Invent qsort_interruptible().

Justin Pryzby reported that some scenarios could cause gathering
of extended statistics to spend many seconds in an un-cancelable
qsort() operation.  To fix, invent qsort_interruptible(), which is
just like qsort_arg() except that it will also do CHECK_FOR_INTERRUPTS
every so often.  This bloats the backend by a couple of kB, which
seems like a good investment.  (We considered just enabling
CHECK_FOR_INTERRUPTS in the existing qsort and qsort_arg functions,
but there are some callers for which that'd demonstrably be unsafe.
Opt-in seems like a better way.)

For now, just apply qsort_interruptible() in statistics collection.
There's probably more places where it could be useful, but we can
always change other call sites as we find problems.

Back-patch to v14.  Before that we didn't have extended stats on
expressions, so that the problem was less severe.  Also, this patch
depends on the sort_template infrastructure introduced in v14.

Tom Lane and Justin Pryzby

Discussion: https://postgr.es/m/20220509000108.GQ28830@telsasoft.com

src/backend/utils/sort/Makefile

@@ -16,6 +16,7 @@ override CPPFLAGS := -I. -I$(srcdir) $(CPPFLAGS)
 
 OBJS = \
 	logtape.o \
+	qsort_interruptible.o \
 	sharedtuplestore.o \
 	sortsupport.o \
 	tuplesort.o \

src/backend/utils/sort/qsort_interruptible.c

@@ -0,0 +1,16 @@
+/*
+ *	qsort_interruptible.c: qsort_arg that includes CHECK_FOR_INTERRUPTS
+ */
+
+#include "postgres.h"
+#include "miscadmin.h"
+
+#define ST_SORT qsort_interruptible
+#define ST_ELEMENT_TYPE_VOID
+#define ST_COMPARATOR_TYPE_NAME qsort_arg_comparator
+#define ST_COMPARE_RUNTIME_POINTER
+#define ST_COMPARE_ARG_TYPE void
+#define ST_SCOPE
+#define ST_DEFINE
+#define ST_CHECK_FOR_INTERRUPTS
+#include "lib/sort_template.h"