src/greenlet/TThreadState.hpp

#ifndef GREENLET_THREAD_STATE_HPP
#define GREENLET_THREAD_STATE_HPP

#include <ctime>
#include <stdexcept>

#include "greenlet_internal.hpp"
#include "greenlet_refs.hpp"
#include "greenlet_thread_support.hpp"

using greenlet::refs::BorrowedObject;
using greenlet::refs::BorrowedGreenlet;
using greenlet::refs::BorrowedMainGreenlet;
using greenlet::refs::OwnedMainGreenlet;
using greenlet::refs::OwnedObject;
using greenlet::refs::OwnedGreenlet;
using greenlet::refs::OwnedList;
using greenlet::refs::PyErrFetchParam;
using greenlet::refs::PyArgParseParam;
using greenlet::refs::ImmortalString;
using greenlet::refs::CreatedModule;
using greenlet::refs::PyErrPieces;
using greenlet::refs::NewReference;

namespace greenlet {
/**
 * Thread-local state of greenlets.
 *
 * Each native thread will get exactly one of these objects,
 * automatically accessed through the best available thread-local
 * mechanism the compiler supports (``thread_local`` for C++11
 * compilers or ``__thread``/``declspec(thread)`` for older GCC/clang
 * or MSVC, respectively.)
 *
 * Previously, we kept thread-local state mostly in a bunch of
 * ``static volatile`` variables in the main greenlet file.. This had
 * the problem of requiring extra checks, loops, and great care
 * accessing these variables if we potentially invoked any Python code
 * that could release the GIL, because the state could change out from
 * under us. Making the variables thread-local solves this problem.
 *
 * When we detected that a greenlet API accessing the current greenlet
 * was invoked from a different thread than the greenlet belonged to,
 * we stored a reference to the greenlet in the Python thread
 * dictionary for the thread the greenlet belonged to. This could lead
 * to memory leaks if the thread then exited (because of a reference
 * cycle, as greenlets referred to the thread dictionary, and deleting
 * non-current greenlets leaked their frame plus perhaps arguments on
 * the C stack). If a thread exited while still having running
 * greenlet objects (perhaps that had just switched back to the main
 * greenlet), and did not invoke one of the greenlet APIs *in that
 * thread, immediately before it exited, without some other thread
 * then being invoked*, such a leak was guaranteed.
 *
 * This can be partly solved by using compiler thread-local variables
 * instead of the Python thread dictionary, thus avoiding a cycle.
 *
 * To fully solve this problem, we need a reliable way to know that a
 * thread is done and we should clean up the main greenlet. On POSIX,
 * we can use the destructor function of ``pthread_key_create``, but
 * there's nothing similar on Windows; a C++11 thread local object
 * reliably invokes its destructor when the thread it belongs to exits
 * (non-C++11 compilers offer ``__thread`` or ``declspec(thread)`` to
 * create thread-local variables, but they can't hold C++ objects that
 * invoke destructors; the C++11 version is the most portable solution
 * I found). When the thread exits, we can drop references and
 * otherwise manipulate greenlets and frames that we know can no
 * longer be switched to. For compilers that don't support C++11
 * thread locals, we have a solution that uses the python thread
 * dictionary, though it may not collect everything as promptly as
 * other compilers do, if some other library is using the thread
 * dictionary and has a cycle or extra reference.
 *
 * There are two small wrinkles. The first is that when the thread
 * exits, it is too late to actually invoke Python APIs: the Python
 * thread state is gone, and the GIL is released. To solve *this*
 * problem, our destructor uses ``Py_AddPendingCall`` to transfer the
 * destruction work to the main thread. (This is not an issue for the
 * dictionary solution.)
 *
 * The second is that once the thread exits, the thread local object
 * is invalid and we can't even access a pointer to it, so we can't
 * pass it to ``Py_AddPendingCall``. This is handled by actually using
 * a second object that's thread local (ThreadStateCreator) and having
 * it dynamically allocate this object so it can live until the
 * pending call runs.
 */


class ThreadState {
private:
    // As of commit 08ad1dd7012b101db953f492e0021fb08634afad
    // this class needed 56 bytes in o Py_DEBUG build
    // on 64-bit macOS 11.
    // Adding the vector takes us up to 80 bytes ()

    /* Strong reference to the main greenlet */
    OwnedMainGreenlet main_greenlet;

    /* Strong reference to the current greenlet. */
    OwnedGreenlet current_greenlet;

    /* Strong reference to the trace function, if any. */
    OwnedObject tracefunc;

    typedef std::vector<PyGreenlet*, PythonAllocator<PyGreenlet*> > deleteme_t;
    /* A vector of raw PyGreenlet pointers representing things that need
       deleted when this thread is running. The vector owns the
       references, but you need to manually INCREF/DECREF as you use
       them. We don't use a vector<refs::OwnedGreenlet> because we
       make copy of this vector, and that would become O(n) as all the
       refcounts are incremented in the copy.
    */
    deleteme_t deleteme;

#ifdef GREENLET_NEEDS_EXCEPTION_STATE_SAVED
    void* exception_state;
#endif

    static std::clock_t _clocks_used_doing_gc;
    static ImmortalString get_referrers_name;
    static PythonAllocator<ThreadState> allocator;

    G_NO_COPIES_OF_CLS(ThreadState);


    // Allocates a main greenlet for the thread state. If this fails,
    // exits the process. Called only during constructing a ThreadState.
    MainGreenlet* alloc_main()
    {
        PyGreenlet* gmain;

        /* create the main greenlet for this thread */
        gmain = reinterpret_cast<PyGreenlet*>(PyType_GenericAlloc(&PyGreenlet_Type, 0));
        if (gmain == NULL) {
            throw PyFatalError("alloc_main failed to alloc"); //exits the process
        }

        MainGreenlet* const main = new MainGreenlet(gmain, this);

        assert(Py_REFCNT(gmain) == 1);
        assert(gmain->pimpl == main);
        return main;
    }


public:
    static void* operator new(size_t UNUSED(count))
    {
        return ThreadState::allocator.allocate(1);
    }

    static void operator delete(void* ptr)
    {
        return ThreadState::allocator.deallocate(static_cast<ThreadState*>(ptr),
                                                 1);
    }

    static void init()
    {
        ThreadState::get_referrers_name = "get_referrers";
        ThreadState::_clocks_used_doing_gc = 0;
    }

    ThreadState()
    {

#ifdef GREENLET_NEEDS_EXCEPTION_STATE_SAVED
        this->exception_state = slp_get_exception_state();
#endif

        // XXX: Potentially dangerous, exposing a not fully
        // constructed object.
        MainGreenlet* const main = this->alloc_main();
        this->main_greenlet = OwnedMainGreenlet::consuming(
            main->self()
        );
        assert(this->main_greenlet);
        this->current_greenlet = main->self();
        // The main greenlet starts with 1 refs: The returned one. We
        // then copied it to the current greenlet.
        assert(this->main_greenlet.REFCNT() == 2);
    }

    inline void restore_exception_state()
    {
#ifdef GREENLET_NEEDS_EXCEPTION_STATE_SAVED
        // It's probably important this be inlined and only call C
        // functions to avoid adding an SEH frame.
        slp_set_exception_state(this->exception_state);
#endif
    }

    inline bool has_main_greenlet() const noexcept
    {
        return bool(this->main_greenlet);
    }

    // Called from the ThreadStateCreator when we're in non-standard
    // threading mode. In that case, there is an object in the Python
    // thread state dictionary that points to us. The main greenlet
    // also traverses into us, in which case it's crucial not to
    // traverse back into the main greenlet.
    int tp_traverse(visitproc visit, void* arg, bool traverse_main=true)
    {
        if (traverse_main) {
            Py_VISIT(main_greenlet.borrow_o());
        }
        if (traverse_main || current_greenlet != main_greenlet) {
            Py_VISIT(current_greenlet.borrow_o());
        }
        Py_VISIT(tracefunc.borrow());
        return 0;
    }

    inline BorrowedMainGreenlet borrow_main_greenlet() const noexcept
    {
        assert(this->main_greenlet);
        assert(this->main_greenlet.REFCNT() >= 2);
        return this->main_greenlet;
    };

    inline OwnedMainGreenlet get_main_greenlet() const noexcept
    {
        return this->main_greenlet;
    }

    /**
     * In addition to returning a new reference to the currunt
     * greenlet, this performs any maintenance needed.
     */
    inline OwnedGreenlet get_current()
    {
        /* green_dealloc() cannot delete greenlets from other threads, so
           it stores them in the thread dict; delete them now. */
        this->clear_deleteme_list();
        //assert(this->current_greenlet->main_greenlet == this->main_greenlet);
        //assert(this->main_greenlet->main_greenlet == this->main_greenlet);
        return this->current_greenlet;
    }

    /**
     * As for non-const get_current();
     */
    inline BorrowedGreenlet borrow_current()
    {
        this->clear_deleteme_list();
        return this->current_greenlet;
    }

    /**
     * Does no maintenance.
     */
    inline OwnedGreenlet get_current() const
    {
        return this->current_greenlet;
    }

    template<typename T, refs::TypeChecker TC>
    inline bool is_current(const refs::PyObjectPointer<T, TC>& obj) const
    {
        return this->current_greenlet.borrow_o() == obj.borrow_o();
    }

    inline void set_current(const OwnedGreenlet& target)
    {
        this->current_greenlet = target;
    }

private:
    /**
     * Deref and remove the greenlets from the deleteme list. Must be
     * holding the GIL.
     *
     * If *murder* is true, then we must be called from a different
     * thread than the one that these greenlets were running in.
     * In that case, if the greenlet was actually running, we destroy
     * the frame reference and otherwise make it appear dead before
     * proceeding; otherwise, we would try (and fail) to raise an
     * exception in it and wind up right back in this list.
     */
    inline void clear_deleteme_list(const bool murder=false)
    {
        if (!this->deleteme.empty()) {
            // It's possible we could add items to this list while
            // running Python code if there's a thread switch, so we
            // need to defensively copy it before that can happen.
            deleteme_t copy = this->deleteme;
            this->deleteme.clear(); // in case things come back on the list
            for(deleteme_t::iterator it = copy.begin(), end = copy.end();
                it != end;
                ++it ) {
                PyGreenlet* to_del = *it;
                if (murder) {
                    // Force each greenlet to appear dead; we can't raise an
                    // exception into it anymore anyway.
                    to_del->pimpl->murder_in_place();
                }

                // The only reference to these greenlets should be in
                // this list, decreffing them should let them be
                // deleted again, triggering calls to green_dealloc()
                // in the correct thread (if we're not murdering).
                // This may run arbitrary Python code and switch
                // threads or greenlets!
                Py_DECREF(to_del);
                if (PyErr_Occurred()) {
                    PyErr_WriteUnraisable(nullptr);
                    PyErr_Clear();
                }
            }
        }
    }

public:

    /**
     * Returns a new reference, or a false object.
     */
    inline OwnedObject get_tracefunc() const
    {
        return tracefunc;
    };


    inline void set_tracefunc(BorrowedObject tracefunc)
    {
        assert(tracefunc);
        if (tracefunc == BorrowedObject(Py_None)) {
            this->tracefunc.CLEAR();
        }
        else {
            this->tracefunc = tracefunc;
        }
    }

    /**
     * Given a reference to a greenlet that some other thread
     * attempted to delete (has a refcount of 0) store it for later
     * deletion when the thread this state belongs to is current.
     */
    inline void delete_when_thread_running(PyGreenlet* to_del)
    {
        Py_INCREF(to_del);
        this->deleteme.push_back(to_del);
    }

    /**
     * Set to std::clock_t(-1) to disable.
     */
    inline static std::clock_t& clocks_used_doing_gc()
    {
        return ThreadState::_clocks_used_doing_gc;
    }

    ~ThreadState()
    {
        if (!PyInterpreterState_Head()) {
            // We shouldn't get here (our callers protect us)
            // but if we do, all we can do is bail early.
            return;
        }

        // We should not have an "origin" greenlet; that only exists
        // for the temporary time during a switch, which should not
        // be in progress as the thread dies.
        //assert(!this->switching_state.origin);

        this->tracefunc.CLEAR();

        // Forcibly GC as much as we can.
        this->clear_deleteme_list(true);

        // The pending call did this.
        assert(this->main_greenlet->thread_state() == nullptr);

        // If the main greenlet is the current greenlet,
        // then we "fell off the end" and the thread died.
        // It's possible that there is some other greenlet that
        // switched to us, leaving a reference to the main greenlet
        // on the stack, somewhere uncollectible. Try to detect that.
        if (this->current_greenlet == this->main_greenlet && this->current_greenlet) {
            assert(this->current_greenlet->is_currently_running_in_some_thread());
            // Drop one reference we hold.
            this->current_greenlet.CLEAR();
            assert(!this->current_greenlet);
            // Only our reference to the main greenlet should be left,
            // But hold onto the pointer in case we need to do extra cleanup.
            PyGreenlet* old_main_greenlet = this->main_greenlet.borrow();
            Py_ssize_t cnt = this->main_greenlet.REFCNT();
            this->main_greenlet.CLEAR();
            if (ThreadState::_clocks_used_doing_gc != std::clock_t(-1)
                && cnt == 2 && Py_REFCNT(old_main_greenlet) == 1) {
                // Highly likely that the reference is somewhere on
                // the stack, not reachable by GC. Verify.
                // XXX: This is O(n) in the total number of objects.
                // TODO: Add a way to disable this at runtime, and
                // another way to report on it.
                std::clock_t begin = std::clock();
                NewReference gc(PyImport_ImportModule("gc"));
                if (gc) {
                    OwnedObject get_referrers = gc.PyRequireAttr(ThreadState::get_referrers_name);
                    OwnedList refs(get_referrers.PyCall(old_main_greenlet));
                    if (refs && refs.empty()) {
                        assert(refs.REFCNT() == 1);
                        // We found nothing! So we left a dangling
                        // reference: Probably the last thing some
                        // other greenlet did was call
                        // 'getcurrent().parent.switch()' to switch
                        // back to us. Clean it up. This will be the
                        // case on CPython 3.7 and newer, as they use
                        // an internal calling conversion that avoids
                        // creating method objects and storing them on
                        // the stack.
                        Py_DECREF(old_main_greenlet);
                    }
                    else if (refs
                             && refs.size() == 1
                             && PyCFunction_Check(refs.at(0))
                             && Py_REFCNT(refs.at(0)) == 2) {
                        assert(refs.REFCNT() == 1);
                        // Ok, we found a C method that refers to the
                        // main greenlet, and its only referenced
                        // twice, once in the list we just created,
                        // once from...somewhere else. If we can't
                        // find where else, then this is a leak.
                        // This happens in older versions of CPython
                        // that create a bound method object somewhere
                        // on the stack that we'll never get back to.
                        if (PyCFunction_GetFunction(refs.at(0).borrow()) == (PyCFunction)green_switch) {
                            BorrowedObject function_w = refs.at(0);
                            refs.clear(); // destroy the reference
                                          // from the list.
                            // back to one reference. Can *it* be
                            // found?
                            assert(function_w.REFCNT() == 1);
                            refs = get_referrers.PyCall(function_w);
                            if (refs && refs.empty()) {
                                // Nope, it can't be found so it won't
                                // ever be GC'd. Drop it.
                                Py_CLEAR(function_w);
                            }
                        }
                    }
                    std::clock_t end = std::clock();
                    ThreadState::_clocks_used_doing_gc += (end - begin);
                }
            }
        }

        // We need to make sure this greenlet appears to be dead,
        // because otherwise deallocing it would fail to raise an
        // exception in it (the thread is dead) and put it back in our
        // deleteme list.
        if (this->current_greenlet) {
            this->current_greenlet->murder_in_place();
            this->current_greenlet.CLEAR();
        }

        if (this->main_greenlet) {
            // Couldn't have been the main greenlet that was running
            // when the thread exited (because we already cleared this
            // pointer if it was). This shouldn't be possible?

            // If the main greenlet was current when the thread died (it
            // should be, right?) then we cleared its self pointer above
            // when we cleared the current greenlet's main greenlet pointer.
            // assert(this->main_greenlet->main_greenlet == this->main_greenlet
            //        || !this->main_greenlet->main_greenlet);
            // // self reference, probably gone
            // this->main_greenlet->main_greenlet.CLEAR();

            // This will actually go away when the ivar is destructed.
            this->main_greenlet.CLEAR();
        }

        if (PyErr_Occurred()) {
            PyErr_WriteUnraisable(NULL);
            PyErr_Clear();
        }

    }

};

ImmortalString ThreadState::get_referrers_name(nullptr);
PythonAllocator<ThreadState> ThreadState::allocator;
std::clock_t ThreadState::_clocks_used_doing_gc(0);


}; // namespace greenlet

#endif