Improve comments about pqsignal().

Explain where pqsignal() came from, what problem it originally solved
without assuming the reader is familiar with historical Unixen, why we
still need it, what it does for us now, and the key differences in
frontend code on Windows.

Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://postgr.es/m/CA%2BhUKG%2BRst1h3uo%2BXRgdRVnWHBa4mmj5gFbmCzZr73s-Fh_5JA%40mail.gmail.com

src/port/pqsignal.c

@@ -11,16 +11,34 @@
  * IDENTIFICATION
  *	  src/port/pqsignal.c
  *
- *	We now assume that all Unix-oid systems have POSIX sigaction(2)
- *	with support for restartable signals (SA_RESTART).  We used to also
- *	support BSD-style signal(2), but there really shouldn't be anything
- *	out there anymore that doesn't have the POSIX API.
+ *	This is the signal() implementation from "Advanced Programming in the UNIX
+ *	Environment", with minor changes.  It was originally a replacement needed
+ *	for old SVR4 systems whose signal() behaved as if sa_flags = SA_RESETHAND |
+ *	SA_NODEFER, also known as "unreliable" signals due to races when the
+ *	handler was reset.
+ *
+ *	By now, all known modern Unix systems have a "reliable" signal() call.
+ *	We still don't want to use it though, because it remains
+ *	implementation-defined by both C99 and POSIX whether the handler is reset
+ *	or signals are blocked when the handler runs, and default restart behavior
+ *	is also unspecified.  Therefore we take POSIX's advice and call sigaction()
+ *	so we can provide explicit sa_flags, but wrap it in this more convenient
+ *	traditional interface style.  It also provides a place to set any extra
+ *	flags we want everywhere, such as SA_NOCLDSTOP.
  *
  *	Windows, of course, is resolutely in a class by itself.  In the backend,
- *	we don't use this file at all; src/backend/port/win32/signal.c provides
- *	pqsignal() for the backend environment.  Frontend programs can use
- *	this version of pqsignal() if they wish, but beware that this does
- *	not provide restartable signals on Windows.
+ *	this relies on pqsigaction() in src/backend/port/win32/signal.c, which
+ *	provides limited emulation of reliable signals.
+ *
+ * 	Frontend programs can use this version of pqsignal() to forward to the
+ * 	native Windows signal() call if they wish, but beware that Windows signals
+ * 	behave quite differently.  Only the 6 signals required by C are supported.
+ * 	SIGINT handlers run in another thread instead of interrupting an existing
+ * 	thread, and the others don't interrupt system calls either, so SA_RESTART
+ * 	is moot.  All except SIGFPE have SA_RESETHAND semantics, meaning the
+ * 	handler is reset to SIG_DFL each time it runs.  The set of things you are
+ * 	allowed to do in a handler is also much more restricted than on Unix,
+ * 	according to the documentation.
  *
  * ------------------------------------------------------------------------
  */