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>