Hi, Max

Thanks for the patch! LGTM

See my comments below.

Sergey

<snipped>

diff --git a/test/tarantool-tests/lj-946-print-errors-from-gc-fin-custom.test.lua b/test/tarantool-tests/lj-946-print-errors-from-gc-fin-custom.test.lua
new file mode 100644
index 00000000..71efc260
--- /dev/null
+++ b/test/tarantool-tests/lj-946-print-errors-from-gc-fin-custom.test.lua
@@ -0,0 +1,42 @@
+local tap = require('tap')
+local test = tap.test('lj-946-print-errors-from-gc-fin-custom'):skipcond({
+  ['Test requires JIT enabled'] = not jit.status(),
+})
+
+test:plan(2)
+
+local ffi = require('ffi')
+local error_in_finalizer = false
+
+local function errfin_handler()
+    error_in_finalizer = true
+end
+
+local function new_bad_cdata()
+  return ffi.gc(ffi.new('char [?]', 1024), 'uncallable string')
+end
+
+local function test_f()
+  collectgarbage('collect')
+  -- Make GC aggressive enough to end the atomic phase before
+  -- exiting the trace.
+  collectgarbage('setstepmul', 400)
+  -- The number of iterations is empirical, just big enough for the
+  -- issue to strike.
+  for _ = 1, 4000 do

Works fine with a number of iterations equal to 300,

checked locally by running test 1000 times.

+    new_bad_cdata()
+  end
+end
+
+jit.opt.start('hotloop=1')
+-- Handler is registered but never called before the patch.
+-- It should be called after the patch.
+jit.attach(errfin_handler, 'errfin')
+local status = pcall(test_f)
+-- We have to stop GC now because any step raises the error due to
+-- cursed cdata objects.
+collectgarbage('stop')
+test:ok(status, 'test function completed successfully')
+test:ok(error_in_finalizer, 'error handler called')
+
+test:done(true)
diff --git a/test/tarantool-tests/lj-946-print-errors-from-gc-fin-default.test.lua b/test/tarantool-tests/lj-946-print-errors-from-gc-fin-default.test.lua
new file mode 100644
index 00000000..dfef11e5
--- /dev/null
+++ b/test/tarantool-tests/lj-946-print-errors-from-gc-fin-default.test.lua
@@ -0,0 +1,11 @@
+local tap = require('tap')
+local test = tap.test('lj-flush-on-trace'):skipcond({
+  ['Test requires JIT enabled'] = not jit.status(),
+})
+
+test:plan(1)
+
+local script = require('utils').exec.makecmd(arg, { redirect = '2>&1' })
+local output = script()
+test:like(output, '.*ERROR in finalizer:.*')

testcase description is missed:

TAP version 13
1..1
ok - nil

+test:done(true)
diff --git a/test/tarantool-tests/lj-946-print-errors-from-gc-fin-default/script.lua b/test/tarantool-tests/lj-946-print-errors-from-gc-fin-default/script.lua
new file mode 100644
index 00000000..fdd9ced1
--- /dev/null
+++ b/test/tarantool-tests/lj-946-print-errors-from-gc-fin-default/script.lua
@@ -0,0 +1,24 @@
+local ffi = require('ffi')
+
+local function new_bad_cdata()
+  return ffi.gc(ffi.new('char [?]', 1024), 'uncallable string')
+end
+
+local function test_f()
+  collectgarbage('collect')
+  -- Make GC aggressive enough to end the atomic phase before
+  -- exiting the trace.
+  collectgarbage('setstepmul', 400)
+  -- The number of iterations is empirical, just big enough for the
+  -- issue to strike.
+  for _ = 1, 4000 do
+    new_bad_cdata()
+  end
+end
+
+jit.opt.start('hotloop=1')
+local status = pcall(test_f)
+-- We have to stop GC now because any step raises the error due to
+-- cursed cdata objects.
+collectgarbage('stop')
+assert(status, 'error is not rethrown')
--
2.39.3 (Apple Git-145)