Do not reuse JS-owned wrapper values.

JavaScript-owned wrapper values cannot be reused because the
QML runtime may choose to destroy the value right after it
is handed a new reference to the same value.

Fixes #68.

Author: niemeyer

bridge.go

@@ -230,8 +230,11 @@ func wrapGoValue(engine *Engine, gvalue interface{}, owner valueOwner) (cvalue u
 
 	painting := cdata.Ref() == atomic.LoadUintptr(&guiPaintRef)
 
+	// Cannot reuse a jsOwner because the QML runtime may choose to destroy
+	// the value _after_ we hand it a new reference to the same value.
+	// See issue #68 for details.
 	prev, ok := engine.values[gvalue]
-	if ok && (prev.owner == owner || owner != cppOwner || painting) {
+	if ok && (prev.owner == cppOwner || painting) {
 		return prev.cvalue
 	}
 
@@ -250,11 +253,12 @@ func wrapGoValue(engine *Engine, gvalue interface{}, owner valueOwner) (cvalue u
 	}
 	fold.cvalue = C.newGoValue(unsafe.Pointer(fold), typeInfo(gvalue), parent)
 	if prev != nil {
-		prev.next = fold
-		fold.prev = prev
-	} else {
-		engine.values[gvalue] = fold
+		// Put new fold first so the single cppOwner, if any, is always the first entry.
+		fold.next = prev
+		prev.prev = fold
 	}
+	engine.values[gvalue] = fold
+
 	//fmt.Printf("[DEBUG] value alive (wrapped): cvalue=%x gvalue=%x/%#v\n", fold.cvalue, addrOf(fold.gvalue), fold.gvalue)
 	stats.valuesAlive(+1)
 	C.engineSetContextForObject(engine.addr, fold.cvalue)

qml_test.go

@@ -1213,6 +1213,21 @@ var tests = []struct {
 		},
 		DoneLog: "Signal has run.",
 	},
+	{
+		Summary: "References handed out must not be GCd (issue #68)",
+		Init: func(c *TestData) {
+			type B struct{ S string }
+			type A struct{ B *B }
+			c.context.SetVar("a", &A{&B{}})
+		},
+		QML: `Item { function f() { var x = [[],[],[]]; gc(); if (!a.b) console.log("BUG"); } }`,
+		Done: func(c *TestData) {
+			for i := 0; i < 100; i++ {
+				c.root.Call("f")
+			}
+		},
+		DoneLog: "!BUG",
+	},
 }
 
 var tablef = flag.String("tablef", "", "if provided, TestTable only runs tests with a summary matching the regexp")