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

1 files changed, 13 insertions, 12 deletions

diff --git a/src/backend/commands/analyze.c b/src/backend/commands/analyze.c
index 2da6b75a155..74888702fb9 100644
--- a/src/backend/commands/analyze.c
+++ b/src/backend/commands/analyze.c
@@ -100,7 +100,7 @@ static VacAttrStats *examine_attribute(Relation onerel, int attnum,
 static int	acquire_sample_rows(Relation onerel, int elevel,
 								HeapTuple *rows, int targrows,
 								double *totalrows, double *totaldeadrows);
-static int	compare_rows(const void *a, const void *b);
+static int	compare_rows(const void *a, const void *b, void *arg);
 static int	acquire_inherited_sample_rows(Relation onerel, int elevel,
 										  HeapTuple *rows, int targrows,
 										  double *totalrows, double *totaldeadrows);
@@ -1306,7 +1306,8 @@ acquire_sample_rows(Relation onerel, int elevel,
 	 * tuples are already sorted.
 	 */
 	if (numrows == targrows)
-		qsort((void *) rows, numrows, sizeof(HeapTuple), compare_rows);
+		qsort_interruptible((void *) rows, numrows, sizeof(HeapTuple),
+							compare_rows, NULL);
 
 	/*
 	 * Estimate total numbers of live and dead rows in relation, extrapolating
@@ -1342,10 +1343,10 @@ acquire_sample_rows(Relation onerel, int elevel,
 }
 
 /*
- * qsort comparator for sorting rows[] array
+ * Comparator for sorting rows[] array
  */
 static int
-compare_rows(const void *a, const void *b)
+compare_rows(const void *a, const void *b, void *arg)
 {
 	HeapTuple	ha = *(const HeapTuple *) a;
 	HeapTuple	hb = *(const HeapTuple *) b;
@@ -1839,7 +1840,7 @@ static void compute_scalar_stats(VacAttrStatsP stats,
 								 int samplerows,
 								 double totalrows);
 static int	compare_scalars(const void *a, const void *b, void *arg);
-static int	compare_mcvs(const void *a, const void *b);
+static int	compare_mcvs(const void *a, const void *b, void *arg);
 static int	analyze_mcv_list(int *mcv_counts,
 							 int num_mcv,
 							 double stadistinct,
@@ -2476,8 +2477,8 @@ compute_scalar_stats(VacAttrStatsP stats,
 		/* Sort the collected values */
 		cxt.ssup = &ssup;
 		cxt.tupnoLink = tupnoLink;
-		qsort_arg((void *) values, values_cnt, sizeof(ScalarItem),
-				  compare_scalars, (void *) &cxt);
+		qsort_interruptible((void *) values, values_cnt, sizeof(ScalarItem),
+							compare_scalars, (void *) &cxt);
 
 		/*
 		 * Now scan the values in order, find the most common ones, and also
@@ -2721,8 +2722,8 @@ compute_scalar_stats(VacAttrStatsP stats,
 						deltafrac;
 
 			/* Sort the MCV items into position order to speed next loop */
-			qsort((void *) track, num_mcv,
-				  sizeof(ScalarMCVItem), compare_mcvs);
+			qsort_interruptible((void *) track, num_mcv, sizeof(ScalarMCVItem),
+								compare_mcvs, NULL);
 
 			/*
 			 * Collapse out the MCV items from the values[] array.
@@ -2885,7 +2886,7 @@ compute_scalar_stats(VacAttrStatsP stats,
 }
 
 /*
- * qsort_arg comparator for sorting ScalarItems
+ * Comparator for sorting ScalarItems
  *
  * Aside from sorting the items, we update the tupnoLink[] array
  * whenever two ScalarItems are found to contain equal datums.  The array
@@ -2922,10 +2923,10 @@ compare_scalars(const void *a, const void *b, void *arg)
 }
 
 /*
- * qsort comparator for sorting ScalarMCVItems by position
+ * Comparator for sorting ScalarMCVItems by position
  */
 static int
-compare_mcvs(const void *a, const void *b)
+compare_mcvs(const void *a, const void *b, void *arg)
 {
 	int			da = ((const ScalarMCVItem *) a)->first;
 	int			db = ((const ScalarMCVItem *) b)->first;