Use fmt for string formatting (GTK client)

[mikedld]

No description provided.

[elfring]

Were changes mixed for different issues in this commit? 🤔

[elfring]

💭 Are you sure that the implementation of this function is async-signal-safe?

[mikedld]

Yes.

[mikedld]

I may have misread the question. The proper answer is: it doesn't matter now since g_warning() is no longer called inside the actual signal handler.

[elfring]

I got the impression from your other response that this short feedback should be reconsidered another bit.
Are there different development views involved according to the term “signal handling” by the programming interfaces “POSIX” and “GLib”? 🤔

[mikedld]

POSIX signals and Glib signals are two completely different things which aren't related to each other. I'm sorry but I'd rather not dive into details here; I believe there's enough information on both topics out there if you're interested to know more.

[mikedld]

Were changes mixed for different issues in this commit? 🤔

Changes to signal_handler() were initially necessary to get rid of printf-style formatting used in g_message() in favor of fmt. I then realised that using fmt in signal handler would be wrong and switched to g_unix_signal_add(), which allows to add handlers that are executed within Glib event loop, not as part of actual signal handling (answering your other question: yes, this is safe). This switch in turn required either passing signal number as part of user_data or via lambda capture, neither of which I wanted to do, so I decided to drop signal number printing altogether which resulted in not needing to use fmt, yielding the code you see in this PR.