Handle syscall failures in wf::compositor_core_impl_t::run

[CarloWood]

This patch address the compiler warnings:

[455/476] Compiling C++ object src/liblibwayfire.a.p/core_core.cpp.o ../../../../github/wayfire/wayfire.git/src/core/core.cpp: In member function ‘virtual pid_t wf::compositor_core_impl_t::run(std::string)’: ../../../../github/wayfire/wayfire.git/src/core/core.cpp:433:10: warning: ignoring return value of ‘int pipe2(int*, int)’ declared with attribute ‘warn_unused_result’ [-Wunused-result]
433 | pipe2(pipe_fd, O_CLOEXEC);
| ~~~~~^~~~~~~~~~~~~~~~~~~~
../../../../github/wayfire/wayfire.git/src/core/core.cpp:467:18: warning: ignoring return value of ‘ssize_t write(int, const void*, size_t)’ declared with attribute ‘warn_unused_result’ [-Wunused-result]
467 | write(pipe_fd[WRITE_END], (void*)(&pid), sizeof(pid));
| ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../../../github/wayfire/wayfire.git/src/core/core.cpp:479:13: warning: ignoring return value of ‘ssize_t read(int, void*, size_t)’ declared with attribute ‘warn_unused_result’ [-Wunused-result]
479 | read(pipe_fd[READ_END], &child_pid, sizeof(child_pid));
| ~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

In order to "handle" a failure to run a command, I'd normally throw an exception that is caught up stream and pops up an informative window to the user describing the error in detail... but this code is kinda C-like, so I opted to just return 0 as the pid_t. The actual PID of a client is NEVER 0, so we can use this perfectly to detect failure.

Why handling failure is correct:

• If pipe2 would fail (which is possible), it leaves pipe_fd untouched and the rest of the code would use uninitialed memory and have Undefined Behaviour.

• If write would fail, then the command just failed: we can't report back a PID.

• Same for the read: if we fail to read the PID then we can't return its value. Returning 0 and handling that as an error (failure to execute the command) makes sense.

• The return value of the executed script was also completely ignored. If the command being run return an non-zero status than now the command is also treated as having failed. It is also possible to never even get to the _exit inside the child. To detect those, WIFEXITED(status) && WEXITSTATUS(status) == 0 are being used.

More sophisticated error handling is possible, but unnecessary: failure never happened so far (apparently) and is just very unlikely in general (apart from perhaps trying to execute non-existing scripts), so what we exactly do isn't that important (as long as I got rid of the pesky compiler warnings). Treating every error as a failure to execute the command is OK.

Given that wf::get_core().run(...) can now return 0 with the meaning that executing the command failed, we also need to handle that gracefully, of course. Hence that command_callback now returns a bool and converts the pid_t into a bool (so that zero now means failure) whenever wf::compositor_core_impl_t::run is executed.

plugins/ipc/ipc.cpp, plugins/ipc/ipc.hpp, plugins/ipc/ipc-method-repository.hpp:
wf::ipc::client_t::send_json now returns a bool (success).

plugins/single_plugins/command.cpp:
command_callback now returns a bool (success).

plugins/ipc/stipc.cpp (stipc_plugin_t::run):
Do not report a PID of zero as if that is the PID.

src/core/core.cpp:
handle syscall errors and return (pid_t)0 upon failure.

[CarloWood]

Force pushed because I rebased after merging with master (wf-touch update).

[ammen99]

overall lgtm, but there are a few smaller things to fix

[CarloWood]

Did I deal with the change requests properly (independent of if I resolved them with the commit), I just mean the clicking here on this github page?

[ammen99]

LGTM now, will test it shortly & merge, thanks!