[GitHub] [cassandra] dcapwell commented on a diff in pull request #2220: CASSANDRA-18337: Operations.migrateReadRequiredOperations fails due to concurrent access when TransactionStatement is prepared

["dcapwell (via GitHub)" <gi...@apache.org>]

dcapwell commented on code in PR #2220:
URL: https://github.com/apache/cassandra/pull/2220#discussion_r1139010319


##########
src/java/org/apache/cassandra/cql3/Operations.java:
##########
@@ -42,29 +44,46 @@ public final class Operations implements Iterable<Operation>
     /**
      * The operations on regular columns.
      */
-    private final List<Operation> regularOperations = new ArrayList<>();
+    private final List<Operation> regularOperations;
 
     /**
      * The operations on static columns.
      */
-    private final List<Operation> staticOperations = new ArrayList<>();
+    private final List<Operation> staticOperations;
 
-    private final List<ReferenceOperation> regularSubstitutions = new ArrayList<>();
-    private final List<ReferenceOperation> staticSubstitutions = new ArrayList<>();
+    private final List<ReferenceOperation> regularSubstitutions;
+    private final List<ReferenceOperation> staticSubstitutions;
 
     public Operations(StatementType type)
     {
         this.type = type;
+        regularOperations = new ArrayList<>();
+        staticOperations = new ArrayList<>();
+        regularSubstitutions = new ArrayList<>();
+        staticSubstitutions = new ArrayList<>();
+    }
+
+    private Operations(Operations other)
+    {
+        type = other.type;
+        regularOperations = new ArrayList<>(other.regularOperations);
+        staticOperations = new ArrayList<>(other.staticOperations);
+        regularSubstitutions = new ArrayList<>(other.regularSubstitutions);
+        staticSubstitutions = new ArrayList<>(other.staticSubstitutions);
     }
 
-    public void migrateReadRequiredOperations()
+    @Nullable
+    public Operations migrateReadRequiredOperations()
     {
-        migrateReadRequiredOperations(staticOperations, staticSubstitutions);
-        migrateReadRequiredOperations(regularOperations, regularSubstitutions);
+        Operations other = new Operations(this);

Review Comment:
   one way to lower memory in the case of no migration is to do 2 passes, first to find if any need to migrate, and the second to migrate them.  This would lower the memory costs in the cases where `requiresRead` isn't found and only adds extra cycles to the cases where it is found....  It was hard for me to say which way to go as I don't really know how common both sides are, so not sure which to bias towards...  so I went with the less-work route and did a tiny change to existing `migrateReadRequiredOperations` at the cost of extra memory.



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