Allow multiple hostaddrs to go with multiple hostnames.

Also fix two other issues, while we're at it:

* In error message on connection failure, if multiple network addresses
were given as the host option, as in "host=127.0.0.1,127.0.0.2", the
error message printed the address twice.

* If there were many more ports than hostnames, the error message would
always claim that there was one port too many, even if there was more than
one. For example, if you gave 2 hostnames and 5 ports, the error message
claimed that you gave 2 hostnames and 3 ports.

Discussion: https://www.postgresql.org/message-id/10badbc6-4d5a-a769-623a-f7ada43e14dd@iki.fi

doc/src/sgml/libpq.sgml

@@ -887,6 +887,42 @@ postgresql://%2Fvar%2Flib%2Fpostgresql/dbname
     host will be tried in turn until a connection is successfully established.
    </para>
    </sect3>
+
+   <sect3 id="libpq-multiple-hosts">
+     <title>Specifying Multiple Hosts</title>
+
+     <para>
+       It is possible to specify multiple hosts to connect to, so that they are 
+       tried in the given order. In the Keyword/Value format, the <literal>host</>, 
+       <literal>hostaddr</>, and <literal>port</> options accept a comma-separated
+       list of values. The same number of elements must be given in each option, such
+       that e.g. the first <literal>hostaddr</> corresponds to the first host name,
+       the second <literal>hostaddr</> corresponds to the second host name, and so
+       forth. As an exception, if only one <literal>port</literal> is specified, it
+       applies to all the hosts.
+     </para>
+
+     <para>
+       In the connection URI format, you can list multiple <literal>host:port</> pairs 
+       separated by commas, in the <literal>host</> component of the URI. In either
+       format, a single hostname can also translate to multiple network addresses. A
+       common example of this is a host that has both an IPv4 and an IPv6 address.
+     </para>
+
+     <para>
+       When multiple hosts are specified, or when a single hostname is 
+       translated to multiple addresses,  all the hosts and addresses will be 
+       tried in order, until one succeeds. If none of the hosts can be reached, 
+       the connection fails. If a connection is established successfully, but 
+       authentication fails, the remaining hosts in the list are not tried.
+     </para>
+
+     <para>
+       If a password file is used, you can have different passwords for 
+       different hosts. All the other connection options are the same for every 
+       host, it is not possible to e.g. specify a different username for 
+       different hosts.
+     </para>
   </sect2>
 
   <sect2 id="libpq-paramkeywords">
@@ -900,7 +936,7 @@ postgresql://%2Fvar%2Flib%2Fpostgresql/dbname
       <term><literal>host</literal></term>
       <listitem>
        <para>
-        Comma-separated list of host names.<indexterm><primary>host name</></>
+        Name of host to connect to.<indexterm><primary>host name</></>
         If a host name begins with a slash, it specifies Unix-domain
         communication rather than TCP/IP communication; the value is the
         name of the directory in which the socket file is stored.  If
@@ -912,6 +948,11 @@ postgresql://%2Fvar%2Flib%2Fpostgresql/dbname
         when <productname>PostgreSQL</> was built). On machines without
         Unix-domain sockets, the default is to connect to <literal>localhost</>.
        </para>
+       <para>
+        A comma-separated list of host names is also accepted, in which case
+        each host name in the list is tried in order. See
+        <xref linkend="libpq-multiple-hosts"> for details.
+       </para>
       </listitem>
      </varlistentry>
 
@@ -965,6 +1006,11 @@ postgresql://%2Fvar%2Flib%2Fpostgresql/dbname
         <xref linkend="libpq-pgpass">).
        </para>
 
+       <para>
+        A comma-separated list of <literal>hostaddrs</> is also accepted, in
+        which case each host in the list is tried in order. See
+        <xref linkend="libpq-multiple-hosts"> for details.
+       </para>
        <para>
         Without either a host name or host address,
         <application>libpq</application> will connect using a
@@ -981,9 +1027,10 @@ postgresql://%2Fvar%2Flib%2Fpostgresql/dbname
         Port number to connect to at the server host, or socket file
         name extension for Unix-domain
         connections.<indexterm><primary>port</></>
-        If the <literal>host</> parameter included multiple, comma-separated
-        hosts, this parameter may specify a list of ports of equal length,
-        or it may specify a single port number to be used for all hosts.
+        If multiple hosts were given in the <literal>host</literal> or
+        <literal>hostaddr</> parameters, this parameter may specify a list
+        of ports of equal length, or it may specify a single port number to
+        be used for all hosts.
        </para>
       </listitem>
      </varlistentry>

src/interfaces/libpq/libpq-int.h

@@ -304,8 +304,9 @@ typedef enum pg_conn_host_type
  */
 typedef struct pg_conn_host
 {
-	char	   *host;			/* host name or address, or socket path */
 	pg_conn_host_type type;		/* type of host */
+	char	   *host;			/* host name or socket path */
+	char	   *hostaddr;		/* host address */
 	char	   *port;			/* port number for this host; if not NULL,
 								 * overrides the PGConn's pgport */
 	char	   *password;		/* password for this host, read from the