Shallow clone trees to save on memory usage

pkg/filetree/filetree.go

@@ -45,7 +45,14 @@ func New() *FileTree {
 	}
 }
 
-// Copy returns a Copy of the current FileTree.
+// Clone returns a shallow Copy of the current FileTree.
+func (t *FileTree) Clone() (ReadWriter, error) {
+	ct := New()
+	ct.tree = t.tree.Clone()
+	return ct, nil
+}
+
+// Copy returns a deep Copy of the current FileTree.
 func (t *FileTree) Copy() (ReadWriter, error) {
 	ct := New()
 	ct.tree = t.tree.Copy()
@@ -781,7 +788,8 @@ func (t *FileTree) Walk(fn func(path file.Path, f filenode.FileNode) error, cond
 
 // Merge takes the given Tree and combines it with the current Tree, preferring files in the other Tree if there
 // are path conflicts. This is the basis function for squashing (where the current Tree is the bottom Tree and the
-// given Tree is the top Tree).
+// given Tree is the top Tree). Note: existing nodes are not mutated during this operation, they are replaced with
+// a node from the other tree.
 //
 //nolint:gocognit,funlen
 func (t *FileTree) Merge(upper Reader) error {

pkg/filetree/interfaces.go

@@ -29,6 +29,7 @@ type PathReader interface {
 
 type Copier interface {
 	Copy() (ReadWriter, error)
+	Clone() (ReadWriter, error)
 }
 
 type Walker interface {

pkg/filetree/union_filetree.go

@@ -21,14 +21,18 @@ func (u *UnionFileTree) Squash() (ReadWriter, error) {
 	case 0:
 		return New(), nil
 	case 1:
-		return u.trees[0].Copy()
+		// important: use clone over copy to reduce memory footprint. If callers need a distinct tree they can call
+		// copy after the fact.
+		return u.trees[0].Clone()
 	}
 
 	var squashedTree ReadWriter
 	var err error
 	for layerIdx, refTree := range u.trees {
 		if layerIdx == 0 {
-			squashedTree, err = refTree.Copy()
+			// important: use clone over copy to reduce memory footprint. If callers need a distinct tree they can call
+			// copy after the fact.
+			squashedTree, err = refTree.Clone()
 			if err != nil {
 				return nil, err
 			}

pkg/tree/tree.go

@@ -2,6 +2,7 @@ package tree
 
 import (
 	"fmt"
+	"maps"
 
 	"github.com/anchore/stereoscope/pkg/tree/node"
 )
@@ -22,6 +23,16 @@ func NewTree() *Tree {
 	}
 }
 
+// Clone returns a shallow copy of the Tree.
+func (t *Tree) Clone() *Tree {
+	ct := NewTree()
+	ct.nodes = maps.Clone(t.nodes)
+	ct.parent = maps.Clone(t.parent)
+	ct.children = maps.Clone(t.children)
+	return ct
+}
+
+// Copy returns a deep copy of the Tree.
 func (t *Tree) Copy() *Tree {
 	ct := NewTree()
 	for k, v := range t.nodes {