pg_basebackup, pg_receivewal: fix failure to find password in ~/.pgpass.

Sloppy refactoring in commit cca97ce caused these programs
to pass dbname = NULL to libpq if there was no "--dbname" switch
on the command line, where before "replication" would be passed.
This didn't break things completely, because the source server doesn't
care about the dbname specified for a physical replication connection.
However, it did cause libpq to fail to match a ~/.pgpass entry that
has "replication" in the dbname field.  Restore the previous behavior
of passing "replication".

Also, closer inspection shows that if you do specify a dbname
in the connection string, that is what will be matched to ~/.pgpass,
not "replication".  This was the pre-existing behavior so we should
not change it, but the SGML docs were pretty misleading about it.
Improve that.

Per bug #18685 from Toshi Harada.  Back-patch to v17 where the
error crept in.

Discussion: https://postgr.es/m/18685-fee2dd142b9688f1@postgresql.org
Discussion: https://postgr.es/m/2702546.1730740456@sss.pgh.pa.us

Author: tglsfdc

doc/src/sgml/ref/pg_basebackup.sgml

@@ -807,14 +807,18 @@ PostgreSQL documentation
         will override any conflicting command line options.
        </para>
        <para>
-        The option is called <literal>--dbname</literal> for consistency with other
+        This option is called <literal>--dbname</literal> for consistency with other
         client applications, but because <application>pg_basebackup</application>
         doesn't connect to any particular database in the cluster, any database
-        name in the connection string will be ignored
-        by <productname>PostgreSQL</productname>. Middleware, or proxies, used in
-        connecting to <productname>PostgreSQL</productname> might however
-        utilize the value. The database name specified in connection string can
-        also be used by <link linkend="logicaldecoding-replication-slots-synchronization">
+        name included in the connection string will be ignored by the server.
+        However, a database name supplied that way overrides the default
+        database name (<literal>replication</literal>) for purposes of
+        looking up the replication connection's password
+        in <filename>~/.pgpass</filename>.  Similarly, middleware or proxies
+        used in connecting to <productname>PostgreSQL</productname> might
+        utilize the name for purposes such as connection routing.  The
+        database name can also be used
+        by <link linkend="logicaldecoding-replication-slots-synchronization">
         logical replication slot synchronization</link>.
        </para>
       </listitem>

doc/src/sgml/ref/pg_receivewal.sgml

@@ -315,13 +315,16 @@ PostgreSQL documentation
         will override any conflicting command line options.
        </para>
        <para>
-        The option is called <literal>--dbname</literal> for consistency with other
+        This option is called <literal>--dbname</literal> for consistency with other
         client applications, but because <application>pg_receivewal</application>
         doesn't connect to any particular database in the cluster, any database
-        name in the connection string will be ignored by
-        <productname>PostgreSQL</productname>. Middleware, or proxies, used in
-        connecting to <productname>PostgreSQL</productname> might however
-        utilize the value.
+        name included in the connection string will be ignored by the server.
+        However, a database name supplied that way overrides the default
+        database name (<literal>replication</literal>) for purposes of
+        looking up the replication connection's password
+        in <filename>~/.pgpass</filename>.  Similarly, middleware or proxies
+        used in connecting to <productname>PostgreSQL</productname> might
+        utilize the name for purposes such as connection routing.
        </para>
       </listitem>
      </varlistentry>

src/bin/pg_basebackup/streamutil.c

@@ -74,7 +74,10 @@ GetConnection(void)
 	PQconninfoOption *conn_opt;
 	char	   *err_msg = NULL;
 
-	/* pg_recvlogical uses dbname only; others use connection_string only. */
+	/*
+	 * pg_recvlogical uses dbname only; others use connection_string only.
+	 * (Note: both variables will be NULL if there's no command line options.)
+	 */
 	Assert(dbname == NULL || connection_string == NULL);
 
 	/*
@@ -120,12 +123,12 @@ GetConnection(void)
 		keywords = pg_malloc0((argcount + 1) * sizeof(*keywords));
 		values = pg_malloc0((argcount + 1) * sizeof(*values));
 		keywords[i] = "dbname";
-		values[i] = dbname;
+		values[i] = (dbname == NULL) ? "replication" : dbname;
 		i++;
 	}
 
 	keywords[i] = "replication";
-	values[i] = dbname == NULL ? "true" : "database";
+	values[i] = (dbname == NULL) ? "true" : "database";
 	i++;
 	keywords[i] = "fallback_application_name";
 	values[i] = progname;