repair psycopg2 (and psycopg) multiple hosts format

Fixed issue in psycopg2 dialect where the "multiple hosts" feature
implemented for :ticket:`4392`, where multiple ``host:port`` pairs could be
passed in the query string as
``?host=host1:port1&host=host2:port2&host=host3:port3`` was not implemented
correctly, as it did not propagate the "port" parameter appropriately.
Connections that didn't use a different "port" likely worked without issue,
and connections that had "port" for some of the entries may have
incorrectly passed on that hostname. The format is now corrected to pass
hosts/ports appropriately.

As part of this change, maintained support for another multihost style that
worked unintentionally, which is comma-separated
``?host=h1,h2,h3&port=p1,p2,p3``. This format is more consistent with
libpq's query-string format, whereas the previous format is inspired by a
different aspect of libpq's URI format but is not quite the same thing.

If the two styles are mixed together, an error is raised as this is
ambiguous.

Fixes: #4392
Change-Id: Ic9cc0b0e6e90725e158d9efe73e042853dd1263f

1 files changed, 35 insertions, 6 deletions

diff --git a/lib/sqlalchemy/dialects/postgresql/psycopg2.py b/lib/sqlalchemy/dialects/postgresql/psycopg2.py
index 2fe1ee15b..6f78dafdd 100644
--- a/lib/sqlalchemy/dialects/postgresql/psycopg2.py
+++ b/lib/sqlalchemy/dialects/postgresql/psycopg2.py
@@ -117,22 +117,51 @@ Specifying multiple fallback hosts
 psycopg2 supports multiple connection points in the connection string.
 When the ``host`` parameter is used multiple times in the query section of
 the URL, SQLAlchemy will create a single string of the host and port
-information provided to make the connections::
+information provided to make the connections.  Tokens may consist of
+``host::port`` or just ``host``; in the latter case, the default port
+is selected by libpq.  In the example below, three host connections
+are specified, for ``HostA::PortA``, ``HostB`` connecting to the default port,
+and ``HostC::PortC``::
 
     create_engine(
-        "postgresql+psycopg2://user:password@/dbname?host=HostA:port1&host=HostB&host=HostC"
+        "postgresql+psycopg2://user:password@/dbname?host=HostA:PortA&host=HostB&host=HostC:PortC"
     )
 
-A connection to each host is then attempted until either a connection is successful
-or all connections are unsuccessful in which case an error is raised.
+As an alternative, libpq query string format also may be used; this specifies
+``host`` and ``port`` as single query string arguments with comma-separated
+lists - the default port can be chosen by indicating an empty value
+in the comma separated list::
+
+    create_engine(
+        "postgresql+psycopg2://user:password@/dbname?host=HostA,HostB,HostC&port=PortA,,PortC"
+    )
+
+With either URL style, connections to each host is attempted based on a
+configurable strategy, which may be configured using the libpq
+``target_session_attrs`` parameter.  Per libpq this defaults to ``any``
+which indicates a connection to each host is then attempted until a connection is successful.
+Other strategies include ``primary``, ``prefer-standby``, etc.  The complete
+list is documented by PostgreSQL at
+`libpq connection strings <https://www.postgresql.org/docs/current/libpq-connect.html#LIBPQ-CONNSTRING>`_.
+
+For example, to indicate two hosts using the ``primary`` strategy::
+
+    create_engine(
+        "postgresql+psycopg2://user:password@/dbname?host=HostA:PortA&host=HostB&host=HostC:PortC&target_session_attrs=primary"
+    )
+
+.. versionchanged:: 1.4.40 Port specification in psycopg2 multiple host format
+   is repaired, previously ports were not correctly interpreted in this context.
+   libpq comma-separated format is also now supported.
 
 .. versionadded:: 1.3.20 Support for multiple hosts in PostgreSQL connection
    string.
 
 .. seealso::
 
-    `PQConnString \
-    <https://www.postgresql.org/docs/current/libpq-connect.html#LIBPQ-CONNSTRING>`_
+    `libpq connection strings <https://www.postgresql.org/docs/current/libpq-connect.html#LIBPQ-CONNSTRING>`_ - please refer
+    to this section in the libpq documentation for complete background on multiple host support.
+
 
 Empty DSN Connections / Environment Variable Connections
 ---------------------------------------------------------