Fix NPE when a message with parsed extension merges from bytes that contain extension data and when lazy mode is enabled.

PiperOrigin-RevId: 904139856

Author: protobuf-github-bot

java/core/src/main/java/com/google/protobuf/MessageReflection.java

@@ -1030,12 +1030,10 @@ public void mergeMessage(
       if (!field.isRepeated()) {
         boolean isLazyField = ExtensionRegistryLite.lazyExtensionEnabled() && field.isExtension();
         if (hasField(field)) {
-          if (isLazyField) {
+          InternalLazyField lazyField = extensions.getLazyField(field);
+          if (isLazyField && lazyField != null) {
             Object unused =
-                setField(
-                    field,
-                    InternalLazyField.mergeFrom(
-                        extensions.getLazyField(field), input, extensionRegistry));
+                setField(field, InternalLazyField.mergeFrom(lazyField, input, extensionRegistry));
             return;
           }
           Object fieldOrBuilder = extensions.getFieldAllowBuilders(field);

java/core/src/test/java/com/google/protobuf/ParserTest.java

@@ -434,4 +434,24 @@ public int read() throws IOException {
       assertThat(e.getClass()).isEqualTo(InterruptedIOException.class);
     }
   }
+
+  @Test
+  public void testMessageWithExtensionsParseFromBytes() throws Exception {
+    TestParsingMerge.Builder builder = TestParsingMerge.newBuilder();
+    ExtensionRegistry registry = ExtensionRegistry.newInstance();
+    UnittestProto.registerAllExtensions(registry);
+    builder
+        .setExtension(
+            TestParsingMerge.optionalExt, TestAllTypes.newBuilder().setOptionalInt32(1).build())
+        .mergeFrom(
+            TestParsingMerge.newBuilder()
+                .setExtension(
+                    TestParsingMerge.optionalExt,
+                    TestAllTypes.newBuilder().setOptionalInt32(2).build())
+                .buildPartial()
+                .toByteString(),
+            registry);
+
+    assertThat(builder.getExtension(TestParsingMerge.optionalExt).getOptionalInt32()).isEqualTo(2);
+  }
 }