bind: do not generate unused Seq objects

Updates golang/go#12619

Change-Id: Ie851795580c82ade3ee70bdb3945b23ca72f57e0
Reviewed-on: https://go-review.googlesource.com/17866
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

bind/java/seq_android.c

@@ -77,6 +77,9 @@ static mem *mem_ensure(mem *m, uint32_t size) {
 }
 
 static mem *mem_get(JNIEnv *env, jobject obj) {
+	if (obj == NULL) {
+		return NULL;
+	}
 	// Storage space for pointer is always 64-bits, even on 32-bit
 	// machines. Cast to uintptr_t to avoid -Wint-to-pointer-cast.
 	return (mem*)(uintptr_t)(*env)->GetLongField(env, obj, memptr_id);
@@ -400,13 +403,20 @@ Java_go_Seq_destroyRef(JNIEnv *env, jclass clazz, jint refnum) {
 
 JNIEXPORT void JNICALL
 Java_go_Seq_send(JNIEnv *env, jclass clazz, jstring descriptor, jint code, jobject src_obj, jobject dst_obj) {
+	uint8_t* req = NULL;
+	size_t reqlen = 0;
 	mem *src = mem_get(env, src_obj);
-	if (src == NULL) {
-		LOG_FATAL("send src is NULL");
+	if (src != NULL) {
+		req = src->buf;
+		reqlen = src->len;
 	}
+
+	uint8_t** res = NULL;
+	size_t* reslen = NULL;
 	mem *dst = mem_get(env, dst_obj);
-	if (dst == NULL) {
-		LOG_FATAL("send dst is NULL");
+	if (dst != NULL) {
+		res = &dst->buf;
+		reslen = &dst->len;
 	}
 
 	GoString desc;
@@ -415,9 +425,13 @@ Java_go_Seq_send(JNIEnv *env, jclass clazz, jstring descriptor, jint code, jobje
 		LOG_FATAL("send GetStringUTFChars failed");
 	}
 	desc.n = (*env)->GetStringUTFLength(env, descriptor);
-	Send(desc, (GoInt)code, src->buf, src->len, &dst->buf, &dst->len);
+
+	Send(desc, (GoInt)code, req, reqlen, res, reslen);
 	(*env)->ReleaseStringUTFChars(env, descriptor, desc.p);
-	unpin_arrays(env, src);  // assume 'src' is no longer needed.
+
+	if (src != NULL) {
+		unpin_arrays(env, src);  // assume 'src' is no longer needed.
+	}
 }
 
 JNIEXPORT void JNICALL

bind/java/seq_android.go

@@ -31,18 +31,26 @@ func Send(descriptor string, code int, req *C.uint8_t, reqlen C.size_t, res **C.
 	if fn == nil {
 		panic(fmt.Sprintf("invalid descriptor(%s) and code(0x%x)", descriptor, code))
 	}
-	in := new(seq.Buffer)
-	if reqlen > 0 {
-		in.Data = (*[maxSliceLen]byte)(unsafe.Pointer(req))[:reqlen]
-	}
-	out := new(seq.Buffer)
-	fn(out, in)
-	// BUG(hyangah): the function returning a go byte slice (so fn writes a pointer into 'out') is unsafe.
-	// After fn is complete here, Go runtime is free to collect or move the pointed byte slice
-	// contents. (Explicitly calling runtime.GC here will surface the problem?)
-	// Without pinning support from Go side, it will be hard to fix it without extra copying.
 
-	seqToBuf(res, reslen, out)
+	var in, out *seq.Buffer
+	if req != nil && reqlen > 0 {
+		in = &seq.Buffer{
+			Data: (*[maxSliceLen]byte)(unsafe.Pointer(req))[:reqlen],
+		}
+	}
+	if res != nil {
+		out = new(seq.Buffer)
+	}
+
+	fn(out, in)
+
+	if res != nil {
+		// BUG(hyangah): the function returning a go byte slice (so fn writes a pointer into 'out') is unsafe.
+		// After fn is complete here, Go runtime is free to collect or move the pointed byte slice
+		// contents. (Explicitly calling runtime.GC here will surface the problem?)
+		// Without pinning support from Go side, it will be hard to fix it without extra copying.
+		seqToBuf(res, reslen, out)
+	}
 }
 
 // DestroyRef is called by Java to inform Go it is done with a reference.