Hi, Sergey!

thanks for the patch! LGTM with a couple of comments below

Sergey

From: Mike Pall <mike>

Reported by Sergey Kaplun.

(cherry picked from commit fbb36bb6bfa88716a47c58bcf9ce9f2ef752abac)

After the previous commit, the VM handler for the 'errfin' VM event
works incorrectly. The error object is taken not from the stack on which
the VM handler is invoked. Hence, it breaks the non-VM stack and returns
an incorrect error message.

This patch fixes the issue by copying the object from the corresponding
Lua stack.

Sergey Kaplun:
* added the description and the test for the problem

Part of tarantool/tarantool#12134
---
 src/lj_gc.c                                   |  6 +++--
 .../lj-1445-errfin-errmsg.test.lua            | 27 +++++++++++++++++++
 2 files changed, 31 insertions(+), 2 deletions(-)
 create mode 100644 test/tarantool-tests/lj-1445-errfin-errmsg.test.lua

diff --git a/src/lj_gc.c b/src/lj_gc.c
index 83ad9ee9..7e4f0a47 100644
--- a/src/lj_gc.c
+++ b/src/lj_gc.c
@@ -519,10 +519,12 @@ static void gc_call_finalizer(global_State *g, lua_State *L,
   if (LJ_HASPROFILE && (oldh & HOOK_PROFILE)) lj_dispatch_update(g);
   g->gc.threshold = oldt;  /* Restore GC threshold. */
   if (errcode) {
+    TValue tmp;
+    copyTV(VL, &tmp, VL->top-1);
+    VL->top--;
     lj_vmevent_send(g, ERRFIN,
-      copyTV(V, V->top++, L->top-1);
+      copyTV(V, V->top++, &tmp);
     );
-    L->top--;
   }
 }
 
diff --git a/test/tarantool-tests/lj-1445-errfin-errmsg.test.lua b/test/tarantool-tests/lj-1445-errfin-errmsg.test.lua
new file mode 100644
index 00000000..def8b79c
--- /dev/null
+++ b/test/tarantool-tests/lj-1445-errfin-errmsg.test.lua
@@ -0,0 +1,27 @@
+local tap = require('tap')
+local test = tap.test('lj-1445-errfin-errmsg')
+

Usually you add a comment with test description and a link to the issue.

But this time a comment is absent. Should we add it?

+test:plan(2)
+
+local EXPECTED_ERR = 'expected err'

s/err/error/

feel free to ignore

+local function bad_fin()
+  error(EXPECTED_ERR)
+end
+local EXPECTED_LOCATION = debug.getinfo(bad_fin).linedefined + 1
+
+local match_err = false
+local match_line = false
+local function errfin_handler(errmsg)
+  match_err = errmsg:match(EXPECTED_ERR)
+  match_line = errmsg:match(EXPECTED_LOCATION)
+end
+
+jit.attach(errfin_handler, 'errfin')
+
+debug.getmetatable(newproxy(true)).__gc = bad_fin

I would define userdata previously for clarity:

local ud = newproxy(true)

and then use it in getmetatable().

feel free to ignore

+collectgarbage()
+
+test:ok(match_err, 'correct error message in errfin handler')
+test:ok(match_line, 'correct source line in errfin handler')
+
+test:done(true)