fn init_runtime(telemetry_config: TelemetryConfig) -> PyResult<RuntimeRef> {
 
     Ok(RuntimeRef {
         runtime: Runtime {
+            pid: std::process::id(),
             core: Arc::new(core),
             metrics_call_buffer,
             log_forwarder_handle,
@@ -197,6 +199,18 @@ impl Runtime {
         let _guard = self.core.tokio_handle().enter();
         pyo3_async_runtimes::generic::future_into_py::<TokioRuntime, _, T>(py, fut)
     }
+
+    pub(crate) fn assert_same_process(&self, action: &'static str) -> PyResult<()> {
+        let current_pid = std::process::id();
+        if self.pid != current_pid {
+            Err(PyAssertionError::new_err(format!(
+                "Cannot {} across forks (original runtime PID is {}, current is {})",
+                action, self.pid, current_pid,
+            )))
+        } else {
+            Ok(())
+        }
+    }
 }
 
 impl Drop for Runtime {

temporalio/bridge/src/worker.rs

@@ -474,6 +474,7 @@ pub fn new_worker(
     config: WorkerConfig,
 ) -> PyResult<WorkerRef> {
     enter_sync!(runtime_ref.runtime);
+    runtime_ref.runtime.assert_same_process("create worker")?;
     let event_loop_task_locals = Arc::new(OnceLock::new());
     let config = convert_worker_config(config, event_loop_task_locals.clone())?;
     let worker = temporal_sdk_core::init_worker(
@@ -495,6 +496,9 @@ pub fn new_replay_worker<'a>(
     config: WorkerConfig,
 ) -> PyResult<Bound<'a, PyTuple>> {
     enter_sync!(runtime_ref.runtime);
+    runtime_ref
+        .runtime
+        .assert_same_process("create replay worker")?;
     let event_loop_task_locals = Arc::new(OnceLock::new());
     let config = convert_worker_config(config, event_loop_task_locals.clone())?;
     let (history_pusher, stream) = HistoryPusher::new(runtime_ref.runtime.clone());
@@ -519,6 +523,7 @@ pub fn new_replay_worker<'a>(
 #[pymethods]
 impl WorkerRef {
     fn validate<'p>(&self, py: Python<'p>) -> PyResult<Bound<PyAny, 'p>> {
+        self.runtime.assert_same_process("use worker")?;
         let worker = self.worker.as_ref().unwrap().clone();
         // Set custom slot supplier task locals so they can run futures.
         // Event loop is assumed to be running at this point.
@@ -538,6 +543,7 @@ impl WorkerRef {
     }
 
     fn poll_workflow_activation<'p>(&self, py: Python<'p>) -> PyResult<Bound<'p, PyAny>> {
+        self.runtime.assert_same_process("use worker")?;
         let worker = self.worker.as_ref().unwrap().clone();
         self.runtime.future_into_py(py, async move {
             let bytes = match worker.poll_workflow_activation().await {
@@ -550,6 +556,7 @@ impl WorkerRef {
     }
 
     fn poll_activity_task<'p>(&self, py: Python<'p>) -> PyResult<Bound<'p, PyAny>> {
+        self.runtime.assert_same_process("use worker")?;
         let worker = self.worker.as_ref().unwrap().clone();
         self.runtime.future_into_py(py, async move {
             let bytes = match worker.poll_activity_task().await {
@@ -562,6 +569,7 @@ impl WorkerRef {
     }
 
     fn poll_nexus_task<'p>(&self, py: Python<'p>) -> PyResult<Bound<'p, PyAny>> {
+        self.runtime.assert_same_process("use worker")?;
         let worker = self.worker.as_ref().unwrap().clone();
         self.runtime.future_into_py(py, async move {
             let bytes = match worker.poll_nexus_task().await {

temporalio/runtime.py

@@ -24,33 +24,83 @@
 import temporalio.bridge.runtime
 import temporalio.common
 
-_default_runtime: Optional[Runtime] = None
+
+class _RuntimeRef:
+    def __init__(
+        self,
+    ) -> None:
+        self._default_runtime: Runtime | None = None
+        self._prevent_default = False
+        self._default_created = False
+
+    def default(self) -> Runtime:
+        if not self._default_runtime:
+            if self._prevent_default:
+                raise RuntimeError(
+                    "Cannot create default Runtime after Runtime.prevent_default has been called"
+                )
+            self._default_runtime = Runtime(telemetry=TelemetryConfig())
+            self._default_created = True
+        return self._default_runtime
+
+    def prevent_default(self):
+        if self._default_created:
+            raise RuntimeError(
+                "Runtime.prevent_default called after default runtime has been created"
+            )
+        self._prevent_default = True
+
+    def set_default(
+        self, runtime: Runtime, *, error_if_already_set: bool = True
+    ) -> None:
+        if self._default_runtime and error_if_already_set:
+            raise RuntimeError("Runtime default already set")
+
+        self._default_runtime = runtime
+
+
+_runtime_ref: _RuntimeRef = _RuntimeRef()
 
 
 class Runtime:
     """Runtime for Temporal Python SDK.
 
-    Users are encouraged to use :py:meth:`default`. It can be set with
+    Most users are encouraged to use :py:meth:`default`. It can be set with
     :py:meth:`set_default`. Every time a new runtime is created, a new internal
     thread pool is created.
 
-    Runtimes do not work across forks.
+    Runtimes do not work across forks. Advanced users should consider using
+    :py:meth:`prevent_default` and `:py:meth`set_default` to ensure each
+    fork creates it's own runtime.
+
     """
 
     @classmethod
     def default(cls) -> Runtime:
-        """Get the default runtime, creating if not already created.
+        """Get the default runtime, creating if not already created. If :py:meth:`prevent_default`
+        is called before this method it will raise a RuntimeError instead of creating a default
+        runtime.
 
         If the default runtime needs to be different, it should be done with
         :py:meth:`set_default` before this is called or ever used.
 
         Returns:
             The default runtime.
         """
-        global _default_runtime
-        if not _default_runtime:
-            _default_runtime = cls(telemetry=TelemetryConfig())
-        return _default_runtime
+        global _runtime_ref
+        return _runtime_ref.default()
+
+    @classmethod
+    def prevent_default(cls):
+        """Prevent :py:meth:`default` from lazily creating a :py:class:`Runtime`.
+
+        Raises a RuntimeError if a default :py:class:`Runtime` has already been created.
+
+        Explicitly setting a default runtime with :py:meth:`set_default` bypasses this setting and
+        future calls to :py:meth:`default` will return the provided runtime.
+        """
+        global _runtime_ref
+        _runtime_ref.prevent_default()
 
     @staticmethod
     def set_default(runtime: Runtime, *, error_if_already_set: bool = True) -> None:
@@ -65,10 +115,9 @@ def set_default(runtime: Runtime, *, error_if_already_set: bool = True) -> None:
             error_if_already_set: If True and default is already set, this will
                 raise a RuntimeError.
         """
-        global _default_runtime
-        if _default_runtime and error_if_already_set:
-            raise RuntimeError("Runtime default already set")
-        _default_runtime = runtime
+        global _runtime_ref
+        _runtime_ref.set_default(runtime, error_if_already_set=error_if_already_set)
+        return
 
     def __init__(self, *, telemetry: TelemetryConfig) -> None:
         """Create a default runtime with the given telemetry config.

tests/conftest.py

@@ -1,7 +1,8 @@
 import asyncio
+import multiprocessing.context
 import os
 import sys
-from typing import AsyncGenerator
+from typing import AsyncGenerator, Iterator
 
 import pytest
 import pytest_asyncio
@@ -133,6 +134,23 @@ async def env(env_type: str) -> AsyncGenerator[WorkflowEnvironment, None]:
     await env.shutdown()
 
 
+@pytest.fixture(scope="session")
+def mp_fork_ctx() -> Iterator[multiprocessing.context.BaseContext | None]:
+    mp_ctx = None
+    try:
+        mp_ctx = multiprocessing.get_context("fork")
+    except ValueError:
+        pass
+
+    try:
+        yield mp_ctx
+    finally:
+        if mp_ctx:
+            for p in mp_ctx.active_children():
+                p.terminate()
+                p.join()
+
+
 @pytest_asyncio.fixture
 async def client(env: WorkflowEnvironment) -> Client:
     return env.client

tests/helpers/fork.py

@@ -0,0 +1,80 @@
+from __future__ import annotations
+
+import asyncio
+import multiprocessing
+import multiprocessing.context
+import sys
+from dataclasses import dataclass
+from typing import Any
+
+import pytest
+
+
+@dataclass
+class _ForkTestResult:
+    status: str
+    err_name: str | None
+    err_msg: str | None
+
+    def __eq__(self, value: object) -> bool:
+        if not isinstance(value, _ForkTestResult):
+            return False
+
+        valid_err_msg = False
+
+        if self.err_msg and value.err_msg:
+            valid_err_msg = (
+                self.err_msg in value.err_msg or value.err_msg in self.err_msg
+            )
+
+        return (
+            value.status == self.status
+            and value.err_name == value.err_name
+            and valid_err_msg
+        )
+
+    @staticmethod
+    def assertion_error(message: str) -> _ForkTestResult:
+        return _ForkTestResult(
+            status="error", err_name="AssertionError", err_msg=message
+        )
+
+
+class _TestFork:
+    _expected: _ForkTestResult
+
+    async def coro(self) -> Any:
+        raise NotImplementedError()
+
+    def entry(self):
+        event_loop = asyncio.new_event_loop()
+        asyncio.set_event_loop(event_loop)
+        try:
+            event_loop.run_until_complete(self.coro())
+            payload = _ForkTestResult(status="ok", err_name=None, err_msg=None)
+        except BaseException as err:
+            payload = _ForkTestResult(
+                status="error", err_name=err.__class__.__name__, err_msg=str(err)
+            )
+
+        self._child_conn.send(payload)
+        self._child_conn.close()
+
+    def run(self, mp_fork_context: multiprocessing.context.BaseContext | None):
+        process_factory = getattr(mp_fork_context, "Process", None)
+
+        if not mp_fork_context or not process_factory:
+            pytest.skip("fork context not available")
+
+        self._parent_conn, self._child_conn = mp_fork_context.Pipe(duplex=False)
+        # start fork
+        child_process = process_factory(target=self.entry, args=(), daemon=False)
+        child_process.start()
+        # close parent's handle on child_conn
+        self._child_conn.close()
+
+        # get run info from pipe
+        payload = self._parent_conn.recv()
+        self._parent_conn.close()
+
+        assert payload == self._expected