mobile/bind: handle null references from Java in Go

The Seq Java class has a special case for null references. Expand
the special case to Go so that null references from Java are properly
translated to nil.

Fixes golang/go#14228

Change-Id: I915d1f843c9db299d6910480f6d10dae0121a3b4
Reviewed-on: https://go-review.googlesource.com/19460
Reviewed-by: David Crawshaw <crawshaw@golang.org>

bind/gengo.go

@@ -352,7 +352,7 @@ func (g *goGen) genRead(valName, seqName string, typ types.Type) {
 			g.Printf("if %s_ref.Num < 0 { // go object \n", valName)
 			g.Printf("   %s = %s_ref.Get().(%s.%s)\n", valName, valName, g.pkg.Name(), o.Name())
 			if hasProxy {
-				g.Printf("} else {  // foreign object \n")
+				g.Printf("} else if %s_ref.Num != seq.NullRefNum {  // foreign object \n", valName)
 				g.Printf("   %s = (*proxy%s)(%s_ref)\n", valName, o.Name(), valName)
 			}
 			g.Printf("}\n")

bind/java/Seq.java

@@ -77,6 +77,8 @@ public class Seq {
 	public native void writeByteArray(byte[] v);
 
 	public void writeRef(Ref ref) {
+		if (ref == null)
+			ref = RefTracker.nullRef;
 		tracker.inc(ref);
 		writeInt32(ref.refnum);
 	}
@@ -172,9 +174,10 @@ public class Seq {
 
 	static final class RefTracker {
 		private static final int REF_OFFSET = 42;
+		private static final int NULL_REFNUM = 41; // also known to bind/seq/ref.go
 
 		// use single Ref for null Seq.Object
-		private static final Ref nullRef = new Ref(REF_OFFSET - 1, null);
+		private static final Ref nullRef = new Ref(NULL_REFNUM, null);
 
 		// Next Java object reference number.
 		//

bind/java/SeqTest.java

@@ -381,4 +381,12 @@ public class SeqTest extends AndroidTestCase {
     String got = n.getErr();
     assertEquals("want back the error message we set", want, got);
   }
+
+  public void testNullReferences() {
+    assertTrue(Testpkg.CallWithNull(null, new Testpkg.NullTest.Stub() {
+      public Testpkg.NullTest Null() {
+        return null;
+      }
+    }));
+  }
 }

bind/java/testpkg/testpkg.go

@@ -204,3 +204,11 @@ func ReadAsset() string {
 	}
 	return string(b)
 }
+
+type NullTest interface {
+	Null() NullTest
+}
+
+func CallWithNull(_null NullTest, nuller NullTest) bool {
+	return _null == nil && nuller.Null() == nil
+}

bind/seq.go

@@ -81,7 +81,7 @@ func seqWrite(o types.Type, name string) string {
 	t := seqType(o)
 	if t == "Ref" {
 		// TODO(crawshaw): do something cleaner, i.e. genWrite.
-		return t + "(" + name + ".ref())"
+		return t + "(" + name + " != null ? " + name + ".ref() : null)"
 	}
 	return t + "(" + name + ")"
 }

bind/seq/ref.go

@@ -19,6 +19,8 @@ type countedObj struct {
 	cnt int32
 }
 
+const NullRefNum = 41 // also known to bind/java/Seq.java
+
 // refs stores Go objects that have been passed to another language.
 var refs struct {
 	sync.Mutex

bind/testdata/interfaces.go.golden

@@ -14,7 +14,7 @@ func proxy_Add3(out, in *seq.Buffer) {
 	param_r_ref := in.ReadRef()
 	if param_r_ref.Num < 0 { // go object
 		param_r = param_r_ref.Get().(interfaces.I)
-	} else { // foreign object
+	} else if param_r_ref.Num != seq.NullRefNum { // foreign object
 		param_r = (*proxyI)(param_r_ref)
 	}
 	res := interfaces.Add3(param_r)
@@ -26,7 +26,7 @@ func proxy_CallErr(out, in *seq.Buffer) {
 	param_e_ref := in.ReadRef()
 	if param_e_ref.Num < 0 { // go object
 		param_e = param_e_ref.Get().(interfaces.Error)
-	} else { // foreign object
+	} else if param_e_ref.Num != seq.NullRefNum { // foreign object
 		param_e = (*proxyError)(param_e_ref)
 	}
 	err := interfaces.CallErr(param_e)

bind/testdata/interfaces.java.golden

@@ -15,7 +15,7 @@ public abstract class Interfaces {
         _out = new go.Seq();
         int _result;
         _in = new go.Seq();
-        _in.writeRef(r.ref());
+        _in.writeRef(r != null ? r.ref() : null);
         Seq.send(DESCRIPTOR, CALL_Add3, _in, _out);
         _result = _out.readInt32();
         return _result;
@@ -26,7 +26,7 @@ public abstract class Interfaces {
         go.Seq _out = null;
         _out = new go.Seq();
         _in = new go.Seq();
-        _in.writeRef(e.ref());
+        _in.writeRef(e != null ? e.ref() : null);
         Seq.send(DESCRIPTOR, CALL_CallErr, _in, _out);
         String _err = _out.readString();
         if (_err != null && !_err.isEmpty()) {
@@ -222,7 +222,7 @@ public abstract class Interfaces {
                 switch (code) {
                 case Proxy.CALL_F: {
                     I1 result = this.F();
-                    out.writeRef(result.ref());
+                    out.writeRef(result != null ? result.ref() : null);
                     return;
                 }
                 default:

bind/testdata/issue10788.java.golden

@@ -66,7 +66,7 @@ public abstract class Issue10788 {
                 go.Seq _out = null;
                 _in = new go.Seq();
                 _in.writeRef(ref);
-                _in.writeRef(s.ref());
+                _in.writeRef(s != null ? s.ref() : null);
                 Seq.send(DESCRIPTOR, CALL_DoSomeWork, _in, _out);
             }
             

bind/testdata/structs.java.golden

@@ -65,7 +65,7 @@ public abstract class Structs {
         _out = new go.Seq();
         S _result;
         _in = new go.Seq();
-        _in.writeRef(s.ref());
+        _in.writeRef(s != null ? s.ref() : null);
         Seq.send(DESCRIPTOR, CALL_Identity, _in, _out);
         _result = new S(_out.readRef());
         return _result;
@@ -77,7 +77,7 @@ public abstract class Structs {
         _out = new go.Seq();
         S _result;
         _in = new go.Seq();
-        _in.writeRef(s.ref());
+        _in.writeRef(s != null ? s.ref() : null);
         Seq.send(DESCRIPTOR, CALL_IdentityWithError, _in, _out);
         _result = new S(_out.readRef());
         String _err = _out.readString();

bind/testdata/vars.go.golden

@@ -93,7 +93,7 @@ func var_setAnInterface(out, in *seq.Buffer) {
 	v_ref := in.ReadRef()
 	if v_ref.Num < 0 { // go object
 		v = v_ref.Get().(vars.I)
-	} else { // foreign object
+	} else if v_ref.Num != seq.NullRefNum { // foreign object
 		v = (*proxyI)(v_ref)
 	}
 	vars.AnInterface = v

bind/testdata/vars.java.golden

@@ -76,7 +76,7 @@ public abstract class Vars {
     
     public static void setAStructPtr(S v) {
         Seq in = new Seq();
-        in.writeRef(v.ref());
+        in.writeRef(v != null ? v.ref() : null);
         Seq.send("vars.AStructPtr", 1, in, null);
     }
     
@@ -154,7 +154,7 @@ public abstract class Vars {
     
     public static void setAnInterface(I v) {
         Seq in = new Seq();
-        in.writeRef(v.ref());
+        in.writeRef(v != null ? v.ref() : null);
         Seq.send("vars.AnInterface", 1, in, null);
     }