Fix missing C stderr output (fprintf(stderr, ...) not shown)

[anutosh491]

Description

Fixes #381

Type of change

Please tick all options which are relevant.

• Bug fix
• New feature
• Added/removed dependencies
• Required documentation updates

[anutosh491]

The Problem here

C++ error output via std::cerr appeared correctly because xeus-cpp redirects the iostream buffers to its xoutput_buffer. However, C-level error output via fprintf(stderr, ...) did not appear. Only printf(...) and fprintf(stdout, ...) worked, because StreamRedirectRAII forwarded the captured stdout stream but silently dropped the captured stderr stream during normal execution.

Hence causing this

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.81%. Comparing base (190caf6) to head (028cec0).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #402      +/-   ##
==========================================
+ Coverage   81.77%   81.81%   +0.04%     
==========================================
  Files          20       20              
  Lines         845      847       +2     
  Branches       87       89       +2     
==========================================
+ Hits          691      693       +2     
  Misses        154      154              

Files with missing lines	Coverage Δ
src/xinterpreter.cpp	90.68% <100.00%> (+0.11%)	⬆️

Files with missing lines	Coverage Δ
src/xinterpreter.cpp	90.68% <100.00%> (+0.11%)	⬆️

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[github-actions]

clang-tidy made some suggestions

[github-actions]

warning: member 'out' of type 'std::string &' (aka 'basic_string &') is a reference [cppcoreguidelines-avoid-const-or-ref-data-members]

      std::string &out;
                   ^

[github-actions]

warning: no header providing "Cpp::kStdErr" is directly included [misc-include-cleaner]

        Cpp::BeginStdStreamCapture(Cpp::kStdErr);
                                        ^

[anutosh491]

Fix introduced

• StreamRedirectRAII now:

  • accepts two references (out, err)
  • begins capture in the correct order (stdout → stderr)
  • ends capture in LIFO order (stderr → stdout)
  • forwards neither inside the destructor

• execute_request_impl now:

  • emits the captured out → std::cout and captured err → std::cerr once
  • flushes them immediately.

This fixes what we want

And C++ based streams worked already

[mcbarton]

I have checked this PR, and it does solve the problem. @anutosh491 make the clang tidy and clang format workflows happy, and I will approve and take in.

[anutosh491]

I have checked this PR, and it does solve the problem. @anutosh491 make the clang tidy and clang format workflows happy, and I will approve and take in.

Thank you, I shall do that soon (afk for sometime)

We technically need to track that we should test such fixes in favour of the C Kernel whenever we end up with a framework for that. I haven't gone through all the discussion but as far as I remember, cppinterop needs to provide us with a utility to differentiate between C++ and C snippets isn't it ?

[mcbarton]

I have checked this PR, and it does solve the problem. @anutosh491 make the clang tidy and clang format workflows happy, and I will approve and take in.

Thank you, I shall do that soon (afk for sometime)

We technically need to track that we should test such fixes in favour of the C Kernel whenever we end up with a framework for that. I haven't gone through all the discussion but as far as I remember, cppinterop needs to provide us with a utility to differentiate between C++ and C snippets isn't it ?

Yes, that is exactly what needs to be done to enable c kernel tests. I have already tested the other small change which would be needed in test_xcpp_kernel.py to run the c kernel tests. If you mention all the things you think the c kernel tests should be testing in the issue already open about adding c tests #394 then I can add them once the cppinterop utility is available.