Meaningful connection errors report a meaningful message

Fixes issue psycopg#173.

NEWS

@@ -6,6 +6,8 @@ What's new in psycopg 2.5.2
 
 - Fixed segfault pickling the exception raised on connection error
   (:ticket:`#170`).
+- Meaningful connection errors report a meaningful message, thanks to
+  Alexey Borzenkov (:ticket:`#173`).
 
 
 What's new in psycopg 2.5.1

psycopg/connection_int.c

@@ -642,6 +642,7 @@ static int
 _conn_poll_connecting(connectionObject *self)
 {
     int res = PSYCO_POLL_ERROR;
+    const char *msg;
 
     Dprintf("conn_poll: poll connecting");
     switch (PQconnectPoll(self->pgconn)) {
@@ -656,7 +657,11 @@ _conn_poll_connecting(connectionObject *self)
         break;
     case PGRES_POLLING_FAILED:
     case PGRES_POLLING_ACTIVE:
-        PyErr_SetString(OperationalError, "asynchronous connection failed");
+        msg = PQerrorMessage(self->pgconn);
+        if (!(msg && *msg)) {
+            msg = "asynchronous connection failed";
+        }
+        PyErr_SetString(OperationalError, msg);
         res = PSYCO_POLL_ERROR;
         break;
     }

tests/test_async.py

@@ -449,6 +449,16 @@ def test_async_cursor_gone(self):
         self.wait(self.conn)
         self.assertEqual(cur.fetchone(), (42,))
 
+    def test_async_connection_error_message(self):
+        try:
+            cnn = psycopg2.connect('dbname=thisdatabasedoesntexist', async=True)
+            self.wait(cnn)
+        except psycopg2.Error, e:
+            self.assertNotEqual(str(e), "asynchronous connection failed",
+                "connection error reason lost")
+        else:
+            self.fail("no exception raised")
+
 
 def test_suite():
     return unittest.TestLoader().loadTestsFromName(__name__)