8000 Fix incorrect return value in pg_size_pretty(bigint) · postgres/postgres@1efcd57 · GitHub
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david-rowleydavid-rowley
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Fix incorrect return value in pg_size_pretty(bigint)
Due to how pg_size_pretty(bigint) was implemented, it's possible that when given a negative number of bytes that the returning value would not match the equivalent positive return value when given the equivalent positive number of bytes. This was due to two separate issues. 1. The function used bit shifting to convert the number of bytes into larger units. The rounding performed by bit shifting is not the same as dividing. For example -3 >> 1 = -2, but -3 / 2 = -1. These two operations are only equivalent with positive numbers. 2. The half_rounded() macro rounded towards positive infinity. This meant that negative numbers rounded towards zero and positive numbers rounded away from zero. Here we fix #1 by dividing the values instead of bit shifting. We fix #2 by adjusting the half_rounded macro always to round away from zero. Additionally, adjust the pg_size_pretty(numeric) function to be more explicit that it's using division rather than bit shifting. A casual observer might have believed bit shifting was used due to a static function being named numeric_shift_right. However, that function was calculating the divisor from the number of bits and performed division. Here we make that more clear. This change is just cosmetic and does not affect the return value of the numeric version of the function. Here we also add a set of regression tests both versions of pg_size_pretty() which test the values directly before and after the function switches to the next unit. This bug was introduced in 8a1fab3. Prior to that negative values were always displayed in bytes. Author: Dean Rasheed, David Rowley Discussion: https://postgr.es/m/CAEZATCXnNW4HsmZnxhfezR5FuiGgp+mkY4AzcL5eRGO4fuadWg@mail.gmail.com Backpatch-through: 9.6, where the bug was introduced.
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‎src/backend/utils/adt/dbsize.c

Lines changed: 21 additions & 18 deletions
Original file line numberDiff line numberDiff line change
@@ -31,8 +31,8 @@
3131
#include "utils/relmapper.h"
3232
#include "utils/syscache.h"
3333

34-
/* Divide by two and round towards positive infinity. */
35-
#define half_rounded(x) (((x) + ((x) < 0 ? 0 : 1)) / 2)
34+
/* Divide by two and round away from zero */
35+
#define half_rounded(x) (((x) + ((x) < 0 ? -1 : 1)) / 2)
3636

3737
/* Return physical size of directory contents, or 0 if dir doesn't exist */
3838
static int64
@@ -547,25 +547,29 @@ pg_size_pretty(PG_FUNCTION_ARGS)
547547
snprintf(buf, sizeof(buf), INT64_FORMAT " bytes", size);
548548
else
549549
{
550-
size >>= 9; /* keep one extra bit for rounding */
550+
/*
551+
* We use divide instead of bit shifting so that behavior matches for
552+
* both positive and negative size values.
553+
*/
554+
size /= (1 << 9); /* keep one extra bit for rounding */
551555
if (Abs(size) < limit2)
552556
snprintf(buf, sizeof(buf), INT64_FORMAT " kB",
553557
half_rounded(size));
554558
else
555559
{
556-
size >>= 10;
560+
size /= (1 << 10);
557561
if (Abs(size) < limit2)
558562
snprintf(buf, sizeof(buf), INT64_FORMAT " MB",
559563
half_rounded(size));
560564
else
561565
{
562-
size >>= 10;
566+
size /= (1 << 10);
563567
if (Abs(size) < limit2)
564568
snprintf(buf, sizeof(buf), INT64_FORMAT " GB",
565569
half_rounded(size));
566570
else
567571
{
568-
size >>= 10;
572+
size /= (1 << 10);
569573
snprintf(buf, sizeof(buf), INT64_FORMAT " TB",
570574
half_rounded(size));
571575
}
@@ -634,15 +638,13 @@ numeric_half_rounded(Numeric n)
634638
}
635639

636640
static Numeric
637-
numeric_shift_right(Numeric n, unsigned count)
641+
numeric_truncated_divide(Numeric n, int64 divisor)
638642
{
639643
Datum d = NumericGetDatum(n);
640-
Datum divisor_int64;
641644
Datum divisor_numeric;
642645
Datum result;
643646

644-
divisor_int64 = Int64GetDatum((int64) (1 << count));
645-
divisor_numeric = DirectFunctionCall1(int8_numeric, divisor_int64);
647+
divisor_numeric = DirectFunctionCall1(int8_numeric, divisor);
646648
result = DirectFunctionCall2(numeric_div_trunc, d, divisor_numeric);
647649
return DatumGetNumeric(result);
648650
}
@@ -665,8 +667,8 @@ pg_size_pretty_numeric(PG_FUNCTION_ARGS)
665667
else
666668
{
667669
/* keep one extra bit for rounding */
668-
/* size >>= 9 */
669-
size = numeric_shift_right(size, 9);
670+
/* size /= (1 << 9) */
671+
size = numeric_truncated_divide(size, 1 << 9);
670672

671673
if (numeric_is_less(numeric_absolute(size), limit2))
672674
{
@@ -675,17 +677,18 @@ pg_size_pretty_numeric(PG_FUNCTION_ARGS)
675677
}
676678
else
677679
{
678-
/* size >>= 10 */
679-
size = numeric_shift_right(size, 10);
680+
/* size /= (1 << 10) */
681+
size = numeric_truncated_divide(size, 1 << 10);
682+
680683
if (numeric_is_less(numeric_absolute(size), limit2))
681684
{
682685
size = numeric_half_rounded(size);
683686
result = psprintf("%s MB", numeric_to_cstring(size));
684687
}
685688
else
686689
{
687-
/* size >>= 10 */
688-
size = numeric_shift_right(size, 10);
690+
/* size /= (1 << 10) */
691+
size = numeric_truncated_divide(size, 1 << 10);
689692

690693
if (numeric_is_less(numeric_absolute(size), limit2))
691694
{
@@ -694,8 +697,8 @@ pg_size_pretty_numeric(PG_FUNCTION_ARGS)
694697
}
695698
else
696699
{
697-
/* size >>= 10 */
698-
size = numeric_shift_right(size, 10);
700+
/* size /= (1 << 10) */
701+
size = numeric_truncated_divide(size, 1 << 10);
699702
size = numeric_half_rounded(size);
700703
result = psprintf("%s TB", numeric_to_cstring(size));
701704
}

‎src/test/regress/expected/dbsize.out

Lines changed: 42 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -35,6 +35,48 @@ SELECT size, pg_size_pretty(size), pg_size_pretty(-1 * size) FROM
3535
1000000000000000.5 | 909 TB | -909 TB
3636
(12 rows)
3737

38+
-- test where units change up
39+
SELECT size, pg_size_pretty(size), pg_size_pretty(-1 * size) FROM
40+
(VALUES (10239::bigint), (10240::bigint),
41+
(10485247::bigint), (10485248::bigint),
42+
(10736893951::bigint), (10736893952::bigint),
43+
(10994579406847::bigint), (10994579406848::bigint),
44+
(11258449312612351::bigint), (11258449312612352::bigint)) x(size);
45+
size | pg_size_pretty | pg_size_pretty
46+
-------------------+----------------+----------------
47+
10239 | 10239 bytes | -10239 bytes
48+
10240 | 10 kB | -10 kB
49+
10485247 | 10239 kB | -10239 kB
50+
10485248 | 10 MB | -10 MB
51+
10736893951 | 10239 MB | -10239 MB
52+
10736893952 | 10 GB | -10 GB
53+
10994579406847 | 10239 GB | -10239 GB
54+
10994579406848 | 10 TB | -10 TB
55+
11258449312612351 | 10239 TB | -10239 TB
56+
11258449312612352 | 10240 TB | -10240 TB
57+
(10 rows)
58+
59+
SELECT size, pg_size_pretty(size), pg_size_pretty(-1 * size) FROM
60+
(VALUES (10239::numeric), (10240::numeric),
61+
(10485247::numeric), (10485248::numeric),
62+
(10736893951::numeric), (10736893952::numeric),
63+
(10994579406847::numeric), (10994579406848::numeric),
64+
(11258449312612351::numeric), (11258449312612352::numeric)) x(size);
65+
size | pg_size_pretty | pg_size_pretty
66+
-------------------+----------------+----------------
67+
10239 | 10239 bytes | -10239 bytes
68+
10240 | 10 kB | -10 kB
69+
10485247 | 10239 kB | -10239 kB
70+
10485248 | 10 MB | -10 MB
71+
10736893951 | 10239 MB | -10239 MB
72+
10736893952 | 10 GB | -10 GB
73+
10994579406847 | 10239 GB | -10239 GB
74+
10994579406848 | 10 TB | -10 TB
75+
11258449312612351 | 10239 TB | -10239 TB
76+
11258449312612352 | 10240 TB | -10240 TB
77+
(10 rows)
78+
79+
-- pg_size_bytes() tests
3880
SELECT size, pg_size_bytes(size) FROM
3981
(VALUES ('1'), ('123bytes'), ('1kB'), ('1MB'), (' 1 GB'), ('1.5 GB '),
4082
('1TB'), ('3000 TB'), ('1e6 MB')) x(size);

‎src/test/regress/sql/dbsize.sql

Lines changed: 16 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -11,6 +11,22 @@ SELECT size, pg_size_pretty(size), pg_size_pretty(-1 * size) FROM
1111
(1000000000.5::numeric), (1000000000000.5::numeric),
1212
(1000000000000000.5::numeric)) x(size);
1313

14+
-- test where units change up
15+
SELECT size, pg_size_pretty(size), pg_size_pretty(-1 * size) FROM
16+
(VALUES (10239::bigint), (10240::bigint),
17+
(10485247::bigint), (10485248::bigint),
18+
(10736893951::bigint), (10736893952::bigint),
19+
(10994579406847::bigint), (10994579406848::bigint),
20+
(11258449312612351::bigint), (11258449312612352::bigint)) x(size);
21+
22+
SELECT size, pg_size_pretty(size), pg_size_pretty(-1 * size) FROM
23+
(VALUES (10239::numeric), (10240::numeric),
24+
(10485247::numeric), (10485248::numeric),
25+
(10736893951::numeric), (10736893952::numeric),
26+
(10994579406847::numeric), (10994579406848::numeric),
27+
(11258449312612351::numeric), (11258449312612352::numeric)) x(size);
28+
29+
-- pg_size_bytes() tests
1430
SELECT size, pg_size_bytes(size) FROM
1531
(VALUES ('1'), ('123bytes'), ('1kB'), ('1MB'), (' 1 GB'), ('1.5 GB '),
1632
('1TB'), ('3000 TB'), ('1e6 MB')) x(size);

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