part: Fix MapTxn reusability

Now that txnIDs are the mechanism to detect if a node can be mutated or not
we need to bump txnID in when committing to allow MapTxn to keep reusing part.Txn.

Rename 'prevTxnID' to 'nextTxnID' and move incrementing the txnID from 'Tree.Txn()'
into 'Txn.Commit()'.

Signed-off-by: Jussi Maki <jussi@isovalent.com>

Author: joamaki

part/tree.go

@@ -19,7 +19,7 @@ type Tree[T any] struct {
 	size      int // the number of objects in the tree
 	opts      options
 	prevTxn   *atomic.Pointer[Txn[T]] // the previous txn for reusing the allocation
-	prevTxnID uint64                  // the transaction ID that produced this tree
+	nextTxnID uint64                  // the next transaction ID to use
 }
 
 // New constructs a new tree.
@@ -74,7 +74,7 @@ func (t *Tree[T]) Txn() *Txn[T] {
 	txn.rootWatch = t.rootWatch
 	txn.size = t.size
 	txn.prevTxn = t.prevTxn
-	txn.txnID = t.prevTxnID + 1
+	txn.txnID = t.nextTxnID
 	return txn
 }
 

part/txn.go

@@ -65,7 +65,7 @@ func (txn *Txn[T]) Clone() Tree[T] {
 		rootWatch: txn.rootWatch,
 		size:      txn.size,
 		prevTxn:   txn.prevTxn,
-		prevTxnID: txn.txnID,
+		nextTxnID: txn.txnID,
 	}
 }
 
@@ -185,13 +185,14 @@ func (txn *Txn[T]) Commit() Tree[T] {
 		validateTree(txn.oldRoot, nil, nil, txn.txnID)
 		validateTree(txn.root, nil, txn.watches, txn.txnID)
 	}
+	txn.txnID++
 	t := Tree[T]{
 		opts:      txn.opts,
 		root:      txn.root,
 		rootWatch: newRootWatch,
 		size:      txn.size,
 		prevTxn:   txn.prevTxn,
-		prevTxnID: txn.txnID,
+		nextTxnID: txn.txnID,
 	}
 	// Store this txn in the tree to reuse the allocation next time.
 	t.prevTxn.Store(txn)