funny (cache (?)) bug in postgres (7.x tested)
Hello!
I found a funny bug in postgres with c functions. (or feature??)
Let's say we have got an function like this:
CREATE FUNCTION hupper(text)
RETURNS text
AS '/fun.so'
LANGUAGE 'c';
and fun.c:
#include <postgresql/postgres.h>
#include <postgresql/utils/elog.h>
#include <postgresql/libpq/libpq-fs.h>
text *hupper (text *a) {
int hossz,i;
hossz=a->vl_len;
for (i=0;i<hossz;i++)
{
char ch;
ch=a->vl_dat[i];
if ((ch>=97)&(ch<=122)) ch=ch-32;
else if (ch=='���') ch='���';
else if (ch=='���') ch='���';
else if (ch=='���') ch='���';
else if (ch=='���') ch='���';
else if (ch=='���') ch='���';
else if (ch=='���') ch='���';
else if (ch=='���') ch='���';
else if (ch=='���') ch='���';
else if (ch=='���') ch='���';
a->vl_dat[i]=ch;
}
return a;
}
We use this to make hungarian upper (=Hupper).
And two select:
gergo=> select mire from mamapenz;
mire
-------
betet
ebed
ebed
ebed
ebed
ebed
(6 rows)
gergo=> select hupper(mire) from mamapenz;
hupper
--------
BETET
EBED
EBED
EBED
EBED
EBED
(6 rows)
this is good, and now:
gergo=> select mire from mamapenz;
^^^^^^^^^^^^^^^^^^^^^
mire
-------
BETET
EBED
EBED
EBED
EBED
EBED
(6 rows)
After once hupper run on the table it will be upper case even I don't use hupper.
It can be fixed with a postgres restart or with 10-20 minutes of waiting.
If this is documented, sorry (but please point out where).
Thanks,
RISKO Gergely
RISKO Gergely <risko@atom.hu> writes:
text *hupper (text *a) {
int hossz,i;hossz=a->vl_len;
for (i=0;i<hossz;i++)
{
char ch;
ch=a->vl_dat[i];
if ((ch>=97)&(ch<=122)) ch=ch-32;
else if (ch=='�') ch='�';
else if (ch=='�') ch='�';
else if (ch=='�') ch='�';
else if (ch=='�') ch='�';
else if (ch=='�') ch='�';
else if (ch=='�') ch='�';
else if (ch=='�') ch='�';
else if (ch=='�') ch='�';
else if (ch=='�') ch='�';
a->vl_dat[i]=ch;
}return a;
}
I think you need to allocate a new TEXT datum and return it. You're
modifying the cached data in place, which is a no-no AFAIK.
-Doug
--
The rain man gave me two cures; he said jump right in,
The first was Texas medicine--the second was just railroad gin,
And like a fool I mixed them, and it strangled up my mind,
Now people just get uglier, and I got no sense of time... --Dylan
Import Notes
Reply to msg id not found: RISKOGergelysmessageofThu28Jun2001213918+0200
On Thu, 28 Jun 2001, RISKO Gergely wrote:
text *hupper (text *a) {
int hossz,i;hossz=a->vl_len;
for (i=0;i<hossz;i++)
{
char ch;
ch=a->vl_dat[i];
if ((ch>=97)&(ch<=122)) ch=ch-32;
else if (ch=='���') ch='���';
else if (ch=='���') ch='���';
else if (ch=='���') ch='���';
else if (ch=='���') ch='���';
else if (ch=='���') ch='���';
else if (ch=='���') ch='���';
else if (ch=='���') ch='���';
else if (ch=='���') ch='���';
else if (ch=='���') ch='���';
a->vl_dat[i]=ch;
}return a;
}
(Rest snipped).
You are not supposed to write directly to the argument you are given. You
must construct a new text value (by allocating space via palloc) and
copy your string there. By overwriting existing values, you potentially
corrupt postgres' cache, resulting in your behaviour.
RISKO Gergely wrote:
[Charset iso-8859-1,iso- unsupported, skipping...]
Hello!
I found a funny bug in postgres with c functions. (or feature??)
Let's say we have got an function like this:
CREATE FUNCTION hupper(text)
RETURNS text
AS '/fun.so'
LANGUAGE 'c';
This is actually neither a feature, nor a bug in Postgres.
Your function is violating some coding rules.
The text argument handed into is actually residing inside the
shared buffer cache. So you're not supposed to change it!
Your function isn't safe for compressed or toasted values. If
you accidentially have big data in that column, it might
crash the backend completely.
A function returning text has to allocate the return value
with palloc().
Look into utils/adt/*.c for examples how to deal correctly
with text attributes.
Jan
--
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