Reuse correct connection parameters for PostgreSQL failover

If the failover callback doesn't provide the connection string to use,
default to the originally used one instead of using empty connection
string which is almost certainly not what we want.

Note that this requires storing the original connection parameters in
postgresql_session_backend, as we don't have access to the ones stored
in the session object and it seems wrong to add backlinks from the
backend to the session object.

Closes #793.

include/soci/postgresql/soci-postgresql.h

@@ -25,6 +25,7 @@
 #endif
 
 #include <soci/soci-backend.h>
+#include "soci/connection-parameters.h"
 #include <libpq-fe.h>
 #include <vector>
 
@@ -395,6 +396,7 @@ struct postgresql_session_backend : details::session_backend
     int statementCount_;
     bool single_row_mode_;
     PGconn * conn_;
+    connection_parameters connectionParameters_;
 };
 
 

src/backends/postgresql/error.cpp

@@ -123,8 +123,11 @@ details::postgresql_result::check_for_data(char const* errMsg) const
 
                         if (retry)
                         {
-                            connection_parameters parameters;
-                            parameters.set_connect_string(newTarget);
+                            connection_parameters parameters =
+                                sessionBackend_.connectionParameters_;
+
+                            if (!newTarget.empty())
+                                parameters.set_connect_string(newTarget);
 
                             sessionBackend_.clean_up();
 

src/backends/postgresql/session.cpp

@@ -9,7 +9,6 @@
 #include "soci/soci-platform.h"
 #include "soci/postgresql/soci-postgresql.h"
 #include "soci/session.h"
-#include "soci/connection-parameters.h"
 #include <libpq/libpq-fs.h> // libpq
 #include <cctype>
 #include <cstdio>
@@ -69,6 +68,7 @@ void postgresql_session_backend::connect(
         "Cannot set extra_float_digits parameter");
 
     conn_ = conn;
+    connectionParameters_ = parameters;
 }
 
 postgresql_session_backend::~postgresql_session_backend()