chore: Support anonymously embedded fields for audit diffs (#5746)

- Anonymously embedded structs are expanded as top level fields.
- Unit tests for anonymously embedded structs

Co-authored-by: Steven Masley <stevenmasley@coder.com>

enterprise/audit/diff.go

@@ -6,6 +6,7 @@ import (
 	"reflect"
 
 	"github.com/google/uuid"
+	"golang.org/x/xerrors"
 
 	"github.com/coder/coder/coderd/audit"
 	"github.com/coder/coder/coderd/database"
@@ -18,11 +19,9 @@ func structName(t reflect.Type) string {
 func diffValues(left, right any, table Table) audit.Map {
 	var (
 		baseDiff = audit.Map{}
-
-		leftV = reflect.ValueOf(left)
-
-		rightV = reflect.ValueOf(right)
-		rightT = reflect.TypeOf(right)
+		rightT   = reflect.TypeOf(right)
+		leftV    = reflect.ValueOf(left)
+		rightV   = reflect.ValueOf(right)
 
 		diffKey = table[structName(rightT)]
 	)
@@ -31,19 +30,25 @@ func diffValues(left, right any, table Table) audit.Map {
 		panic(fmt.Sprintf("dev error: type %q (type %T) attempted audit but not auditable", rightT.Name(), right))
 	}
 
-	for i := 0; i < rightT.NumField(); i++ {
-		if !rightT.Field(i).IsExported() {
-			continue
-		}
+	// allFields contains all top level fields of the struct.
+	allFields, err := flattenStructFields(leftV, rightV)
+	if err != nil {
+		// This should never happen. Only structs should be flattened. If an
+		// error occurs, an unsupported or non-struct type was passed in.
+		panic(fmt.Sprintf("dev error: failed to flatten struct fields: %v", err))
+	}
 
+	for _, field := range allFields {
 		var (
-			leftF  = leftV.Field(i)
-			rightF = rightV.Field(i)
+			leftF  = field.LeftF
+			rightF = field.RightF
 
 			leftI  = leftF.Interface()
 			rightI = rightF.Interface()
+		)
 
-			diffName = rightT.Field(i).Tag.Get("json")
+		var (
+			diffName = field.FieldType.Tag.Get("json")
 		)
 
 		atype, ok := diffKey[diffName]
@@ -145,6 +150,64 @@ func convertDiffType(left, right any) (newLeft, newRight any, changed bool) {
 	}
 }
 
+// fieldDiff has all the required information to return an audit diff for a
+// given field.
+type fieldDiff struct {
+	FieldType reflect.StructField
+	LeftF     reflect.Value
+	RightF    reflect.Value
+}
+
+// flattenStructFields will return all top level fields for a given structure.
+// Only anonymously embedded structs will be recursively flattened such that their
+// fields are returned as top level fields. Named nested structs will be returned
+// as a single field.
+// Conflicting field names need to be handled by the caller.
+func flattenStructFields(leftV, rightV reflect.Value) ([]fieldDiff, error) {
+	// Dereference pointers if the field is a pointer field.
+	if leftV.Kind() == reflect.Ptr {
+		leftV = derefPointer(leftV)
+		rightV = derefPointer(rightV)
+	}
+
+	if leftV.Kind() != reflect.Struct {
+		return nil, xerrors.Errorf("%q is not a struct, kind=%s", leftV.String(), leftV.Kind())
+	}
+
+	var allFields []fieldDiff
+	rightT := rightV.Type()
+
+	// Loop through all top level fields of the struct.
+	for i := 0; i < rightT.NumField(); i++ {
+		if !rightT.Field(i).IsExported() {
+			continue
+		}
+
+		var (
+			leftF  = leftV.Field(i)
+			rightF = rightV.Field(i)
+		)
+
+		if rightT.Field(i).Anonymous {
+			// Anonymous fields are recursively flattened.
+			anonFields, err := flattenStructFields(leftF, rightF)
+			if err != nil {
+				return nil, xerrors.Errorf("flatten anonymous field %q: %w", rightT.Field(i).Name, err)
+			}
+			allFields = append(allFields, anonFields...)
+			continue
+		}
+
+		// Single fields append as is.
+		allFields = append(allFields, fieldDiff{
+			LeftF:     leftF,
+			RightF:    rightF,
+			FieldType: rightT.Field(i),
+		})
+	}
+	return allFields, nil
+}
+
 // derefPointer deferences a reflect.Value that is a pointer to its underlying
 // value. It dereferences recursively until it finds a non-pointer value. If the
 // pointer is nil, it will be coerced to the zero value of the underlying type.