From ebd6636a1e90dd12cb0ef112e7f1e0ca511f5d3b Mon Sep 17 00:00:00 2001
From: "Yukihiro \"Matz\" Matsumoto" <matz@ruby-lang.org>
Date: Thu, 15 Jun 2017 23:20:36 +0900
Subject: Call write barriers for stack-modified fibers; fix #3699

---
 mrbgems/mruby-fiber/src/fiber.c | 21 +++++++++++++--------
 src/vm.c                        | 10 +++++++++-
 2 files changed, 22 insertions(+), 9 deletions(-)

diff --git a/mrbgems/mruby-fiber/src/fiber.c b/mrbgems/mruby-fiber/src/fiber.c
index a0faafe28..8a6146dbe 100644
--- a/mrbgems/mruby-fiber/src/fiber.c
+++ b/mrbgems/mruby-fiber/src/fiber.c
@@ -171,6 +171,16 @@ fiber_check_cfunc(mrb_state *mrb, struct mrb_context *c)
   }
 }
 
+static void
+fiber_switch_context(mrb_state *mrb, struct mrb_context *c)
+{
+  if (mrb->c->fib) {
+    mrb_write_barrier(mrb, (struct RBasic*)mrb->c->fib);
+  }
+  c->status = MRB_FIBER_RUNNING;
+  mrb->c = c;
+}
+
 static mrb_value
 fiber_switch(mrb_state *mrb, mrb_value self, mrb_int len, const mrb_value *a, mrb_bool resume, mrb_bool vmexec)
 {
@@ -207,9 +217,7 @@ fiber_switch(mrb_state *mrb, mrb_value self, mrb_int len, const mrb_value *a, mr
   else {
     value = fiber_result(mrb, a, len);
   }
-  mrb_write_barrier(mrb, (struct RBasic*)c->fib);
-  c->status = MRB_FIBER_RUNNING;
-  mrb->c = c;
+  fiber_switch_context(mrb, c);
 
   if (vmexec) {
     c->vmexec = TRUE;
@@ -308,10 +316,8 @@ fiber_transfer(mrb_state *mrb, mrb_value self)
 
   if (c == mrb->root_c) {
     mrb->c->status = MRB_FIBER_TRANSFERRED;
-    mrb->c = c;
-    c->status = MRB_FIBER_RUNNING;
+    fiber_switch_context(mrb, c);
     MARK_CONTEXT_MODIFY(c);
-    mrb_write_barrier(mrb, (struct RBasic*)c->fib);
     return fiber_result(mrb, a, len);
   }
 
@@ -336,13 +342,12 @@ mrb_fiber_yield(mrb_state *mrb, mrb_int len, const mrb_value *a)
   fiber_check_cfunc(mrb, c);
   c->prev->status = MRB_FIBER_RUNNING;
   c->status = MRB_FIBER_SUSPENDED;
-  mrb->c = c->prev;
+  fiber_switch_context(mrb, c->prev);
   c->prev = NULL;
   if (c->vmexec) {
     c->vmexec = FALSE;
     mrb->c->ci->acc = CI_ACC_RESUMED;
   }
-  mrb_write_barrier(mrb, (struct RBasic*)c->fib);
   MARK_CONTEXT_MODIFY(mrb->c);
   return fiber_result(mrb, a, len);
 }
diff --git a/src/vm.c b/src/vm.c
index e3460d742..d69617be5 100644
--- a/src/vm.c
+++ b/src/vm.c
@@ -867,7 +867,12 @@ mrb_vm_run(mrb_state *mrb, struct RProc *proc, mrb_value self, unsigned int stac
   if (c->ci - c->cibase > cioff) {
     c->ci = c->cibase + cioff;
   }
-  mrb->c = c;
+  if (mrb->c != c) {
+    if (mrb->c->fib) {
+      mrb_write_barrier(mrb, (struct RBasic*)mrb->c->fib);
+    }
+    mrb->c = c;
+  }
   return result;
 }
 
@@ -1819,6 +1824,9 @@ RETRY_TRY_BLOCK:
               else {
                 struct mrb_context *c = mrb->c;
 
+                if (c->fib) {
+                  mrb_write_barrier(mrb, (struct RBasic*)c->fib);
+                }
                 mrb->c = c->prev;
                 c->prev = NULL;
                 goto L_RAISE;
-- 
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