plugin/loop: avoid panic on invalid server block (#7568)

Ignore invalid ServerBlockKeys in loop plugin that fail
normalization. Retain the default “.” zone instead of
indexing into an empty slice. This prevents an
index-out-of-range panic triggered by malformed
inputs such as "unix://".

Added tests to validate and increase test coverage.

Signed-off-by: Ville Vesilehto <ville@vesilehto.fi>

plugin/loop/setup_test.go

@@ -17,3 +17,32 @@ func TestSetup(t *testing.T) {
 		t.Fatal("Expected errors, but got none")
 	}
 }
+
+func TestParseServerBlockKeys(t *testing.T) {
+	tests := []struct {
+		name   string
+		key    string
+		want   string
+		wantOk bool
+	}{
+		{name: "valid domain", key: "example.org", want: "example.org.", wantOk: true},
+		{name: "invalid scheme", key: "unix://", want: ".", wantOk: true},
+		{name: "empty", key: "", want: ".", wantOk: true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			c := caddy.NewTestController("dns", `loop`)
+			if tt.key != "" {
+				c.ServerBlockKeys = []string{tt.key}
+			}
+			l, err := parse(c)
+			if (err == nil) != tt.wantOk {
+				t.Fatalf("parse err=%v, wantOk=%v", err, tt.wantOk)
+			}
+			if l.zone != tt.want {
+				t.Fatalf("zone=%q, want %q", l.zone, tt.want)
+			}
+		})
+	}
+}