Re: Question about integer out of range in function

Condor <condor@xxxxxxxxxx> · Sun, 16 May 2021 20:54:20 +0300

On 14-05-2021 17:42, Tom Lane wrote:
Condor <condor@xxxxxxxxxx> writes:
      new_time = fromtime * 1000; -- here is line 19

Are you entirely certain that you counted lines correctly?
If new_time and fromtime are both declared bigint, and
fromtime is on the order of

# select extract(epoch from now())::bigint;
  extract
------------
 1621003124
(1 row)

then there's no way that that statement fails on integer overflow.

What I'm wondering about is the next line:

      unix_time = (new_time + rand_int)::BIGINT;

You've not shown us the declaration of unix_time ...

			regards, tom lane

Hello Tom,

I can't figure out where the problem is. The only explanation I can 
guess is that my expectations are plpgsql will sum variables like asm or 
c, well  .. let me explain.

CREATE FUNCTION generate_ulid(fromtime int default 0) -- Yes is INT, 
that is the problem. Explain bellow.
RETURNS TEXT
AS $$
DECLARE
  unix_time  BIGINT;
  ulid       BYTEA;
  rand_int   INTEGER;
  new_time   BIGINT;

BEGIN
  -- 6 timestamp bytes
  IF fromtime = 0 THEN
    unix_time = (EXTRACT(EPOCH FROM NOW()) * 1000)::BIGINT;
  ELSE
     rand_int = (random() * 1000)::INT;
     new_time = fromtime * 1000; -- here is line 19
     unix_time = (new_time + rand_int)::BIGINT;
  END IF;

new_time is bigint, fromtime is int so I expect to be like assembly or c 
:

new_time = fromtime;
new_time * 1000;

/*
mov %rip, %eax
imul $0x3e8, %eax, %eax
mov %eax, %rip
*/

but I guest it's seems plpgsql do:

fromtime = fromtime * 1000;
new_time = fromtime;

Here is the complete func:

CREATE EXTENSION IF NOT EXISTS pgcrypto;

DROP FUNCTION IF EXISTS generate_ulid(int) CASCADE;
DROP FUNCTION IF EXISTS generate_ulid(BIGINT) CASCADE;
DROP FUNCTION IF EXISTS generate_ulid() CASCADE;

CREATE FUNCTION generate_ulid(fromtime int default 0)
RETURNS TEXT
AS $$
DECLARE
  -- Crockford's Base32
  encoding   BYTEA = '0123456789ABCDEFGHJKMNPQRSTVWXYZ';
  timestamp  BYTEA = E'\\000\\000\\000\\000\\000\\000';
  output     TEXT = '';

  unix_time  BIGINT;
  ulid       BYTEA;
  rand_int   INTEGER;
  new_time   BIGINT;

BEGIN
  -- 6 timestamp bytes
  IF fromtime = 0 THEN
    unix_time = (EXTRACT(EPOCH FROM NOW()) * 1000)::BIGINT;
  ELSE
     rand_int = (random() * 1000)::INT;
     new_time = fromtime * 1000; -- I think this is the broken line
     unix_time = (new_time + rand_int)::BIGINT;
 -- the line will look like this after debug purpose: unix_time = 
((fromtime::BIGINT * 1000) + (random() * 1000)::INT)::BIGINT;
  END IF;
  timestamp = SET_BYTE(timestamp, 0, (unix_time >> 
40)::BIT(8)::INTEGER);
  timestamp = SET_BYTE(timestamp, 1, (unix_time >> 
32)::BIT(8)::INTEGER);
  timestamp = SET_BYTE(timestamp, 2, (unix_time >> 
24)::BIT(8)::INTEGER);
  timestamp = SET_BYTE(timestamp, 3, (unix_time >> 
16)::BIT(8)::INTEGER);
  timestamp = SET_BYTE(timestamp, 4, (unix_time >> 8)::BIT(8)::INTEGER);
  timestamp = SET_BYTE(timestamp, 5, unix_time::BIT(8)::INTEGER);

  -- 10 entropy bytes
  ulid = timestamp || gen_random_bytes(10);

  -- Encode the timestamp
  output = output || CHR(GET_BYTE(encoding, (GET_BYTE(ulid, 0) & 224) >> 
5));
  output = output || CHR(GET_BYTE(encoding, (GET_BYTE(ulid, 0) & 31)));
  output = output || CHR(GET_BYTE(encoding, (GET_BYTE(ulid, 1) & 248) >> 
3));
  output = output || CHR(GET_BYTE(encoding, ((GET_BYTE(ulid, 1) & 7) << 
2) | ((GET_BYTE(ulid, 2) & 192) >> 6)));
  output = output || CHR(GET_BYTE(encoding, (GET_BYTE(ulid, 2) & 62) >> 
1));
  output = output || CHR(GET_BYTE(encoding, ((GET_BYTE(ulid, 2) & 1) << 
4) | ((GET_BYTE(ulid, 3) & 240) >> 4)));
  output = output || CHR(GET_BYTE(encoding, ((GET_BYTE(ulid, 3) & 15) << 
1) | ((GET_BYTE(ulid, 4) & 128) >> 7)));
  output = output || CHR(GET_BYTE(encoding, (GET_BYTE(ulid, 4) & 124) >> 
2));
  output = output || CHR(GET_BYTE(encoding, ((GET_BYTE(ulid, 4) & 3) << 
3) | ((GET_BYTE(ulid, 5) & 224) >> 5)));
  output = output || CHR(GET_BYTE(encoding, (GET_BYTE(ulid, 5) & 31)));

  -- Encode the entropy
  output = output || CHR(GET_BYTE(encoding, (GET_BYTE(ulid, 6) & 248) >> 
3));
  output = output || CHR(GET_BYTE(encoding, ((GET_BYTE(ulid, 6) & 7) << 
2) | ((GET_BYTE(ulid, 7) & 192) >> 6)));
  output = output || CHR(GET_BYTE(encoding, (GET_BYTE(ulid, 7) & 62) >> 
1));
  output = output || CHR(GET_BYTE(encoding, ((GET_BYTE(ulid, 7) & 1) << 
4) | ((GET_BYTE(ulid, 8) & 240) >> 4)));
  output = output || CHR(GET_BYTE(encoding, ((GET_BYTE(ulid, 8) & 15) << 
1) | ((GET_BYTE(ulid, 9) & 128) >> 7)));
  output = output || CHR(GET_BYTE(encoding, (GET_BYTE(ulid, 9) & 124) >> 
2));
  output = output || CHR(GET_BYTE(encoding, ((GET_BYTE(ulid, 9) & 3) << 
3) | ((GET_BYTE(ulid, 10) & 224) >> 5)));
  output = output || CHR(GET_BYTE(encoding, (GET_BYTE(ulid, 10) & 31)));
  output = output || CHR(GET_BYTE(encoding, (GET_BYTE(ulid, 11) & 248) 
>> 3));
  output = output || CHR(GET_BYTE(encoding, ((GET_BYTE(ulid, 11) & 7) << 
2) | ((GET_BYTE(ulid, 12) & 192) >> 6)));
  output = output || CHR(GET_BYTE(encoding, (GET_BYTE(ulid, 12) & 62) >> 
1));
  output = output || CHR(GET_BYTE(encoding, ((GET_BYTE(ulid, 12) & 1) << 
4) | ((GET_BYTE(ulid, 13) & 240) >> 4)));
  output = output || CHR(GET_BYTE(encoding, ((GET_BYTE(ulid, 13) & 15) 
<< 1) | ((GET_BYTE(ulid, 14) & 128) >> 7)));
  output = output || CHR(GET_BYTE(encoding, (GET_BYTE(ulid, 14) & 124) 
>> 2));
  output = output || CHR(GET_BYTE(encoding, ((GET_BYTE(ulid, 14) & 3) << 
3) | ((GET_BYTE(ulid, 15) & 224) >> 5)));
  output = output || CHR(GET_BYTE(encoding, (GET_BYTE(ulid, 15) & 31)));

  RETURN output;
END
$$
LANGUAGE plpgsql
VOLATILE;

Regards,
HS