Record update uses inplace beam directive unsafely in specific scenario, which causes a variable to be mutated #10367
Description
Describe the bug
When the below code is compiled in OTP 28.1.1, it uses the inplace beam directive, but this can cause P2 to be updated before it is used for the final time.
I tried to simplify the case, or otherwise determine what specifically about this code causes the mis-compilation, but this is the best I could get. Any simplifying changes to the code cause the compiled code to behave correctly again.
To Reproduce
-module(test_0).
-export([update_test/0]).
-record(r, {x, y}).
gen() ->
[#r{x = a}, #r{x = b, y = dict:new()}].
update([_, #r{x = b} = P2]) ->
P2#r{x = a};
update([_, #r{x = a} = P2]) ->
P2#r{x = b}.
update_test() ->
[P1, P2] = gen(),
Expected = P2#r{x = a},
timer:sleep(0),
Expected = update([P1, P2]).Compiled and run with
erlc test_0.erl && erl -pa ./ -noshell -eval 'test_0:update_test(),init:stop().'The final line fails with a badmatch, because Expected has #r{x = a}, but the result of update/2 gives #r{x = b}.
Since P2 was defined with #r{x = b}, and update/2 swaps b for a, we would expect that update([P1, P2]) should return #r{x = a}.
In fact, P2 gets updated by Expected = P2#r{x = a}, because the beam code uses inplace:
{beam_file,test_0,
[{module_info,0,11},{module_info,1,13},{update_test,0,7}],
[{vsn,[39080679506587980296061558520440968194]}],
[{version,"9.0.2"},
{options,[]},
...
{function,update_test,0,7,
...
{update_record,{atom,inplace},
3,
{tr,{x,0},{t_tuple,0,false,#{}}},
{y,0},
{list,[2,{atom,a}]}},Expected behavior
The record update of P2 in update_test should not be compiled with the inplace directive, or if it did, it should certainly not affect usages of P2 after.
This can be seen to be correct in the beam code when compiled with OTP 27.3.4.3:
{beam_file,test_0,
[{module_info,0,11},{module_info,1,13},{update_test,0,7}],
[{vsn,[181881847084282917466935693926325942879]}],
[{version,"8.6.1.2"},
{options,[]},
...
{function,update_test,0,7,
...
{update_record,{atom,reuse},
3,
{tr,{x,0},{t_tuple,0,false,#{}}},
{y,0},
{list,[2,{atom,a}]}},Affected versions
Affects: OTP 28.1.1
Doesn't affect: OTP 27.3.4.3