libpq: Return of NULL from PQexec

On Mon, Sep 12, 2011 at 10:40 AM, Mark Hills <Mark.Hills@framestore.com> wrote:

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>

yeah -- libpq's handling of errors and result state is (put
charitably) pretty clunky -- a modernized api would probably lean on
thread local storage for global error state and return sane errors in
OOM conditions. libpq simply demands wrapping if you want a clean
api. anyways, +1 on the patch and the rationale -- the idea is to not
manually check NULL but to try and rely on the API -- this removes
(stupid) logic from userland.

merlin