From 4c73eab92351e0942dc27f84fec1bf8c94619d02 Mon Sep 17 00:00:00 2001
From: Yorick van Pelt <yorick@yorickvanpelt.nl>
Date: Fri, 10 Feb 2023 15:04:17 +0100
Subject: [PATCH] DisableGC: replace by CoroutineContext, std::shared_ptr<void>

---
 src/libexpr/eval.cc      | 20 +++++++++++---------
 src/libutil/serialise.cc | 38 ++++++++++++++++++++------------------
 src/libutil/serialise.hh | 10 +++-------
 3 files changed, 34 insertions(+), 34 deletions(-)

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index a5a198926..01680afff 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -325,20 +325,22 @@ static Symbol getName(const AttrName & name, EvalState & state, Env & env)
     }
 }
 
-
-class BoehmDisableGC : public DisableGC {
+#if HAVE_BOEHMGC
+/* Disable GC while this object lives. Used by CoroutineContext.
+ *
+ * Boehm keeps a count of GC_disable() and GC_enable() calls,
+ * and only enables GC when the count matches.
+ */
+class BoehmDisableGC {
 public:
     BoehmDisableGC() {
-#if HAVE_BOEHMGC
         GC_disable();
-#endif
     };
-    virtual ~BoehmDisableGC() override {
-#if HAVE_BOEHMGC
+    ~BoehmDisableGC() {
         GC_enable();
-#endif
     };
 };
+#endif
 
 static bool gcInitialised = false;
 
@@ -365,8 +367,8 @@ void initGC()
 
 
     /* Used to disable GC when entering coroutines on macOS */
-    DisableGC::create = []() {
-        return std::dynamic_pointer_cast<DisableGC>(std::make_shared<BoehmDisableGC>());
+    create_disable_gc = []() -> std::shared_ptr<void> {
+        return std::make_shared<BoehmDisableGC>();
     };
 
     /* Set the initial heap size to something fairly big (25% of
diff --git a/src/libutil/serialise.cc b/src/libutil/serialise.cc
index 531100c01..7ef24d458 100644
--- a/src/libutil/serialise.cc
+++ b/src/libutil/serialise.cc
@@ -186,8 +186,21 @@ static DefaultStackAllocator defaultAllocatorSingleton;
 StackAllocator *StackAllocator::defaultAllocator = &defaultAllocatorSingleton;
 
 
-std::shared_ptr<DisableGC> (*DisableGC::create)() = []() {
-    return std::dynamic_pointer_cast<DisableGC>(std::make_shared<DisableGC>());
+std::shared_ptr<void> (*create_disable_gc)() = []() -> std::shared_ptr<void> {
+    return {};
+};
+
+/* This class is used for entry and exit hooks on coroutines */
+class CoroutineContext {
+#if __APPLE__
+    /* Disable GC when entering the coroutine on macOS, since it doesn't find the main thread stack in this case.
+     * std::shared_ptr<void> performs type-erasure, so it will call the right
+     * deleter. */
+    const std::shared_ptr<void> disable_gc = create_disable_gc();
+#endif
+public:
+    CoroutineContext() {};
+    ~CoroutineContext() {};
 };
 
 std::unique_ptr<FinishSink> sourceToSink(std::function<void(Source &)> fun)
@@ -211,10 +224,7 @@ std::unique_ptr<FinishSink> sourceToSink(std::function<void(Source &)> fun)
             cur = in;
 
             if (!coro) {
-#if __APPLE__
-                /* Disable GC when entering the coroutine on macOS, since it doesn't find the main thread stack in this case */
-                auto disablegc = DisableGC::create();
-#endif
+                CoroutineContext ctx;
                 coro = coro_t::push_type(VirtualStackAllocator{}, [&](coro_t::pull_type & yield) {
                     LambdaSource source([&](char *out, size_t out_len) {
                         if (cur.empty()) {
@@ -236,9 +246,7 @@ std::unique_ptr<FinishSink> sourceToSink(std::function<void(Source &)> fun)
             if (!*coro) { abort(); }
 
             if (!cur.empty()) {
-#if __APPLE__
-                auto disablegc = DisableGC::create();
-#endif
+                CoroutineContext ctx;
                 (*coro)(false);
             }
         }
@@ -248,9 +256,7 @@ std::unique_ptr<FinishSink> sourceToSink(std::function<void(Source &)> fun)
             if (!coro) return;
             if (!*coro) abort();
             {
-#if __APPLE__
-                auto disablegc = DisableGC::create();
-#endif
+                CoroutineContext ctx;
                 (*coro)(true);
             }
             if (*coro) abort();
@@ -284,9 +290,7 @@ std::unique_ptr<Source> sinkToSource(
         size_t read(char * data, size_t len) override
         {
             if (!coro) {
-#if __APPLE__
-                auto disablegc = DisableGC::create();
-#endif
+                CoroutineContext ctx;
                 coro = coro_t::pull_type(VirtualStackAllocator{}, [&](coro_t::push_type & yield) {
                     LambdaSink sink([&](std::string_view data) {
                         if (!data.empty()) yield(std::string(data));
@@ -299,9 +303,7 @@ std::unique_ptr<Source> sinkToSource(
 
             if (pos == cur.size()) {
                 if (!cur.empty()) {
-#if __APPLE__
-                    auto disablegc = DisableGC::create();
-#endif
+                    CoroutineContext ctx;
                     (*coro)();
                 }
                 cur = coro->get();
diff --git a/src/libutil/serialise.hh b/src/libutil/serialise.hh
index fb1f14a3b..58b9499db 100644
--- a/src/libutil/serialise.hh
+++ b/src/libutil/serialise.hh
@@ -502,13 +502,9 @@ struct StackAllocator {
 };
 
 /* Disabling GC when entering a coroutine (on macos).
-   ::create is to avoid boehm gc dependency in libutil.
+   mutable to avoid boehm gc dependency in libutil.
  */
-class DisableGC {
-public:
-    DisableGC() {};
-    virtual ~DisableGC() {};
-    static std::shared_ptr<DisableGC> (*create)();
-};
+extern std::shared_ptr<void> (*create_disable_gc)();
+
 
 }