prashantwason commented on a change in pull request #4739:
URL: https://github.com/apache/hudi/pull/4739#discussion_r802064734



##########
File path: 
hudi-common/src/main/java/org/apache/hudi/metadata/HoodieMetadataPayload.java
##########
@@ -362,28 +375,33 @@ private HoodieMetadataColumnStats 
combineColumnStatsMetadatat(HoodieMetadataPayl
     return filesystemMetadata.entrySet().stream().filter(e -> 
e.getValue().getIsDeleted() == isDeleted);
   }
 
-  private Map<String, HoodieMetadataFileInfo> 
combineFilesystemMetadata(HoodieMetadataPayload previousRecord) {
+  private Map<String, HoodieMetadataFileInfo> 
combineFileSystemMetadata(HoodieMetadataPayload previousRecord) {
     Map<String, HoodieMetadataFileInfo> combinedFileInfo = new HashMap<>();
+
+    // First, add all files listed in the previous record
     if (previousRecord.filesystemMetadata != null) {
       combinedFileInfo.putAll(previousRecord.filesystemMetadata);
     }
 
+    // Second, merge in the files listed in the new record
     if (filesystemMetadata != null) {
-      filesystemMetadata.forEach((filename, fileInfo) -> {
-        // If the filename wasnt present then we carry it forward
-        if (!combinedFileInfo.containsKey(filename)) {
-          combinedFileInfo.put(filename, fileInfo);
-        } else {
-          if (fileInfo.getIsDeleted()) {
-            // file deletion
-            combinedFileInfo.remove(filename);
-          } else {
-            // file appends.
-            combinedFileInfo.merge(filename, fileInfo, (oldFileInfo, 
newFileInfo) -> {
-              return new HoodieMetadataFileInfo(oldFileInfo.getSize() + 
newFileInfo.getSize(), false);
-            });
-          }
-        }
+      validatePayload(type, filesystemMetadata);
+
+      filesystemMetadata.forEach((key, fileInfo) -> {
+        combinedFileInfo.merge(key, fileInfo,
+            // Combine previous record w/ the new one, new records taking 
precedence over
+            // the old one
+            //
+            // NOTE: That if previous listing contains the file that is being 
deleted by the tombstone
+            //       record (`IsDeleted` = true) in the new one, we simply 
delete the file from the resulting
+            //       listing as well as drop the tombstone itself.
+            //       However, if file is not present in the previous record we 
have to persist tombstone
+            //       record in the listing to make sure we carry forward 
information that this file
+            //       was deleted. This special case could occur since the 
merging flow is 2-stage:
+            //          - First we merge records from all of the delta 
log-files
+            //          - Then we merge records from base-files with the delta 
ones (coming as a result
+            //          of the previous step)
+            (oldFileInfo, newFileInfo) -> newFileInfo.getIsDeleted() ? null : 
newFileInfo);

Review comment:
       We can add a validation here that oldFileInfo should not have 
getIsDeleted()
   checkState(!oldFileInfo.getIsDeleted())




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