[core] introduce Placeholder for Blob File Format

paimon-common/src/main/java/org/apache/paimon/data/BlobPlaceholder.java

@@ -0,0 +1,59 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.paimon.data;
+
+import org.apache.paimon.fs.SeekableInputStream;
+
+import java.io.IOException;
+import java.io.Serializable;
+
+/**
+ * The placeholder blob, mainly for blob update in data-evolution. It should never be exposed to
+ * users.
+ */
+public class BlobPlaceholder implements Blob, Serializable {
+
+    private static final long serialVersionUID = 1L;
+
+    public static final BlobPlaceholder INSTANCE = new BlobPlaceholder();
+
+    private BlobPlaceholder() {}
+
+    private Object readResolve() {
+        return INSTANCE;
+    }
+
+    @Override
+    public byte[] toData() {
+        throw new UnsupportedOperationException(
+                "Should never call this method for placeholder blob.");
+    }
+
+    @Override
+    public BlobDescriptor toDescriptor() {
+        throw new UnsupportedOperationException(
+                "Should never call this method for placeholder blob.");
+    }
+
+    @Override
+    public SeekableInputStream newInputStream() throws IOException {
+        throw new UnsupportedOperationException(
+                "Should never call this method for placeholder blob.");
+    }
+}

paimon-common/src/main/java/org/apache/paimon/reader/DataEvolutionFileReader.java

@@ -66,7 +66,7 @@ public DataEvolutionFileReader(
                 rowOffsets.length == fieldOffsets.length,
                 "Row offsets and field offsets must have the same length");
         checkArgument(rowOffsets.length > 0, "Row offsets must not be empty");
-        checkArgument(readers != null && readers.length > 1, "Readers should be more than 1");
+        checkArgument(readers != null && readers.length >= 1, "should not pass empty readers.");
         this.rowOffsets = rowOffsets;
         this.fieldOffsets = fieldOffsets;
         this.readers = readers;