Subprocess.h: use vector instead of dynamically-sized VLA

[katrinafyi]

addresses warning present in recent clang versions (i was using clang 19).

souffle> /tmp/nix-build-souffle-2.4.1.drv-0/source/src/include/souffle/utility/SubProcess.h:81:29: warning: variable length arrays in C++ are a Clang extension [-Wvla-cxx-extension]
souffle>    81 |             char* argv_temp[argv.size() + 2];
souffle>       |                             ^~~~~~~~~~~~~~~
souffle> /tmp/nix-build-souffle-2.4.1.drv-0/source/src/include/souffle/utility/SubProcess.h:201:21: note: in instantiation of function template specialization 'souffle::execute<tcb::span<std::pair<const char *, const char *>> &, void>' requested here
souffle>   201 |     return souffle::execute(program, argv_ptr, envp_ptr);
souffle>       |                     ^
souffle> /tmp/nix-build-souffle-2.4.1.drv-0/source/src/MainDriver.cpp:165:17: note: in instantiation of function template specialization 'souffle::execute<std::map<const char *, std::string> &, void>' requested here
souffle>   165 |     auto exit = execute(binaryFilename, {}, env);
souffle>       |                 ^
souffle> /tmp/nix-build-souffle-2.4.1.drv-0/source/src/include/souffle/utility/span.h:473:56: note: function parameter 'argv' with unknown value cannot be used in a constant expression
souffle>   473 |     constexpr size_type size() const noexcept { return storage_.size; }
souffle>       |                                                        ^
souffle> /tmp/nix-build-souffle-2.4.1.drv-0/source/src/include/souffle/utility/SubProcess.h:81:29: note: in call to 'argv.size()'
souffle>    81 |             char* argv_temp[argv.size() + 2];
souffle>       |                             ^~~~~~~~~~~
souffle> /tmp/nix-build-souffle-2.4.1.drv-0/source/src/include/souffle/utility/SubProcess.h:67:61: note: declared here
souffle>    67 |         std::string const& program, span<char const* const> argv = {}, Envp&& envp = {}) {
souffle>       |                                                             ^

the VLA warnings have always existed but were recently added to the Wall group: https://ddanilov.me/default-non-standard-features/

[quentin]

Hi, thanks for the fix!

[katrinafyi]

No worries! The failing clang-format check isn't near the lines I've changed. Do you still want me to apply the style changes?

[quentin]

No worries! The failing clang-format check isn't near the lines I've changed. Do you still want me to apply the style changes?

Yes please. It happens because clang-format only runs on modified files, but the version of clang-format since the last check of this file may have changed and with that, sometimes, formatting rules get a slight update.

Ideally when something like this happens we should add a dedicated formatting rule in our project to fix the expected formatting.