go/zodb: Expose retrieving ZODB class name of a Go object as public API

It is useful in situations where one need to print potentially Broken
objects properly, because %T does not print that detail and using %#v
creates too much noise.

go/zodb/connection.go

@@ -206,7 +206,7 @@ func (conn *Connection) get(class string, oid Oid) (IPersistent, error) {
 	conn.cache.Unlock()
 
 	if checkClass {
-		if cls := zclassOf(obj); class != cls {
+		if cls := ClassOf(obj); class != cls {
 			var err error = &wrongClassError{class, cls}
 			xerr.Contextf(&err, "get %s", Xid{conn.at, oid})
 			return nil, err

go/zodb/persistent.go

@@ -278,15 +278,38 @@ type zclass struct {
 var classTab = make(map[string]*zclass)       // {} class -> zclass
 var typeTab  = make(map[reflect.Type]*zclass) // {} type  -> zclass
 
-// zclassOf returns ZODB class of a Go object.
+// ClassOf returns ZODB class of a Go object.
 //
-// If ZODB class was not registered for obj's type, "" is returned.
-func zclassOf(obj IPersistent) string {
-	zc, ok := typeTab[reflect.TypeOf(obj).Elem()]
-	if !ok {
-		return ""
+// The following is returned:
+//
+//	- if obj's type was registered (RegisterClass) -- corresponding class.
+//	- for Broken objects -- ZODB.Broken("<broken-class>").
+//	- else -- ZODB.Go("<fully-qualified-type(obj)>")
+func ClassOf(obj IPersistent) string {
+	zb, broken := obj.(*Broken)
+	if broken {
+		return fmt.Sprintf("ZODB.Broken(%q)", zb.class)
 	}
+
+	typ := reflect.TypeOf(obj)
+	typ = typ.Elem() // *MyPersistent -> MyPersistent
+	zc, ok := typeTab[typ]
+	if ok {
 		return zc.class
+	}
+
+	// the type was not registered to ZODB
+	fullType := typ.PkgPath()
+	if typ.PkgPath() != "" {
+		fullType += "."
+	}
+	fullType += typ.Name()
+	if fullType == "" {
+		// fallback, since it is possible if the type is anonymous
+		// XXX not fully qualified
+		fullType = fmt.Sprintf("*%T", typ)
+	}
+	return fmt.Sprintf("ZODB.Go(%q)", fullType)
 }
 
 var rIPersistent = reflect.TypeOf((*IPersistent)(nil)).Elem() // typeof(IPersistent)

go/zodb/persistent_test.go

-// Copyright (C) 2018  Nexedi SA and Contributors.
+// Copyright (C) 2018-2019  Nexedi SA and Contributors.
 //                          Kirill Smelkov <kirr@nexedi.com>
 //
 // This program is free software: you can Use, Study, Modify and Redistribute
@@ -51,6 +51,11 @@ func (o *myObjectState) PySetState(pystate interface{}) error {
 	return nil
 }
 
+// Peristent that is not registered to ZODB.
+type Unregistered struct {
+	Persistent
+}
+
 func init() {
 	t := reflect.TypeOf
 	RegisterClass("t.zodb.MyObject", t(MyObject{}), t(myObjectState{}))
@@ -120,7 +125,7 @@ func TestPersistent(t *testing.T) {
 	}
 
 	checkObj(obj, nil, 11, InvalidTid, GHOST, 0, nil)
-	assert.Equal(zclassOf(obj), "t.zodb.MyObject")
+	assert.Equal(ClassOf(obj), "t.zodb.MyObject")
 
 	// t.zodb.MyOldObject -> *MyObject
 	xobj = newGhost("t.zodb.MyOldObject", 12, nil)
@@ -130,7 +135,11 @@ func TestPersistent(t *testing.T) {
 	}
 
 	checkObj(obj, nil, 12, InvalidTid, GHOST, 0, nil)
-	assert.Equal(zclassOf(obj), "t.zodb.MyObject")
+	assert.Equal(ClassOf(obj), "t.zodb.MyObject")
+
+	// ClassOf(unregistered-obj)
+	obj2 := &Unregistered{}
+	assert.Equal(ClassOf(obj2), `ZODB.Go("lab.nexedi.com/kirr/neo/go/zodb.Unregistered")`)
 
 
 	// TODO activate	- jar has to load, state changes