Set 'loading' and 'shutdown_asap' to volatile sig_atomic_t type (#7845)
Set 'loading' and 'shutdown_asap' to volatile sig_atomic_t type (#7845)

We may access and modify these two variables in signal handler function,
to guarantee them async-signal-safe, so we should set them to volatile
sig_atomic_t type.

It doesn't look like this could have caused any real issue, and it seems that
signals are handled in main thread on most platforms. But we want to follow C
and POSIX standard in signal handler function.
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