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

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

maedhroz commented on code in PR #2220:
URL: https://github.com/apache/cassandra/pull/2220#discussion_r1139180984


##########
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:
   I guess you could do something like this...
   
   ```
   @Nullable
   public Operations migrateReadRequiredOperations()
   {
       Operations other = null;
   
       if (regularOperations.stream().anyMatch(Operation::requiresRead))
       {
           other = new Operations(this);
           migrateReadRequiredOperations(other.regularOperations, other.regularSubstitutions);
           migrateReadRequiredOperations(other.staticOperations, other.staticSubstitutions);
       }
       else if (staticOperations.stream().anyMatch(Operation::requiresRead))
       {
           other = new Operations(this);
           migrateReadRequiredOperations(other.staticOperations, other.staticSubstitutions);
       }
       
       return other;
   }
   ```
   This is cheaper in the case where we don't need any migrations, and operations on static columns are probably the exceptional case...just not sure what the probabilities are, as you say.



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