dsn: default Net to tcp in NewConfig so Addr-only configs round-trip

dsn.go

@@ -264,13 +264,19 @@ func (cfg *Config) FormatDSN() string {
 	}
 
 	// [protocol[(address)]]
-	if len(cfg.Net) > 0 {
-		buf.WriteString(cfg.Net)
-		if len(cfg.Addr) > 0 {
-			buf.WriteByte('(')
-			buf.WriteString(cfg.Addr)
-			buf.WriteByte(')')
+	if len(cfg.Addr) > 0 {
+		net := cfg.Net
+		if net == "" {
+			net = "tcp"
 		}
+		buf.WriteString(net)
+		buf.WriteByte('(')
+		buf.WriteString(cfg.Addr)
+		buf.WriteByte(')')
+	} else if cfg.Net != "" && cfg.Net != "tcp" {
+		// Preserve an explicit non-default protocol when there's no
+		// address, so e.g. Net="unix" still round-trips.
+		buf.WriteString(cfg.Net)
 	}
 
 	// /dbname

dsn_test.go

@@ -330,6 +330,43 @@ func TestDSNUnsafeCollation(t *testing.T) {
 	}
 }
 
+func TestFormatDSN_NetWithoutAddr(t *testing.T) {
+	// An explicit non-default Net should still appear in the formatted
+	// DSN when Addr is empty, so the Config round-trips.
+	cases := []struct {
+		name string
+		net  string
+		want string
+	}{
+		{"unix without addr", "unix", "unix/"},
+		// tcp is the default; dropping it is a no-op on parse.
+		{"tcp without addr", "tcp", "/"},
+		{"empty net empty addr", "", "/"},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			cfg := NewConfig()
+			cfg.Net = tc.net
+			cfg.Addr = ""
+			if got := cfg.FormatDSN(); got != tc.want {
+				t.Errorf("FormatDSN() = %q, want %q", got, tc.want)
+			}
+		})
+	}
+}
+
+func TestFormatDSN_AddrWithoutNet(t *testing.T) {
+	// Direct check of the bug from #1616: an Addr-only Config should
+	// format with the default tcp protocol so it round-trips.
+	cfg := NewConfig()
+	cfg.Addr = "myhost:3306"
+	got := cfg.FormatDSN()
+	want := "tcp(myhost:3306)/"
+	if got != want {
+		t.Errorf("FormatDSN() = %q, want %q", got, want)
+	}
+}
+
 func TestParamsAreSorted(t *testing.T) {
 	expected := "/dbname?interpolateParams=true&foobar=baz&quux=loo"
 	cfg := NewConfig()