fix/boehmgc-coroutine-sp-fallback.diff

diff --git a/pthread_stop_world.c b/pthread_stop_world.c
index 1cee6a0b..46c3acd9 100644
--- a/pthread_stop_world.c
+++ b/pthread_stop_world.c
@@ -674,6 +674,8 @@ GC_INNER void GC_push_all_stacks(void)
     struct GC_traced_stack_sect_s *traced_stack_sect;
     pthread_t self = pthread_self();
     word total_size = 0;
+    size_t stack_limit;
+    pthread_attr_t pattr;
 
     if (!EXPECT(GC_thr_initialized, TRUE))
       GC_thr_init();
@@ -723,6 +725,28 @@ GC_INNER void GC_push_all_stacks(void)
           hi = p->altstack + p->altstack_size;
           /* FIXME: Need to scan the normal stack too, but how ? */
           /* FIXME: Assume stack grows down */
+        } else {
+          if (pthread_getattr_np(p->id, &pattr)) {
+            ABORT("GC_push_all_stacks: pthread_getattr_np failed!");
+          }
+          if (pthread_attr_getstacksize(&pattr, &stack_limit)) {
+            ABORT("GC_push_all_stacks: pthread_attr_getstacksize failed!");
+          }
+          // 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.
+          // We assume that the coroutine has similarly added its entire stack.
+          // This could be made accurate by cooperating with the application
+          // via new functions and/or callbacks.
+          #ifndef STACK_GROWS_UP
+            if (lo >= hi || lo < hi - stack_limit) { // sp outside stack
+              lo = hi - stack_limit;
+            }
+          #else
+          #error "STACK_GROWS_UP not supported in boost_coroutine2 (as of june 2021), so we don't support it in Nix."
+          #endif
         }
         GC_push_all_stack_sections(lo, hi, traced_stack_sect);
 #       ifdef STACK_GROWS_UP