hang instead of pthread_exit during interpreter shutdown

see python/cpython#87135 and
rust-lang/rust#135929

Author: Ariel Ben-Yehuda

pyo3-ffi/Cargo.toml

@@ -42,6 +42,7 @@ generate-import-lib = ["pyo3-build-config/python3-dll-a"]
 paste = "1"
 
 [build-dependencies]
+cc = "1.2"
 pyo3-build-config = { path = "../pyo3-build-config", version = "=0.23.3", features = ["resolve-config"] }
 
 [lints]

pyo3-ffi/build.rs

@@ -182,6 +182,22 @@ fn emit_link_config(interpreter_config: &InterpreterConfig) -> Result<()> {
     Ok(())
 }
 
+fn do_cc(interpreter_config: &InterpreterConfig) -> Result<()> {
+    let implementation_def = match interpreter_config.implementation {
+        PythonImplementation::CPython => "PYTHON_IS_CPYTHON",
+        PythonImplementation::PyPy => "PYTHON_IS_PYPY",
+        PythonImplementation::GraalPy => "PYTHON_IS_GRAALPY",
+    };
+    println!("cargo:rerun-if-changed=src/acquire_gil.cpp");
+    cc::Build::new()
+        .cpp(true)
+        .file("src/acquire_gil.cpp")
+        .define(implementation_def, None)
+        .cpp_link_stdlib("stdc++")
+        .compile("acquire_gil");
+    Ok(())
+}
+
 /// Prepares the PyO3 crate for compilation.
 ///
 /// This loads the config from pyo3-build-config and then makes some additional checks to improve UX
@@ -218,6 +234,8 @@ fn configure_pyo3() -> Result<()> {
     // Emit cfgs like `invalid_from_utf8_lint`
     print_feature_cfgs();
 
+    do_cc(&interpreter_config)?;
+
     Ok(())
 }
 

pyo3-ffi/src/acquire_gil.cpp

@@ -0,0 +1,45 @@
+#if defined(_WIN32)
+#include <windows.h>
+#include <synchapi.h>
+#else
+#include <unistd.h>
+#endif
+
+#if defined(PYTHON_IS_PYPY)
+#define gil_func_name PyPyGILState_Ensure
+#define wrapped_func_name PyPyGILState_Ensure_Safe
+#else
+#define gil_func_name PyGILState_Ensure
+#define wrapped_func_name PyGILState_Ensure_Safe
+#endif
+
+extern "C" {
+    int wrapped_func_name(void);
+    int gil_func_name(void);
+};
+
+#if !defined(_WIN32)
+// mark the wrapped function as visibility("hidden") to avoid causing namespace pollution
+__attribute__((visibility("hidden")))
+#endif
+int wrapped_func_name(void) {
+    // Do the equivalent of https://github.com/python/cpython/issues/87135 (included
+    // in Python 3.14) to avoid pthread_exit unwinding the current thread, which tends
+    // to cause undefined behavior in Rust.
+    //
+    // Unfortunately, I don't know of a way to do a catch(...) from Rust.
+    try {
+        return gil_func_name();
+    } catch(...) {
+        while(1) {
+#if defined(_WIN32)
+            SleepEx(INFINITE, TRUE);
+#elif defined(__wasi__)
+            sleep(9999999);  // WASI doesn't have pause() ?!
+#else
+            pause();
+#endif
+        }
+    }
+}
+

pyo3-ffi/src/pystate.rs

@@ -74,7 +74,13 @@ pub enum PyGILState_STATE {
 }
 
 extern "C" {
-    #[cfg_attr(PyPy, link_name = "PyPyGILState_Ensure")]
+    // The PyGILState_Ensure function will call pthread_exit during interpreter shutdown,
+    // which causes undefined behavior. Redirect to the "safe" version that hangs instead,
+    // as Python 3.14 does.
+    //
+    // See https://github.com/rust-lang/rust/issues/135929
+    #[cfg_attr(PyPy, link_name = "PyPyGILState_Ensure_Safe")]
+    #[cfg_attr(not(PyPy), link_name = "PyGILState_Ensure_Safe")]
     pub fn PyGILState_Ensure() -> PyGILState_STATE;
     #[cfg_attr(PyPy, link_name = "PyPyGILState_Release")]
     pub fn PyGILState_Release(arg1: PyGILState_STATE);