template-engine: must initialize flash attributes only when there is one flash cookie

- fix #3607

Author: jknack

Diff for: jooby/src/main/java/io/jooby/Context.java

@@ -154,6 +154,13 @@ public interface Context extends Registry {
    */
   @NonNull FlashMap flash();
 
+  /**
+   * Flash map or null when no flash cookie exists.
+   *
+   * @return Flash map or null when no flash cookie exists.
+   */
+  @Nullable FlashMap flashOrNull();
+
   /**
    * Get a flash attribute.
    *

Diff for: jooby/src/main/java/io/jooby/DefaultContext.java

@@ -110,6 +110,12 @@ default boolean matches(String pattern) {
                 FlashMap.NAME, key -> FlashMap.create(this, getRouter().getFlashCookie().clone()));
   }
 
+  @Nullable @Override
+  default FlashMap flashOrNull() {
+    var flashCookie = cookie(getRouter().getFlashCookie().getName());
+    return flashCookie.isMissing() ? null : flash();
+  }
+
   /**
    * Get a flash attribute.
    *

Diff for: jooby/src/main/java/io/jooby/ForwardingContext.java

@@ -673,6 +673,11 @@ public FlashMap flash() {
     return ctx.flash();
   }
 
+  @Nullable @Override
+  public FlashMap flashOrNull() {
+    return ctx.flashOrNull();
+  }
+
   @NonNull @Override
   public Value flash(@NonNull String name) {
     return ctx.flash(name);

Diff for: jooby/src/main/java/io/jooby/TemplateEngine.java

@@ -39,7 +39,7 @@ public interface TemplateEngine extends MessageEncoder {
   @Override
   default DataBuffer encode(@NonNull Context ctx, @NonNull Object value) throws Exception {
     // initialize flash and session attributes (if any)
-    ctx.flash();
+    ctx.flashOrNull();
     ctx.sessionOrNull();
 
     ctx.setDefaultResponseType(MediaType.html);

Diff for: jooby/src/test/java/io/jooby/Issue3607.java

@@ -0,0 +1,36 @@
+/*
+ * Jooby https://jooby.io
+ * Apache License Version 2.0 https://jooby.io/LICENSE.txt
+ * Copyright 2014 Edgar Espina
+ */
+package io.jooby;
+
+import static org.mockito.Mockito.*;
+
+import org.junit.jupiter.api.Test;
+
+import io.jooby.buffer.DataBuffer;
+import io.jooby.buffer.DefaultDataBufferFactory;
+
+public class Issue3607 {
+
+  private static class TemplateEngineImpl implements TemplateEngine {
+    @Override
+    public DataBuffer render(Context ctx, ModelAndView<?> modelAndView) throws Exception {
+      // do nothing
+      return DefaultDataBufferFactory.sharedInstance.wrap(new byte[0]);
+    }
+  }
+
+  @Test
+  public void shouldNotGenerateEmptyFlashMap() throws Exception {
+    var ctx = mock(Context.class);
+
+    var templateEngine = new TemplateEngineImpl();
+    templateEngine.encode(ctx, ModelAndView.map("index.html"));
+
+    verify(ctx, times(1)).flashOrNull();
+    verify(ctx, times(1)).sessionOrNull();
+    verify(ctx, times(1)).setDefaultResponseType(MediaType.html);
+  }
+}

Diff for: tests/src/test/java/io/jooby/i3607/Issue3607.java

@@ -0,0 +1,52 @@
+/*
+ * Jooby https://jooby.io
+ * Apache License Version 2.0 https://jooby.io/LICENSE.txt
+ * Copyright 2014 Edgar Espina
+ */
+package io.jooby.i3607;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+import java.util.Map;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.atomic.AtomicBoolean;
+
+import io.jooby.FlashMap;
+import io.jooby.ModelAndView;
+import io.jooby.junit.ServerTest;
+import io.jooby.junit.ServerTestRunner;
+import io.jooby.pebble.PebbleModule;
+
+public class Issue3607 {
+  @ServerTest
+  public void shouldNotGenerateEmptyFlashMap(ServerTestRunner runner) throws InterruptedException {
+    var latch = new CountDownLatch(1);
+    var mustBeNull = new AtomicBoolean(false);
+    runner
+        .define(
+            app -> {
+              app.install(new PebbleModule());
+              app.use(
+                  next ->
+                      ctx -> {
+                        ctx.onComplete(
+                            done -> {
+                              mustBeNull.set(!done.getAttributes().containsKey(FlashMap.NAME));
+                              latch.countDown();
+                            });
+                        return next.apply(ctx);
+                      });
+              app.get("/3607", ctx -> ModelAndView.map("index.pebble", Map.of("name", "Pebble")));
+            })
+        .ready(
+            client -> {
+              client.get(
+                  "/3607",
+                  rsp -> {
+                    assertEquals("Hello Pebble!", rsp.body().string().trim());
+                  });
+            });
+    latch.await();
+    assertTrue(mustBeNull.get(), "Flash map must be null");
+  }
+}