Thread: replace strtok()

Under the topic of getting rid of thread-unsafe functions in the backend 
[0], here is a patch series to deal with strtok().

Of course, strtok() is famously not thread-safe and can be replaced by 
strtok_r().  But it also has the wrong semantics in some cases, because 
it considers adjacent delimiters to be one delimiter.  So if you parse

     SCRAM-SHA-256$<iterations>:<salt>$<storedkey>:<serverkey>

with strtok(), then

     SCRAM-SHA-256$$<iterations>::<salt>$$<storedkey>::<serverkey>

parses just the same.  In many cases, this is arguably wrong and could 
hide mistakes.

So I'm suggesting to use strsep() in those places.  strsep() is 
nonstandard but widely available.

There are a few places where strtok() has the right semantics, such as 
parsing tokens separated by whitespace.  For those, I'm using strtok_r().

A reviewer job here would be to check whether I made that distinction 
correctly in each case.

On the portability side, I'm including a port/ replacement for strsep() 
and some workaround to get strtok_r() for Windows.  I have included 
these here as separate patches for clarity.


[0]: 
https://www.postgresql.org/message-id/856e5ec3-879f-42ee-8258-8bcc6ec9bdea@eisentraut.org

At Tue, 18 Jun 2024 09:18:28 +0200, Peter Eisentraut <peter@eisentraut.org> wrote in 
> Under the topic of getting rid of thread-unsafe functions in the
> backend [0], here is a patch series to deal with strtok().
> 
> Of course, strtok() is famously not thread-safe and can be replaced by
> strtok_r().  But it also has the wrong semantics in some cases,
> because it considers adjacent delimiters to be one delimiter.  So if
> you parse
> 
>     SCRAM-SHA-256$<iterations>:<salt>$<storedkey>:<serverkey>
> 
> with strtok(), then
> 
>     SCRAM-SHA-256$$<iterations>::<salt>$$<storedkey>::<serverkey>
> 
> parses just the same.  In many cases, this is arguably wrong and could
> hide mistakes.
> 
> So I'm suggesting to use strsep() in those places.  strsep() is
> nonstandard but widely available.
> 
> There are a few places where strtok() has the right semantics, such as
> parsing tokens separated by whitespace.  For those, I'm using
> strtok_r().

I agree with the distinction.

> A reviewer job here would be to check whether I made that distinction
> correctly in each case.

0001 and 0002 look correct to me regarding that distinction. They
applied correctly to the master HEAD and all tests passed on Linux.

> On the portability side, I'm including a port/ replacement for
> strsep() and some workaround to get strtok_r() for Windows.  I have
> included these here as separate patches for clarity.

0003 looks fine and successfully built and seems working on an MSVC
build.

About 0004, Cygwin seems to have its own strtok_r, but I haven't
checked how that fact affects the build.

> [0]:
> https://www.postgresql.org/message-id/856e5ec3-879f-42ee-8258-8bcc6ec9bdea@eisentraut.org

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center

On 18.06.24 13:43, Ranier Vilela wrote:
> But I would like to see more const char * where this is possible.
> 
> For example, in pg_locale.c
> IMO, the token variable can be const char *.
> 
> At least strchr expects a const char * as the first parameter.

This would not be future-proof.  In C23, if you pass a const char * into 
strchr(), you also get a const char * as a result.  And in this case, we 
do write into the area pointed to by the result.  So with a const char 
*token, this whole thing would not compile cleanly under C23.

> I found another implementation of strsep, it seems lighter to me.
> I will attach it for consideration, however, I have not done any testing.

Yeah, surely there are many possible implementations.  I'm thinking, 
since we already took other str*() functions from OpenBSD, it makes 
sense to do this here as well, so we have only one source to deal with.

Peter Eisentraut <peter@eisentraut.org> writes:
> On 18.06.24 13:43, Ranier Vilela wrote:
>> I found another implementation of strsep, it seems lighter to me.
>> I will attach it for consideration, however, I have not done any testing.

> Yeah, surely there are many possible implementations.  I'm thinking, 
> since we already took other str*() functions from OpenBSD, it makes 
> sense to do this here as well, so we have only one source to deal with.

Why not use strpbrk?  That's equally thread-safe, it's been there
since C89, and it doesn't have the problem that you can't find out
which of the delimiter characters was found.

            regards, tom lane

On Sat, Jun 22, 2024 at 11:48:21AM -0400, Tom Lane wrote:
> Peter Eisentraut <peter@eisentraut.org> writes:
> > On 18.06.24 13:43, Ranier Vilela wrote:
> >> I found another implementation of strsep, it seems lighter to me.
> >> I will attach it for consideration, however, I have not done any testing.
>
> > Yeah, surely there are many possible implementations.  I'm thinking,
> > since we already took other str*() functions from OpenBSD, it makes
> > sense to do this here as well, so we have only one source to deal with.
>
> Why not use strpbrk?  That's equally thread-safe, it's been there
> since C89, and it doesn't have the problem that you can't find out
> which of the delimiter characters was found.

Yeah, strpbrk() has been used in the tree as far as 2003 without any
port/ implementation.
--
Michael

On 24.06.24 02:34, Michael Paquier wrote:
> On Sat, Jun 22, 2024 at 11:48:21AM -0400, Tom Lane wrote:
>> Peter Eisentraut <peter@eisentraut.org> writes:
>>> On 18.06.24 13:43, Ranier Vilela wrote:
>>>> I found another implementation of strsep, it seems lighter to me.
>>>> I will attach it for consideration, however, I have not done any testing.
>>
>>> Yeah, surely there are many possible implementations.  I'm thinking,
>>> since we already took other str*() functions from OpenBSD, it makes
>>> sense to do this here as well, so we have only one source to deal with.
>>
>> Why not use strpbrk?  That's equally thread-safe, it's been there
>> since C89, and it doesn't have the problem that you can't find out
>> which of the delimiter characters was found.
> 
> Yeah, strpbrk() has been used in the tree as far as 2003 without any
> port/ implementation.

The existing uses of strpbrk() are really just checking whether some 
characters exist in a string, more like an enhanced strchr().  I don't 
see any uses for tokenizing a string like strtok() or strsep() would do. 
  I think that would look quite cumbersome.  So I think a simpler and 
more convenient abstraction like strsep() would still be worthwhile.