[Tarantool-patches] [PATCH luajit v2] Throw any errors before stack changes in trace stitching.

[Maxim Kokryashkin]

From: Mike Pall <mike>

Thanks to doujiang24.

(cherry-picked from commit 3f9389edc6cdf3f78a6896d550c236860aed62b2)

The Lua stack is changed in the `recff_stitch`, so if the trace
is aborted, for example, when the `maxsnap` is reached, and an
error is thrown from there, the execution continues with an
unbalanced stack.

This patch adds a check for snapshot overflow to the
`recff_stitch` to overcome the issue for the `LJ_TRERR_SNAPOV`
case, but other cases remain unresolved and should be fixed in
other series. For a detailed description of the remaining cases,
see the issue[1].

[1]: https://github.com/LuaJIT/LuaJIT/issues/1166

Maxim Kokryashkin:
* added the description and the test for the problem

Part of tarantool/tarantool#9595
---
Changes in v2:
- Fixed comments as per review by Sergey Kaplun

Branch: https://github.com/tarantool/luajit/tree/fckxorg/lj-720-errors-before-stitch

 src/lj_ffrecord.c                             |  4 ++++
 .../lj-720-errors-before-stitch.test.lua      | 20 +++++++++++++++++++
 2 files changed, 24 insertions(+)
 create mode 100644 test/tarantool-tests/lj-720-errors-before-stitch.test.lua

diff --git a/src/lj_ffrecord.c b/src/lj_ffrecord.c
index 1b2ddb72..e3ed80fb 100644
--- a/src/lj_ffrecord.c
+++ b/src/lj_ffrecord.c
@@ -107,6 +107,10 @@ static void recff_stitch(jit_State *J)
   const BCIns *pc = frame_pc(base-1);
   TValue *pframe = frame_prevl(base-1);

+  /* Check for this now. Throwing in lj_record_stop messes up the stack. */
+  if (J->cur.nsnap >= (MSize)J->param[JIT_P_maxsnap])
+    lj_trace_err(J, LJ_TRERR_SNAPOV);
+
   /* Move func + args up in Lua stack and insert continuation. */
   memmove(&base[1], &base[-1-LJ_FR2], sizeof(TValue)*nslot);
   setframe_ftsz(nframe, ((char *)nframe - (char *)pframe) + FRAME_CONT);
diff --git a/test/tarantool-tests/lj-720-errors-before-stitch.test.lua b/test/tarantool-tests/lj-720-errors-before-stitch.test.lua
new file mode 100644
index 00000000..00fb0b9b
--- /dev/null
+++ b/test/tarantool-tests/lj-720-errors-before-stitch.test.lua
@@ -0,0 +1,20 @@
+local tap = require('tap')
+local test = tap.test('lj-720-errors-before-stitch'):skipcond({
+  ['Test requires JIT enabled'] = not jit.status(),
+})
+test:plan(1)
+
+-- `math.modf` recording is NYI.
+local modf = math.modf
+jit.opt.start('hotloop=1', 'maxsnap=1')
+
+-- The loop has only two iterations: the first to detect its
+-- hotness and the second to record it. The snapshot limit is
+-- set to one and is certainly reached.
+for _ = 1, 2 do
+  -- Forcify stitch.
+  modf(1.2)
+end
+
+test:ok(true, 'stack is balanced')
+test:done(true)
--
2.43.0