Re: [PATCH] libpq: Return of NULL from PQexec

Thanks. Applied and back-patched to 9.1.

---------------------------------------------------------------------------

Mark Hills wrote:
> I'm sending the patch below for inclusion.
>
> Originally I ran this by pgsql-hackers, just to confirm the docs (and not
> the code) were in need of the fix, which seems to be the case:
>
> http://archives.postgresql.org/pgsql-hackers/2011-09/msg00614.php
>
> Thanks
>
> --
> Mark
>
> ---------- Forwarded message ----------
> Date: Mon, 12 Sep 2011 16:40:28 +0100 (BST)
> From: Mark Hills <mark.hills@framestore.com>
> To: pgsql-hackers@postgresql.org
> Subject: libpq: Return of NULL from PQexec
>
> The libpq documentation for PQexec states:
>
>   "If a null pointer is returned, it should be treated like a
>    PGRES_FATAL_ERROR result"
>
> But this contradicts the example programs; eg. from Example Program 1:
>
>    /* Start a transaction block */
>    res = PQexec(conn, "BEGIN");
>    if (PQresultStatus(res) != PGRES_COMMAND_OK)
>    {
>        fprintf(stderr, "BEGIN command failed: %s", PQerrorMessage(conn));
>        PQclear(res);
>        exit_nicely(conn);
>    }
>
> which does not check if (res != NULL).
>
> The example is not buggy -- PQresultStatus, PQerrorMessage and,
> importantly, PQclear deal with the NULL value; eg. src/libpq/fq-exec.c:
>
>   ExecStatusType
>   PQresultStatus(const PGresult *res)
>   {
>       if (!res)
>           return PGRES_FATAL_ERROR;
>       return res->resultStatus;
>   }
>
> So I took the example to be correct, and attempted to fix the
> documentation in the patch below. I hope this is useful.
>
> In a straw-poll of code I could find using libpq, I found most follows the
> example and is reliant on libpq for its NULL check.
>
> The same also applies to PQconnect functions -- and PQstatus, PQfinish,
> which I also tried to clarify in the documentation.
>
> Thanks
>
> --
> Mark
>
>
> diff --git a/doc/src/sgml/libpq.sgml b/doc/src/sgml/libpq.sgml
> index 163a893..57be7e1 100644
> --- a/doc/src/sgml/libpq.sgml
> +++ b/doc/src/sgml/libpq.sgml
> @@ -62,7 +62,7 @@
>     return a non-null object pointer, unless perhaps there is too
>     little memory even to allocate the <structname>PGconn</> object.
>     The <function>PQstatus</> function should be called to check
> -   whether a connection was successfully made before queries are sent
> +   the return value for a successful connection before queries are sent
>     via the connection object.
>
>     <warning>
> @@ -1748,8 +1748,10 @@ PGresult *PQexec(PGconn *conn, const char *command);
>          Returns a <structname>PGresult</structname> pointer or possibly a null
>          pointer.  A non-null pointer will generally be returned except in
>          out-of-memory conditions or serious errors such as inability to send
> -        the command to the server.  If a null pointer is returned, it should
> -        be treated like a <symbol>PGRES_FATAL_ERROR</symbol> result.  Use
> +        the command to the server.  The <function>PQresultStatus</> function
> +        should be called to check the return value for any errors (including
> +        the value of a null pointer, in which case it will return
> +        <symbol>PGRES_FATAL_ERROR</symbol>).  Use
>          <function>PQerrorMessage</function> to get more information about such
>          errors.
>         </para>
>
> --
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--
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + It's impossible for everything to be true. +