AVRO-4170: Improve sync marker error message in DataFileStream

Author: wangyum

lang/java/avro/src/main/java/org/apache/avro/file/DataFileStream.java

@@ -330,7 +330,10 @@ DataBlock nextRawBlock(DataBlock reuse) throws IOException {
     vin.readFixed(syncBuffer);
     availableBlock = false;
     if (!Arrays.equals(syncBuffer, header.sync))
-      throw new IOException("Invalid sync!");
+      throw new IOException("Invalid sync marker! The sync marker in the data block doesn't match the "
+          + "file header's sync marker. This likely indicates data corruption, truncated file, "
+          + "or incorrectly concatenated Avro files. Verify file integrity and ensure proper "
+          + "file transmission or creation.");
     return reuse;
   }
 

lang/java/avro/src/test/java/org/apache/avro/TestDataFileCorruption.java

@@ -87,7 +87,10 @@ void corruptedFile() throws IOException {
       assertEquals("fig", r.next().toString());
       assertFalse(r.hasNext());
     } catch (AvroRuntimeException e) {
-      assertEquals("Invalid sync!", e.getCause().getMessage());
+      assertEquals("Invalid sync marker! The sync marker in the data block doesn't match the "
+          + "file header's sync marker. This likely indicates data corruption, truncated file, "
+          + "or incorrectly concatenated Avro files. Verify file integrity and ensure proper "
+          + "file transmission or creation.", e.getCause().getMessage());
     }
 
   }