Avoid calling gettext() in signal handlers.

It seems highly unlikely that gettext() can be relied on to be
async-signal-safe.  psql used to understand that, but someone got
it wrong long ago in the src/bin/scripts/ version of handle_sigint,
and then the bad idea was perpetuated when those two versions were
unified into src/fe_utils/cancel.c.

I'm unsure why there have not been field complaints about this
... maybe gettext() is signal-safe once it's translated at least
one message?  But we have no business assuming any such thing.

In cancel.c (v13 and up), I preserved our ability to localize
"Cancel request sent" messages by invoking gettext() before
the signal handler is set up.  In earlier branches I just made
src/bin/scripts/ not localize those messages, as psql did then.

(Just for extra unsafety, the src/bin/scripts/ version was
invoking fprintf() from a signal handler.  Sigh.)

Noted while fixing signal-safety issues in PQcancel() itself.
Back-patch to all supported branches.

Discussion: https://postgr.es/m/2937814.1641960929@sss.pgh.pa.us

src/fe_utils/cancel.c

@@ -42,6 +42,13 @@
  */
 static PGcancel *volatile cancelConn = NULL;
 
+/*
+ * Predetermined localized error strings --- needed to avoid trying
+ * to call gettext() from a signal handler.
+ */
+static const char *cancel_sent_msg = NULL;
+static const char *cancel_not_sent_msg = NULL;
+
 /*
  * CancelRequested is set when we receive SIGINT (or local equivalent).
  * There is no provision in this module for resetting it; but applications
@@ -158,11 +165,11 @@ handle_sigint(SIGNAL_ARGS)
 	{
 		if (PQcancel(cancelConn, errbuf, sizeof(errbuf)))
 		{
-			write_stderr(_("Cancel request sent\n"));
+			write_stderr(cancel_sent_msg);
 		}
 		else
 		{
-			write_stderr(_("Could not send cancel request: "));
+			write_stderr(cancel_not_sent_msg);
 			write_stderr(errbuf);
 		}
 	}
@@ -179,6 +186,9 @@ void
 setup_cancel_handler(void (*callback) (void))
 {
 	cancel_callback = callback;
+	cancel_sent_msg = _("Cancel request sent\n");
+	cancel_not_sent_msg = _("Could not send cancel request: ");
+
 	pqsignal(SIGINT, handle_sigint);
 }
 
@@ -203,11 +213,11 @@ consoleHandler(DWORD dwCtrlType)
 		{
 			if (PQcancel(cancelConn, errbuf, sizeof(errbuf)))
 			{
-				write_stderr(_("Cancel request sent\n"));
+				write_stderr(cancel_sent_msg);
 			}
 			else
 			{
-				write_stderr(_("Could not send cancel request: "));
+				write_stderr(cancel_not_sent_msg);
 				write_stderr(errbuf);
 			}
 		}
@@ -225,6 +235,8 @@ void
 setup_cancel_handler(void (*callback) (void))
 {
 	cancel_callback = callback;
+	cancel_sent_msg = _("Cancel request sent\n");
+	cancel_not_sent_msg = _("Could not send cancel request: ");
 
 	InitializeCriticalSection(&cancelConnLock);