Make ProjectElementContainers.DeepCopyFrom copy all descendants (dotnet#7454)

When "DeepCopy"ing ProjectItemGroupElements, we copied attributes but not children, which meant we would XML embedded in metadata if present.

Fixes dotnet#7435

Co-authored-by: Rainer Sigwald <[email protected]>

Author: Forgind

src/Build.OM.UnitTests/Construction/ProjectItemGroupElement_tests.cs

@@ -5,6 +5,7 @@
 using System.Xml;
 
 using Microsoft.Build.Construction;
+using Shouldly;
 using Xunit;
 
 #nullable disable
@@ -70,6 +71,49 @@ public void ReadItemGroupTwoItems()
             Assert.Equal("i2", items[1].Include);
         }
 
+        [Fact]
+        public void DeepCopyFromItemGroupWithMetadata()
+        {
+            string content = @"
+                    <Project>
+                        <ItemGroup>
+                            <i Include='i1'>
+                              <M>metadataValue</M>
+                            </i>
+                            <i Include='i2'>
+                              <M>
+                                <Some>
+                                    <Xml With='Nesting' />
+                                </Some>
+                              </M>
+                            </i>
+                        </ItemGroup>
+                    </Project>
+                ";
+
+            ProjectRootElement project = ProjectRootElement.Create(XmlReader.Create(new StringReader(content)));
+            ProjectItemGroupElement group = (ProjectItemGroupElement)Helpers.GetFirst(project.Children);
+
+            ProjectRootElement newProject = ProjectRootElement.Create();
+            ProjectItemGroupElement newItemGroup = project.AddItemGroup();
+
+            newItemGroup.DeepCopyFrom(group);
+
+            var items = Helpers.MakeList(newItemGroup.Items);
+
+            items.Count.ShouldBe(2);
+
+            items[0].Include.ShouldBe("i1");
+            ProjectMetadataElement metadataElement = items[0].Metadata.ShouldHaveSingleItem();
+            metadataElement.Name.ShouldBe("M");
+            metadataElement.Value.ShouldBe("metadataValue");
+
+            items[1].Include.ShouldBe("i2");
+            metadataElement = items[1].Metadata.ShouldHaveSingleItem();
+            metadataElement.Name.ShouldBe("M");
+            metadataElement.Value.ShouldBe("<Some><Xml With=\"Nesting\" /></Some>");
+        }
+
         /// <summary>
         /// Set the condition value
         /// </summary>

src/Build/Construction/ProjectElementContainer.cs

@@ -407,13 +407,46 @@ protected internal virtual ProjectElementContainer DeepClone(ProjectRootElement
                 }
                 else
                 {
-                    clone.AppendChild(child.Clone(clone.ContainingProject));
+                    ProjectElement childClone = child.Clone(clone.ContainingProject);
+                    clone.AppendChild(childClone);
+                    if (childClone.XmlElement is not null)
+                    {
+                        AppendAttributesAndChildren(childClone.XmlElement, child.XmlElement);
+                    }
                 }
             }
 
             return clone;
         }
 
+        private void AppendAttributesAndChildren(XmlNode appendTo, XmlNode appendFrom)
+        {
+            appendTo.RemoveAll();
+            // Copy over the attributes from the template element.
+            if (appendFrom.Attributes is not null)
+            {
+                foreach (XmlAttribute attribute in appendFrom.Attributes)
+                {
+                    XmlAttribute attr = appendTo.OwnerDocument.CreateAttribute(attribute.LocalName, attribute.NamespaceURI);
+                    attr.Value = attribute.Value;
+                    appendTo.Attributes.Append(attr);
+                }
+            }
+
+            // If this element has pure text content, copy that over.
+            if (appendFrom.ChildNodes.Count == 1 && appendFrom.FirstChild.NodeType == XmlNodeType.Text)
+            {
+                appendTo.AppendChild(appendTo.OwnerDocument.CreateTextNode(appendFrom.FirstChild.Value));
+            }
+
+            foreach (XmlNode child in appendFrom.ChildNodes)
+            {
+                XmlNode childClone = appendTo.OwnerDocument.CreateNode(child.NodeType, child.Prefix, child.Name, child.NamespaceURI);
+                appendTo.AppendChild(childClone);
+                AppendAttributesAndChildren(childClone, child);
+            }
+        }
+
         internal static ProjectElementContainer DeepClone(ProjectElementContainer xml, ProjectRootElement factory, ProjectElementContainer parent)
         {
             return xml.DeepClone(factory, parent);