libexpr: Use GC_set_sp_corrector instead of patch

Manually tested by printing to stderr in both branches (sp in os
stack, or not), and triggering a GC in a filterSource function,
e.g.:

    let
      generateTree = n: if n == 0 then "ha" else { left = generateTree (n - 1); right = generateTree (n - 1); };
    in
      builtins.deepSeq (generateTree 18) ...

Note that the darwin still uses the strategy of disabling GC, despite
having an implementation that compiles. The proper solution will be
enabled and tested later.

src/libexpr/eval.cc

@@ -31,6 +31,7 @@
 #include <sstream>
 #include <cstring>
 #include <optional>
+#include <pthread.h>
 #include <unistd.h>
 #include <sys/time.h>
 #include <fstream>
@@ -250,6 +251,50 @@ class BoehmGCStackAllocator : public StackAllocator {
 
 static BoehmGCStackAllocator boehmGCStackAllocator;
 
+/**
+ * When a thread goes into a coroutine, we lose its original sp until
+ * control flow returns to the thread.
+ * While in the coroutine, the sp points outside the thread stack,
+ * so we can detect this and push the entire thread stack instead,
+ * as an approximation.
+ * The coroutine's stack is covered by `BoehmGCStackAllocator`.
+ * This is not an optimal solution, because the garbage is scanned when a
+ * coroutine is active, for both the coroutine and the original thread stack.
+ * However, the implementation is quite lean, and usually we don't have active
+ * coroutines during evaluation, so this is acceptable.
+ */
+void fixupBoehmStackPointer(void ** sp_ptr, void * pthread_id) {
+    void *& sp = *sp_ptr;
+    pthread_attr_t pattr;
+    size_t osStackSize;
+    void * osStackLow;
+    void * osStackBase;
+
+    #ifdef __APPLE__
+    osStackSize = pthread_get_stacksize_np((pthread_t)pthread_id);
+    osStackLow = pthread_get_stackaddr_np((pthread_t)pthread_id);
+    #else
+    if (pthread_attr_init(&pattr)) {
+        throw Error("fixupBoehmStackPointer: pthread_attr_init failed");
+    }
+    if (pthread_getattr_np((pthread_t)pthread_id, &pattr)) {
+        throw Error("fixupBoehmStackPointer: pthread_getattr_np failed");
+    }
+    if (pthread_attr_getstack(&pattr, &osStackLow, &osStackSize)) {
+        throw Error("fixupBoehmStackPointer: pthread_attr_getstack failed");
+    }
+    if (pthread_attr_destroy(&pattr)) {
+        throw Error("fixupBoehmStackPointer: pthread_attr_destroy failed");
+    }
+    #endif
+    osStackBase = (char *)osStackLow + osStackSize;
+    // NOTE: We assume the stack grows down, as it does on all architectures we support.
+    //       Architectures that grow the stack up are rare.
+    if (sp >= osStackBase || sp < osStackLow) { // lo is outside the os stack
+        sp = osStackBase;
+    }
+}
+
 #endif
 
 
@@ -303,16 +348,21 @@ void initGC()
 
     GC_set_oom_fn(oomHandler);
 
+    // TODO: Comment suggests an implementation that works on darwin and windows
+    //       https://github.com/ivmai/bdwgc/issues/362#issuecomment-1936672196
+    #ifndef __APPLE__
+    GC_set_sp_corrector(&fixupBoehmStackPointer);
+    #endif
+
     StackAllocator::defaultAllocator = &boehmGCStackAllocator;
 
-
-#if NIX_BOEHM_PATCH_VERSION != 1
-    printTalkative("Unpatched BoehmGC, disabling GC inside coroutines");
-    /* Used to disable GC when entering coroutines on macOS */
-    create_coro_gc_hook = []() -> std::shared_ptr<void> {
-        return std::make_shared<BoehmDisableGC>();
-    };
-#endif
+    if (!GC_get_sp_corrector()) {
+        printTalkative("BoehmGC on this platform does not support sp_corrector; will disable GC inside coroutines");
+        /* Used to disable GC when entering coroutines on macOS */
+        create_coro_gc_hook = []() -> std::shared_ptr<void> {
+            return std::make_shared<BoehmDisableGC>();
+        };
+    }
 
     /* Set the initial heap size to something fairly big (25% of
        physical RAM, up to a maximum of 384 MiB) so that in most cases

src/libexpr/local.mk

@@ -15,7 +15,9 @@ libexpr_SOURCES := \
 
 INCLUDE_libexpr := -I $(d)
 
-libexpr_CXXFLAGS += $(INCLUDE_libutil) $(INCLUDE_libstore) $(INCLUDE_libfetchers) $(INCLUDE_libmain) $(INCLUDE_libexpr)
+libexpr_CXXFLAGS += \
+	$(INCLUDE_libutil) $(INCLUDE_libstore) $(INCLUDE_libfetchers) $(INCLUDE_libmain) $(INCLUDE_libexpr) \
+	-DGC_THREADS
 
 libexpr_LIBS = libutil libstore libfetchers