part: Only reuse watches hash map when it is small

[ upstream commit d02e729 ]

This avoids carrying over a large allocation [txn.watches] from a
massive one-off transaction into future transactions.

This should mitigate cilium/cilium#44310.

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

Author: joamaki

part/txn.go

@@ -182,17 +182,26 @@ func (txn *Txn[T]) Commit() *Tree[T] {
 	return t
 }
 
+// watchesReuseThreshold is the threshold at which the [txn.watches] hash
+// map is reused for next transaction.
+const watchesReuseThreshold = 64
+
 // Notify closes the watch channels of nodes that were
 // mutated as part of this transaction. Must be called before
 // Tree.Txn() is used again.
 func (txn *Txn[T]) Notify() {
 	for ch := range txn.watches {
 		close(ch)
 	}
-	clear(txn.watches)
 	if !txn.dirty && len(txn.watches) > 0 {
 		panic("BUG: watch channels marked but txn not dirty")
 	}
+	// Clear or reallocate the watches hash map for the next transaction.
+	if len(txn.watches) <= watchesReuseThreshold {
+		clear(txn.watches)
+	} else {
+		txn.watches = make(map[chan struct{}]struct{})
+	}
 	if txn.dirty && txn.rootWatch != nil {
 		close(txn.rootWatch)
 		txn.rootWatch = nil