Pybind11 causes clang 3.3 and clang 3.4 to segfault

[bstaletic]

Issue description

As it stands now, compiling with clang 3.4 (assuming the same goes for clang 3.3) makes clang segfault. To work around this, ycm-core/ycmd#925 had to drop support for clang 3.3 and clang 3.4.

Looking at the current ycm_core.cpp and the last year's segfaults (today's segfaults don't look much different to me), @jagerman thought it had something to do with the static variable used in exception binding and told me to try the following in pybind11.h.

NAMESPACE_BEGIN(detail)
template <typename CppException>
exception<CppException> &get_exception_object() {
    static exception<CppException> ex;
    return ex;
}
NAMESPACE_END(detail)

/**
 * Registers a Python exception in `m` of the given `name` and installs an exception translator to
 * translate the C++ exception to the created Python exception using the exceptions what() method.
 * This is intended for simple exception translations; for more complex translation, register the
 * exception object and translator directly.
 */
template <typename CppException>
exception<CppException> &register_exception(handle scope,
                                            const char *name,
                                            PyObject *base = PyExc_Exception) {
    auto &ex = detail::get_exception_object<CppException>();
    ex = exception<CppException>(scope, name, base);
    register_exception_translator([](std::exception_ptr p) {
        if (!p) return;
        try {
            std::rethrow_exception(p);
        } catch (const CppException &e) {
            detail::get_exception_object<CppException>()(e.what());
        }
    });
    return ex;
}

This did solve the segmentation fault (at least on clang 3.4), but is now causing compilation errors.

Reproducible example code

Unfortunately, I can't make this minimal. Something, somewhere in ycmd libclang completer bindings is making clang 3.3 segfault, but I am willing to run tests on travis.

Since travis only supports clang 3.4, I'll try to package clang 3.3 for my distro so that we can test it. I still can't promise I'll manage to get clang 3.3 ready for testing. I will be using https://github.com/bstaletic/ycmd/tree/pybind11_segfault to test whatever we come up with.

[jagerman]

Add: exception() = default; to the 'class exception' just above that code; that should fix the compilation error.

If this solves the problem I'll clean up the patch to #ifdef this approach for clang < 3.5. (Edit: it's not particularly intrusive, and cleaner to have just one code path so I didn't #ifdef it.)

[jagerman]

If this solves the problem I'll clean up the patch to #ifdef this approach for clang < 3.5.

... which is horrific thanks to Apple deciding that clang's macros should reflect Apple's Xcode marketing versions. Blech.

[jagerman]

The issue here is almost certainly https://bugs.llvm.org/show_bug.cgi?id=19052, indeed present in 3.4 and fixed in 3.5.

[bstaletic]

Yup, that fixed it.
https://travis-ci.org/bstaletic/ycmd/builds/362630239

Thanks for looking into this.

[bstaletic]

For completeness, here's the diff that fixed it:

diff --git a/cpp/pybind11/pybind11/pybind11.h b/cpp/pybind11/pybind11/pybind11.h
index 977045d6..c685e06c 100644
--- a/cpp/pybind11/pybind11/pybind11.h
+++ b/cpp/pybind11/pybind11/pybind11.h
@@ -1650,6 +1650,7 @@ void register_exception_translator(ExceptionTranslator&& translator) {
 template <typename type>
 class exception : public object {
 public:
+    exception() = default;
     exception(handle scope, const char *name, PyObject *base = PyExc_Exception) {
         std::string full_name = scope.attr("__name__").cast<std::string>() +
                                 std::string(".") + name;
@@ -1666,6 +1667,14 @@ public:
     }
 };

+NAMESPACE_BEGIN(detail)
+template <typename CppException>
+exception<CppException> &get_exception_object() {
+    static exception<CppException> ex;
+    return ex;
+}
+NAMESPACE_END(detail)
+
 /**
  * Registers a Python exception in `m` of the given `name` and installs an exception translator to
  * translate the C++ exception to the created Python exception using the exceptions what() method.
@@ -1676,13 +1685,14 @@ template <typename CppException>
 exception<CppException> &register_exception(handle scope,
                                             const char *name,
                                             PyObject *base = PyExc_Exception) {
-    static exception<CppException> ex(scope, name, base);
+    auto &ex = detail::get_exception_object<CppException>();
+    ex = exception<CppException>(scope, name, base);
     register_exception_translator([](std::exception_ptr p) {
         if (!p) return;
         try {
             std::rethrow_exception(p);
         } catch (const CppException &e) {
-            ex(e.what());
+            detail::get_exception_object<CppException>()(e.what());
         }
     });
     return ex;

[jagerman]

I cleaned it up a bit for PR #1350; once that passes the CI tests I'll merge.