proc: fix embedded field search (#2320)

Both structMember and findMethod implemented a depth-first search in
embedded fields but the Go specification requires a breadth-first
search. They also allowed promotion of fields in the concrete type of
embedded interfaces even though this is not allowed by Go.
Furthermore they both lacked protection from infinite recursion
when a type embeds itself and the user requests a non-existent field.

Fixes #2316

pkg/proc/variables.go

@@ -1060,59 +1060,61 @@ func (v *Variable) structMember(memberName string) (*Variable, error) {
 			v = &v.Children[0]
 		}
 	}
-	structVar := v.maybeDereference()
-	structVar.Name = v.Name
-	if structVar.Unreadable != nil {
-		return structVar, nil
-	}
 
-	switch t := structVar.RealType.(type) {
-	case *godwarf.StructType:
-		for _, field := range t.Field {
-			if field.Name != memberName {
-				continue
-			}
-			return structVar.toField(field)
+	queue := []*Variable{v}
+	seen := map[string]struct{}{} // prevent infinite loops
+	first := true
+
+	for len(queue) > 0 {
+		v := queue[0]
+		queue = append(queue[:0], queue[1:]...)
+		if _, isseen := seen[v.RealType.String()]; isseen {
+			continue
 		}
-		// Check for embedded field only if field was
-		// not a regular struct member
-		for _, field := range t.Field {
-			isEmbeddedStructMember :=
-				field.Embedded ||
-					(field.Type.Common().Name == field.Name) ||
-					(len(field.Name) > 1 &&
-						field.Name[0] == '*' &&
-						field.Type.Common().Name[1:] == field.Name[1:])
-			if !isEmbeddedStructMember {
-				continue
-			}
-			// Check for embedded field referenced by type name
-			parts := strings.Split(field.Name, ".")
-			if len(parts) > 1 && parts[1] == memberName {
+		seen[v.RealType.String()] = struct{}{}
+
+		structVar := v.maybeDereference()
+		structVar.Name = v.Name
+		if structVar.Unreadable != nil {
+			return structVar, nil
+		}
+
+		switch t := structVar.RealType.(type) {
+		case *godwarf.StructType:
+			for _, field := range t.Field {
+				if field.Name == memberName {
+					return structVar.toField(field)
+				}
+				isEmbeddedStructMember :=
+					field.Embedded ||
+						(field.Type.Common().Name == field.Name) ||
+						(len(field.Name) > 1 &&
+							field.Name[0] == '*' &&
+							field.Type.Common().Name[1:] == field.Name[1:])
+				if !isEmbeddedStructMember {
+					continue
+				}
 				embeddedVar, err := structVar.toField(field)
 				if err != nil {
 					return nil, err
 				}
-				return embeddedVar, nil
+				// Check for embedded field referenced by type name
+				parts := strings.Split(field.Name, ".")
+				if len(parts) > 1 && parts[1] == memberName {
+					return embeddedVar, nil
+				}
+				embeddedVar.Name = structVar.Name
+				queue = append(queue, embeddedVar)
 			}
-			// Recursively check for promoted fields on the embedded field
-			embeddedVar, err := structVar.toField(field)
-			if err != nil {
-				return nil, err
-			}
-			embeddedVar.Name = structVar.Name
-			embeddedField, _ := embeddedVar.structMember(memberName)
-			if embeddedField != nil {
-				return embeddedField, nil
+		default:
+			if first {
+				return nil, fmt.Errorf("%s (type %s) is not a struct", vname, structVar.TypeString())
 			}
 		}
-		return nil, fmt.Errorf("%s has no member %s", vname, memberName)
-	default:
-		if v.Name == "" {
-			return nil, fmt.Errorf("type %s is not a struct", structVar.TypeString())
-		}
-		return nil, fmt.Errorf("%s (type %s) is not a struct", vname, structVar.TypeString())
+		first = false
 	}
+
+	return nil, fmt.Errorf("%s has no member %s", vname, memberName)
 }
 
 func readVarEntry(entry *godwarf.Tree, image *Image) (name string, typ godwarf.Type, err error) {