Re: Question about integer out of range in function

On 14-05-2021 17:42, Tom Lane wrote:
> Condor <condor@stz-bg.com> 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