Allow dbname in pg_basebackup/pg_receivewal connstring

As physical replication work at the cluster level and not database
level, any dbname in the connection string is ignored. Proxies and
middleware used in connecting to the cluster might however need to
know the dbname in order to make the correct routing decision for
the connection.

With this the startup packet will include the dbname parameter.

Author: Jelte Fennema-Nio <me@jeltef.nl>
Reviewed-by: Tristen Raab <tristen.raab@highgo.ca>
Reviewed-by: Jim Jones <jim.jones@uni-muenster.de>
Discussion: https://postgr.es/m/CAGECzQTw-dZkVT_RELRzfWRzY714-VaTjoBATYfZq93R8C-auA@mail.gmail.com

doc/src/sgml/ref/pg_basebackup.sgml

@@ -778,7 +778,10 @@ PostgreSQL documentation
         The 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.
+        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.
        </para>
       </listitem>
      </varlistentry>

doc/src/sgml/ref/pg_receivewal.sgml

@@ -316,8 +316,11 @@ PostgreSQL documentation
        <para>
         The 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, database
+        doesn't connect to any particular database in the cluster, any database
-        name in the connection string will be ignored.
+        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.
        </para>
       </listitem>
      </varlistentry>

src/bin/pg_basebackup/streamutil.c

@@ -79,9 +79,6 @@ GetConnection(void)
 	/*
 	 * Merge the connection info inputs given in form of connection string,
 	 * options and default values (dbname=replication, replication=true, etc.)
-	 * Explicitly discard any dbname value in the connection string;
-	 * otherwise, PQconnectdbParams() would interpret that value as being
-	 * itself a connection string.
 	 */
 	i = 0;
 	if (connection_string)
@@ -92,18 +89,24 @@ GetConnection(void)
 
 		for (conn_opt = conn_opts; conn_opt->keyword != NULL; conn_opt++)
 		{
-			if (conn_opt->val != NULL && conn_opt->val[0] != '\0' &&
+			if (conn_opt->val != NULL && conn_opt->val[0] != '\0')
-				strcmp(conn_opt->keyword, "dbname") != 0)
 				argcount++;
 		}
 
 		keywords = pg_malloc0((argcount + 1) * sizeof(*keywords));
 		values = pg_malloc0((argcount + 1) * sizeof(*values));
 
+		/*
+		 * Set dbname here already, so it can be overridden by a dbname in the
+		 * connection string.
+		 */
+		keywords[i] = "dbname";
+		values[i] = "replication";
+		i++;
+
 		for (conn_opt = conn_opts; conn_opt->keyword != NULL; conn_opt++)
 		{
-			if (conn_opt->val != NULL && conn_opt->val[0] != '\0' &&
+			if (conn_opt->val != NULL && conn_opt->val[0] != '\0')
-				strcmp(conn_opt->keyword, "dbname") != 0)
 			{
 				keywords[i] = conn_opt->keyword;
 				values[i] = conn_opt->val;
@@ -115,11 +118,11 @@ GetConnection(void)
 	{
 		keywords = pg_malloc0((argcount + 1) * sizeof(*keywords));
 		values = pg_malloc0((argcount + 1) * sizeof(*values));
+		keywords[i] = "dbname";
+		values[i] = dbname;
+		i++;
 	}
 
-	keywords[i] = "dbname";
-	values[i] = dbname == NULL ? "replication" : dbname;
-	i++;
 	keywords[i] = "replication";
 	values[i] = dbname == NULL ? "true" : "database";
 	i++;
@@ -171,7 +174,11 @@ GetConnection(void)
 			values[i] = NULL;
 		}
 
-		tmpconn = PQconnectdbParams(keywords, values, true);
+		/*
+		 * Only expand dbname when we did not already parse the argument as a
+		 * connection string ourselves.
+		 */
+		tmpconn = PQconnectdbParams(keywords, values, !connection_string);
 
 		/*
 		 * If there is too little memory even to allocate the PGconn object