Re: BUG #16330: psql accesses null pointer in connect.c:do_connect

On Mon, Mar 30, 2020 at 02:20:48AM +0000, PG Bug reporting form wrote:
> If the connection to postmaster is closed, then trying to re-connect to
> another one leads to SIGSEGV.
>
> REPRODUCE:
> $ psql
> -> \conninfo
> You are connected to database "hugh" as user "hugh" via socket in
> "/run/postgresql" at port "5432".
> *shut down server with commands like "systemctl stop postgresql"*
> -> \conninfo
> You are currently not connected to a database.
> -> \c a b c d
> [1]    984978 segmentation fault (core dumped)  psql

Indeed.  Reproduced the problem here on HEAD and REL_12_STABLE, though
you need to execute a command after stopping the server to let psql
know that you are not connected to a database.

> ANALYSIS:
> PQhost(o_conn) returns NULL, and strcmp(host, NULL) raises SIGSEGV.

Yes, the problem comes from this commit (added Alvaro in CC):
commit: 6e5f8d489acccdc50a35a1b7db8e72b5ad579253
author: Alvaro Herrera <alvherre@alvh.no-ip.org>
date: Mon, 19 Nov 2018 14:34:12 -0300
psql: Show IP address in \conninfo

And more particularly this bit that ignores the possibility of
PQhost() being NULL:
-   if (!user && reuse_previous)
-       user = PQuser(o_conn);
-   if (!host && reuse_previous)
-       host = PQhost(o_conn);
-   if (!port && reuse_previous)
-       port = PQport(o_conn);
+   if (reuse_previous)
+   {
+       if (!user)
+           user = PQuser(o_conn);
+       if (host && strcmp(host, PQhost(o_conn)) == 0)
+       {
+           /*
+            * if we are targetting the same host, reuse its hostaddr for
+            * consistency
+            */
+           hostaddr = PQhostaddr(o_conn);

A fix like the attached should be sufficient as if we know that
PQhost() is NULL for the old connection we cannot use the old
hostaddr.  Alvaro, what do you think?
--
Michael