Allow btree comparison functions to return INT_MIN.

Historically we forbade datatype-specific comparison functions from
returning INT_MIN, so that it would be safe to invert the sort order
just by negating the comparison result.  However, this was never
really safe for comparison functions that directly return the result
of memcmp(), strcmp(), etc, as POSIX doesn't place any such restriction
on those library functions.  Buildfarm results show that at least on
recent Linux on s390x, memcmp() actually does return INT_MIN sometimes,
causing sort failures.

The agreed-on answer is to remove this restriction and fix relevant
call sites to not make such an assumption; code such as "res = -res"
should be replaced by "INVERT_COMPARE_RESULT(res)".  The same is needed
in a few places that just directly negated the result of memcmp or
strcmp.

To help find places having this problem, I've also added a compile option
to nbtcompare.c that causes some of the commonly used comparators to
return INT_MIN/INT_MAX instead of their usual -1/+1.  It'd likely be
a good idea to have at least one buildfarm member running with
"-DSTRESS_SORT_INT_MIN".  That's far from a complete test of course,
but it should help to prevent fresh introductions of such bugs.

This is a longstanding portability hazard, so back-patch to all supported
branches.

Discussion: https://postgr.es/m/20180928185215.ffoq2xrq5d3pafna@alap3.anarazel.de

11 files changed, 86 insertions, 55 deletions

diff --git a/contrib/ltree/ltree_op.c b/contrib/ltree/ltree_op.c
index cb59d21ced4..d1067f81465 100644
--- a/contrib/ltree/ltree_op.c
+++ b/contrib/ltree/ltree_op.c
@@ -45,17 +45,24 @@ ltree_compare(const ltree *a, const ltree *b)
 	ltree_level *bl = LTREE_FIRST(b);
 	int			an = a->numlevel;
 	int			bn = b->numlevel;
-	int			res = 0;
 
 	while (an > 0 && bn > 0)
 	{
+		int			res;
+
 		if ((res = memcmp(al->name, bl->name, Min(al->len, bl->len))) == 0)
 		{
 			if (al->len != bl->len)
 				return (al->len - bl->len) * 10 * (an + 1);
 		}
 		else
+		{
+			if (res < 0)
+				res = -1;
+			else
+				res = 1;
 			return res * 10 * (an + 1);
+		}
 
 		an--;
 		bn--;
@@ -146,7 +153,7 @@ inner_isparent(const ltree *c, const ltree *p)
 	{
 		if (cl->len != pl->len)
 			return false;
-		if (memcmp(cl->name, pl->name, cl->len))
+		if (memcmp(cl->name, pl->name, cl->len) != 0)
 			return false;
 
 		pn--;
diff --git a/contrib/pgcrypto/imath.c b/contrib/pgcrypto/imath.c
index 61a01e2b710..6449dd34b0b 100644
--- a/contrib/pgcrypto/imath.c
+++ b/contrib/pgcrypto/imath.c
@@ -1254,11 +1254,9 @@ mp_int_compare(mp_int a, mp_int b)
 		 * If they're both zero or positive, the normal comparison applies; if
 		 * both negative, the sense is reversed.
 		 */
-		if (sa == MP_ZPOS)
-			return cmp;
-		else
-			return -cmp;
-
+		if (sa != MP_ZPOS)
+			INVERT_COMPARE_RESULT(cmp);
+		return cmp;
 	}
 	else
 	{
@@ -1314,10 +1312,9 @@ mp_int_compare_value(mp_int z, int value)
 	{
 		cmp = s_vcmp(z, value);
 
-		if (vsign == MP_ZPOS)
-			return cmp;
-		else
-			return -cmp;
+		if (vsign != MP_ZPOS)
+			INVERT_COMPARE_RESULT(cmp);
+		return cmp;
 	}
 	else
 	{
diff --git a/src/backend/access/nbtree/nbtcompare.c b/src/backend/access/nbtree/nbtcompare.c
index 8ce800945ac..d915539df79 100644
--- a/src/backend/access/nbtree/nbtcompare.c
+++ b/src/backend/access/nbtree/nbtcompare.c
@@ -22,11 +22,10 @@
  *
  *	The result is always an int32 regardless of the input datatype.
  *
- *	Although any negative int32 (except INT_MIN) is acceptable for reporting
- *	"<", and any positive int32 is acceptable for reporting ">", routines
+ *	Although any negative int32 is acceptable for reporting "<",
+ *	and any positive int32 is acceptable for reporting ">", routines
  *	that work on 32-bit or wider datatypes can't just return "a - b".
- *	That could overflow and give the wrong answer.  Also, one must not
- *	return INT_MIN to report "<", since some callers will negate the result.
+ *	That could overflow and give the wrong answer.
  *
  *	NOTE: it is critical that the comparison function impose a total order
  *	on all non-NULL values of the data type, and that the datatype's
@@ -44,13 +43,31 @@
  *	during an index access won't be recovered till end of query.  This
  *	primarily affects comparison routines for toastable datatypes;
  *	they have to be careful to free any detoasted copy of an input datum.
+ *
+ *	NOTE: we used to forbid comparison functions from returning INT_MIN,
+ *	but that proves to be too error-prone because some platforms' versions
+ *	of memcmp() etc can return INT_MIN.  As a means of stress-testing
+ *	callers, this file can be compiled with STRESS_SORT_INT_MIN defined
+ *	to cause many of these functions to return INT_MIN or INT_MAX instead of
+ *	their customary -1/+1.  For production, though, that's not a good idea
+ *	since users or third-party code might expect the traditional results.
  *-------------------------------------------------------------------------
  */
 #include "postgres.h"
 
+#include <limits.h>
+
 #include "utils/builtins.h"
 #include "utils/sortsupport.h"
 
+#ifdef STRESS_SORT_INT_MIN
+#define A_LESS_THAN_B		INT_MIN
+#define A_GREATER_THAN_B	INT_MAX
+#else
+#define A_LESS_THAN_B		(-1)
+#define A_GREATER_THAN_B	1
+#endif
+
 
 Datum
 btboolcmp(PG_FUNCTION_ARGS)
@@ -95,11 +112,11 @@ btint4cmp(PG_FUNCTION_ARGS)
 	int32		b = PG_GETARG_INT32(1);
 
 	if (a > b)
-		PG_RETURN_INT32(1);
+		PG_RETURN_INT32(A_GREATER_THAN_B);
 	else if (a == b)
 		PG_RETURN_INT32(0);
 	else
-		PG_RETURN_INT32(-1);
+		PG_RETURN_INT32(A_LESS_THAN_B);
 }
 
 static int
@@ -109,11 +126,11 @@ btint4fastcmp(Datum x, Datum y, SortSupport ssup)
 	int32		b = DatumGetInt32(y);
 
 	if (a > b)
-		return 1;
+		return A_GREATER_THAN_B;
 	else if (a == b)
 		return 0;
 	else
-		return -1;
+		return A_LESS_THAN_B;
 }
 
 Datum
@@ -132,11 +149,11 @@ btint8cmp(PG_FUNCTION_ARGS)
 	int64		b = PG_GETARG_INT64(1);
 
 	if (a > b)
-		PG_RETURN_INT32(1);
+		PG_RETURN_INT32(A_GREATER_THAN_B);
 	else if (a == b)
 		PG_RETURN_INT32(0);
 	else
-		PG_RETURN_INT32(-1);
+		PG_RETURN_INT32(A_LESS_THAN_B);
 }
 
 static int
@@ -146,11 +163,11 @@ btint8fastcmp(Datum x, Datum y, SortSupport ssup)
 	int64		b = DatumGetInt64(y);
 
 	if (a > b)
-		return 1;
+		return A_GREATER_THAN_B;
 	else if (a == b)
 		return 0;
 	else
-		return -1;
+		return A_LESS_THAN_B;
 }
 
 Datum
@@ -169,11 +186,11 @@ btint48cmp(PG_FUNCTION_ARGS)
 	int64		b = PG_GETARG_INT64(1);
 
 	if (a > b)
-		PG_RETURN_INT32(1);
+		PG_RETURN_INT32(A_GREATER_THAN_B);
 	else if (a == b)
 		PG_RETURN_INT32(0);
 	else
-		PG_RETURN_INT32(-1);
+		PG_RETURN_INT32(A_LESS_THAN_B);
 }
 
 Datum
@@ -183,11 +200,11 @@ btint84cmp(PG_FUNCTION_ARGS)
 	int32		b = PG_GETARG_INT32(1);
 
 	if (a > b)
-		PG_RETURN_INT32(1);
+		PG_RETURN_INT32(A_GREATER_THAN_B);
 	else if (a == b)
 		PG_RETURN_INT32(0);
 	else
-		PG_RETURN_INT32(-1);
+		PG_RETURN_INT32(A_LESS_THAN_B);
 }
 
 Datum
@@ -197,11 +214,11 @@ btint24cmp(PG_FUNCTION_ARGS)
 	int32		b = PG_GETARG_INT32(1);
 
 	if (a > b)
-		PG_RETURN_INT32(1);
+		PG_RETURN_INT32(A_GREATER_THAN_B);
 	else if (a == b)
 		PG_RETURN_INT32(0);
 	else
-		PG_RETURN_INT32(-1);
+		PG_RETURN_INT32(A_LESS_THAN_B);
 }
 
 Datum
@@ -211,11 +228,11 @@ btint42cmp(PG_FUNCTION_ARGS)
 	int16		b = PG_GETARG_INT16(1);
 
 	if (a > b)
-		PG_RETURN_INT32(1);
+		PG_RETURN_INT32(A_GREATER_THAN_B);
 	else if (a == b)
 		PG_RETURN_INT32(0);
 	else
-		PG_RETURN_INT32(-1);
+		PG_RETURN_INT32(A_LESS_THAN_B);
 }
 
 Datum
@@ -225,11 +242,11 @@ btint28cmp(PG_FUNCTION_ARGS)
 	int64		b = PG_GETARG_INT64(1);
 
 	if (a > b)
-		PG_RETURN_INT32(1);
+		PG_RETURN_INT32(A_GREATER_THAN_B);
 	else if (a == b)
 		PG_RETURN_INT32(0);
 	else
-		PG_RETURN_INT32(-1);
+		PG_RETURN_INT32(A_LESS_THAN_B);
 }
 
 Datum
@@ -239,11 +256,11 @@ btint82cmp(PG_FUNCTION_ARGS)
 	int16		b = PG_GETARG_INT16(1);
 
 	if (a > b)
-		PG_RETURN_INT32(1);
+		PG_RETURN_INT32(A_GREATER_THAN_B);
 	else if (a == b)
 		PG_RETURN_INT32(0);
 	else
-		PG_RETURN_INT32(-1);
+		PG_RETURN_INT32(A_LESS_THAN_B);
 }
 
 Datum
@@ -253,11 +270,11 @@ btoidcmp(PG_FUNCTION_ARGS)
 	Oid			b = PG_GETARG_OID(1);
 
 	if (a > b)
-		PG_RETURN_INT32(1);
+		PG_RETURN_INT32(A_GREATER_THAN_B);
 	else if (a == b)
 		PG_RETURN_INT32(0);
 	else
-		PG_RETURN_INT32(-1);
+		PG_RETURN_INT32(A_LESS_THAN_B);
 }
 
 static int
@@ -267,11 +284,11 @@ btoidfastcmp(Datum x, Datum y, SortSupport ssup)
 	Oid			b = DatumGetObjectId(y);
 
 	if (a > b)
-		return 1;
+		return A_GREATER_THAN_B;
 	else if (a == b)
 		return 0;
 	else
-		return -1;
+		return A_LESS_THAN_B;
 }
 
 Datum
@@ -299,9 +316,9 @@ btoidvectorcmp(PG_FUNCTION_ARGS)
 		if (a->values[i] != b->values[i])
 		{
 			if (a->values[i] > b->values[i])
-				PG_RETURN_INT32(1);
+				PG_RETURN_INT32(A_GREATER_THAN_B);
 			else
-				PG_RETURN_INT32(-1);
+				PG_RETURN_INT32(A_LESS_THAN_B);
 		}
 	}
 	PG_RETURN_INT32(0);
diff --git a/src/backend/access/nbtree/nbtsearch.c b/src/backend/access/nbtree/nbtsearch.c
index 527efdfb315..108b0e27c08 100644
--- a/src/backend/access/nbtree/nbtsearch.c
+++ b/src/backend/access/nbtree/nbtsearch.c
@@ -484,7 +484,7 @@ _bt_compare(Relation rel,
 													 scankey->sk_argument));
 
 			if (!(scankey->sk_flags & SK_BT_DESC))
-				result = -result;
+				INVERT_COMPARE_RESULT(result);
 		}
 
 		/* if the keys are unequal, return the difference */
diff --git a/src/backend/access/nbtree/nbtutils.c b/src/backend/access/nbtree/nbtutils.c
index ef889b9e5bc..88d0b12f93f 100644
--- a/src/backend/access/nbtree/nbtutils.c
+++ b/src/backend/access/nbtree/nbtutils.c
@@ -509,7 +509,7 @@ _bt_compare_array_elements(const void *a, const void *b, void *arg)
 											  cxt->collation,
 											  da, db));
 	if (cxt->reverse)
-		compare = -compare;
+		INVERT_COMPARE_RESULT(compare);
 	return compare;
 }
 
@@ -1638,7 +1638,7 @@ _bt_check_rowcompare(ScanKey skey, IndexTuple tuple, TupleDesc tupdesc,
 													subkey->sk_argument));
 
 		if (subkey->sk_flags & SK_BT_DESC)
-			cmpresult = -cmpresult;
+			INVERT_COMPARE_RESULT(cmpresult);
 
 		/* Done comparing if unequal, else advance to next column */
 		if (cmpresult != 0)
diff --git a/src/backend/executor/nodeIndexscan.c b/src/backend/executor/nodeIndexscan.c
index 11de6f11298..0dfbfbd5eb7 100644
--- a/src/backend/executor/nodeIndexscan.c
+++ b/src/backend/executor/nodeIndexscan.c
@@ -412,9 +412,10 @@ reorderqueue_cmp(const pairingheap_node *a, const pairingheap_node *b,
 	ReorderTuple *rtb = (ReorderTuple *) b;
 	IndexScanState *node = (IndexScanState *) arg;
 
-	return -cmp_orderbyvals(rta->orderbyvals, rta->orderbynulls,
-							rtb->orderbyvals, rtb->orderbynulls,
-							node);
+	/* exchange argument order to invert the sort order */
+	return cmp_orderbyvals(rtb->orderbyvals, rtb->orderbynulls,
+						   rta->orderbyvals, rta->orderbynulls,
+						   node);
 }
 
 /*
diff --git a/src/backend/executor/nodeMergeAppend.c b/src/backend/executor/nodeMergeAppend.c
index bdf76808a8a..100dbbbf53b 100644
--- a/src/backend/executor/nodeMergeAppend.c
+++ b/src/backend/executor/nodeMergeAppend.c
@@ -250,7 +250,10 @@ heap_compare_slots(Datum a, Datum b, void *arg)
 									  datum2, isNull2,
 									  sortKey);
 		if (compare != 0)
-			return -compare;
+		{
+			INVERT_COMPARE_RESULT(compare);
+			return compare;
+		}
 	}
 	return 0;
 }
diff --git a/src/bin/pg_rewind/filemap.c b/src/bin/pg_rewind/filemap.c
index 60c85a4b662..366b97de91f 100644
--- a/src/bin/pg_rewind/filemap.c
+++ b/src/bin/pg_rewind/filemap.c
@@ -680,7 +680,7 @@ final_filemap_cmp(const void *a, const void *b)
 		return -1;
 
 	if (fa->action == FILE_ACTION_REMOVE)
-		return -strcmp(fa->path, fb->path);
+		return strcmp(fb->path, fa->path);
 	else
 		return strcmp(fa->path, fb->path);
 }
diff --git a/src/include/access/nbtree.h b/src/include/access/nbtree.h
index 42928d49f9b..d8c14e0493a 100644
--- a/src/include/access/nbtree.h
+++ b/src/include/access/nbtree.h
@@ -444,8 +444,7 @@ typedef struct xl_btree_newroot
  *	When a new operator class is declared, we require that the user
  *	supply us with an amproc procedure (BTORDER_PROC) for determining
  *	whether, for two keys a and b, a < b, a = b, or a > b.  This routine
- *	must return < 0, 0, > 0, respectively, in these three cases.  (It must
- *	not return INT_MIN, since we may negate the result before using it.)
+ *	must return < 0, 0, > 0, respectively, in these three cases.
  *
  *	To facilitate accelerated sorting, an operator class may choose to
  *	offer a second procedure (BTSORTSUPPORT_PROC).  For full details, see
diff --git a/src/include/c.h b/src/include/c.h
index 2481ded8385..86e4b549754 100644
--- a/src/include/c.h
+++ b/src/include/c.h
@@ -961,6 +961,14 @@ typedef NameData *Name;
  */
 
 /*
+ * Invert the sign of a qsort-style comparison result, ie, exchange negative
+ * and positive integer values, being careful not to get the wrong answer
+ * for INT_MIN.  The argument should be an integral variable.
+ */
+#define INVERT_COMPARE_RESULT(var) \
+	((var) = ((var) < 0) ? 1 : -(var))
+
+/*
  * Use this, not "char buf[BLCKSZ]", to declare a field or local variable
  * holding a page buffer, if that page might be accessed as a page and not
  * just a string of bytes.  Otherwise the variable might be under-aligned,
diff --git a/src/include/utils/sortsupport.h b/src/include/utils/sortsupport.h
index a984edda58f..23f77ca3f99 100644
--- a/src/include/utils/sortsupport.h
+++ b/src/include/utils/sortsupport.h
@@ -96,8 +96,7 @@ typedef struct SortSupportData
 	 * Comparator function has the same API as the traditional btree
 	 * comparison function, ie, return <0, 0, or >0 according as x is less
 	 * than, equal to, or greater than y.  Note that x and y are guaranteed
-	 * not null, and there is no way to return null either.  Do not return
-	 * INT_MIN, as callers are allowed to negate the result before using it.
+	 * not null, and there is no way to return null either.
 	 *
 	 * This may be either the authoritative comparator, or the abbreviated
 	 * comparator.  Core code may switch this over the initial preference of
@@ -237,7 +236,7 @@ ApplySortComparator(Datum datum1, bool isNull1,
 	{
 		compare = (*ssup->comparator) (datum1, datum2, ssup);
 		if (ssup->ssup_reverse)
-			compare = -compare;
+			INVERT_COMPARE_RESULT(compare);
 	}
 
 	return compare;
@@ -275,7 +274,7 @@ ApplySortAbbrevFullComparator(Datum datum1, bool isNull1,
 	{
 		compare = (*ssup->abbrev_full_comparator) (datum1, datum2, ssup);
 		if (ssup->ssup_reverse)
-			compare = -compare;
+			INVERT_COMPARE_RESULT(compare);
 	}
 
 	return compare;