backend crash when saving a pv image generated from a FITS cube

[kswang1029]

Describe the bug
The backend crashes when saving a PV image generated from a FITS cube with a specific set of headers. This is reported by a user with v3b2 using the centos8 AppImage. The same issue can be reproduced with macOS too.

To Reproduce
Steps to reproduce the behavior:

1. load the attached test FITS cube
2. generate a PV image
3. save the PV image as FITS
4. backend should crash

Expected behavior
Backend should not crash and saving PV should be successful.

Screenshots or videos
N/A

Platform info (please complete the following information):

• OS: macOS Monterey
• Browser: only tried Chrome
• Version: 97.0.4692.71

Additional context
backend crash report from macOS

Thread 0 Crashed:: Dispatch queue: com.apple.main-thread
0 libc++.1.dylib 0x7ff81625743d std::__1::basic_string<char, std::__1::char_traits, std::__1::allocator >::operator=(std::__1::basic_string<char, std::__1::char_traits, std::__1::allocator > const&) + 9
1 libcasa_coordinates.5.dylib 0x10a397576 casacore::ObsInfo::copy_other(casacore::ObsInfo const&) + 38
2 libcasa_coordinates.5.dylib 0x10a39772e casacore::ObsInfo::operator=(casacore::ObsInfo const&) + 14
3 libcasa_coordinates.5.dylib 0x10a335450 casacore::CoordinateSystem::copy(casacore::CoordinateSystem const&) + 144
4 libcasa_coordinates.5.dylib 0x10a336304 casacore::CoordinateSystem::CoordinateSystem(casacore::CoordinateSystem const&) + 756
5 carta_backend 0x10765a9a4 GetSpectralCoordPreferences(casacore::ImageInterface, bool&, bool&, bool&, bool&) + 68 (Casacore.cc:121)
6 carta_backend 0x10743e471 carta::Frame::ExportFITSImage(casacore::ImageInterface&, std::__1::__fs::filesystem::path, casacore::String&) + 49 (Frame.cc:1928)
7 carta_backend 0x10744b9b5 carta::Frame::SaveFile(std::__1::basic_string<char, std::__1::char_traits, std::__1::allocator > const&, CARTA::SaveFile const&, CARTA::SaveFileAck&, std::__1::shared_ptrcarta::Region) + 1829 (Frame.cc:1840)
8 carta_backend 0x1075f9f58 carta::Session::OnSaveFile(CARTA::SaveFile const&, unsigned int) + 1880 (Session.cc:1294)
9 carta_backend 0x107604ad3 carta::SessionManager::OnMessage(uWS::WebSocket<false, true, carta::PerSocketData>, std::__1::basic_string_view<char, std::__1::char_traits >, uWS::OpCode) + 2547 (SessionManager.cc:372)
10 carta_backend 0x10760e9e4 ofats::any_detail::any_invocable_impl<void, false, uWS::WebSocket<false, true, carta::PerSocketData>, std::__1::basic_string_view<char, std::__1::char_traits >, uWS::OpCode>::call(uWS::WebSocket<false, true, carta::PerSocketData>, std::__1::basic_string_view<char, std::__1::char_traits >, uWS::OpCode) + 17 (MoveOnlyFunction.h:247) [inlined]
11 carta_backend 0x10760e9e4 ofats::any_invocable<void (uWS::WebSocket<false, true, carta::PerSocketData>, std::__1::basic_string_view<char, std::__1::char_traits >, uWS::OpCode)>::operator()(uWS::WebSocket<false, true, carta::PerSocketData>, std::__1::basic_string_view<char, std::__1::char_traits >, uWS::OpCode) + 17 (MoveOnlyFunction.h:354) [inlined]
12 carta_backend 0x10760e9e4 uWS::WebSocketContext<false, true, carta::PerSocketData>::handleFragment(char*, unsigned long, unsigned int, int, bool, uWS::WebSocketState, void) + 1300 (WebSocketContext.h:93)
13 carta_backend 0x10760dc56 bool uWS::WebSocketProtocol<true, uWS::WebSocketContext<false, true, carta::PerSocketData> >::consumeMessage<6u, unsigned char>(unsigned char, char*&, unsigned int&, uWS::WebSocketState, void) + 726 (WebSocketProtocol.h:328)
14 carta_backend 0x10760d8d0 uWS::WebSocketProtocol<true, uWS::WebSocketContext<false, true, carta::PerSocketData> >::consume(char*, unsigned int, uWS::WebSocketState, void) + 272 (WebSocketProtocol.h:424)
15 carta_backend 0x10760d76a auto uWS::WebSocketContext<false, true, carta::PerSocketData>::init()::'lambda'(auto*, char*, int)::operator()<us_socket_t>(auto*, char*, int) const + 138 (WebSocketContext.h:286)
16 carta_backend 0x10776aca5 us_loop_run + 133 (epoll_kqueue.c:147)
17 carta_backend 0x107609d67 uWS::Loop::run() + 8 (Loop.h:159) [inlined]
18 carta_backend 0x107609d67 uWS::run() + 15 (Loop.h:176) [inlined]
19 carta_backend 0x107609d67 uWS::TemplatedApp::run() + 15 (App.h:393) [inlined]
20 carta_backend 0x107609d67 carta::SessionManager::RunApp() + 343 (SessionManager.cc:555)
21 carta_backend 0x107552ed3 main + 4931 (Main.cc:208)
22 dyld 0x1162674fe start + 462

test.fits.zip

[kswang1029]

@pford would you be able to investigate this for the v3-beta3 release?

[kswang1029]

After some more tests with a mockup cube, the root cause is identified.

• if the input FITS cube is 4D as ra-dec-channel-stokes, then the generated PV image can be saved without problems
• if the input FITS cube is 3D as ra-dec-channel, then the generated PV image cannot be saved resulting a backend crash.

So I guess this is something to do with the Stokes axis. Does the backend always assume there is a Stokes axis in the saved PV image?