Don't send dbname= as a startup parameter when database= is used

Fixes #1256

Author: arp242

connector.go

@@ -758,14 +758,12 @@ func (cfg *Config) setFromTag(o map[string]string, tag string) error {
 
 	// Set run-time; we delete map keys as they're set in the struct.
 	if tag == "postgres" {
-		// Make sure database= sets dbname=; in startup() we send database for
-		// dbname, and if we have both set it's inconsistent as the loop order
-		// is a map.
+		// Make sure database= sets dbname=, as that previously worked (kind of
+		// by accident).
+		// TODO(v2): remove
 		if d, ok := o["database"]; ok {
+			cfg.Database = d
 			delete(o, "database")
-			if o["dbname"] == "" {
-				o["dbname"] = d
-			}
 		}
 		cfg.Runtime = o
 	}

connector_test.go

@@ -113,8 +113,23 @@ func TestNewConnector(t *testing.T) {
 		defer db.Close()
 		useConn(t, db)
 	})
+
+	t.Run("database=", func(t *testing.T) {
+		// Make sure database= consistently take precedence over dbname=
+		for i := 0; i < 10; i++ {
+			err := pqtest.MustDB(t, "database=err").Ping()
+			if err == nil || err.Error() != `pq: database "err" does not exist (3D000)` {
+				t.Errorf("wrong error: %s", err)
+			}
+			err = pqtest.MustDB(t, "dbname=one database=two").Ping()
+			if err == nil || err.Error() != `pq: database "two" does not exist (3D000)` {
+				t.Errorf("wrong error: %s", err)
+			}
+		}
+	})
 }
 
+// TODO: this can be merged with TestNewConfig, I think?
 func TestParseOpts(t *testing.T) {
 	tests := []struct {
 		in      string
@@ -130,6 +145,7 @@ func TestParseOpts(t *testing.T) {
 		{"dbname=データベース password=パスワード", map[string]string{"dbname": "データベース", "password": "パスワード"}, ""},
 		{"dbname=hello user=''", map[string]string{"dbname": "hello", "user": ""}, ""},
 		{"user='' dbname=hello", map[string]string{"dbname": "hello", "user": ""}, ""},
+
 		// The last option value is an empty string if there's no non-whitespace after its =
 		{"dbname=hello user=   ", map[string]string{"dbname": "hello", "user": ""}, ""},
 
@@ -229,6 +245,8 @@ func TestRuntimeParameters(t *testing.T) {
 		})
 	}
 }
+
+// TODO: this can be merged with TestNewConfig, I think?
 func TestParseEnviron(t *testing.T) {
 	tests := []struct {
 		in   []string