libutils: wp::fromExisting bugfix

This API was tested before, but it wasn't used until it is needed in
libbinder. Previously it passed the tests because wp::operator==
compares weak_ptrs which are never deleted, but it doesn't check the
value of m_ptr as well. This assumes that the RefBase implementation is
self-consistent.

Future considerations:
- add additional CHECK (perf?)
- add an additional optional CHECK?
- update all refbase tests to use an embellished form of this operator

Bug: 184190315
Test: libutils_test, boot and kill process when libbinder is using this
API

Change-Id: I66c97386d769529d5efae48e06775d4b4a344025

libutils/RefBase_test.cpp

@@ -242,12 +242,12 @@ TEST(RefBase, ReplacedComparison) {
 }
 
 TEST(RefBase, AssertWeakRefExistsSuccess) {
-    // uses some other refcounting method, or non at all
     bool isDeleted;
     sp<Foo> foo = sp<Foo>::make(&isDeleted);
     wp<Foo> weakFoo = foo;
 
     EXPECT_EQ(weakFoo, wp<Foo>::fromExisting(foo.get()));
+    EXPECT_EQ(weakFoo.unsafe_get(), wp<Foo>::fromExisting(foo.get()).unsafe_get());
 
     EXPECT_FALSE(isDeleted);
     foo = nullptr;
@@ -255,7 +255,7 @@ TEST(RefBase, AssertWeakRefExistsSuccess) {
 }
 
 TEST(RefBase, AssertWeakRefExistsDeath) {
-    // uses some other refcounting method, or non at all
+    // uses some other refcounting method, or none at all
     bool isDeleted;
     Foo* foo = new Foo(&isDeleted);
 

libutils/include/utils/RefBase.h

@@ -547,6 +547,7 @@ wp<T> wp<T>::fromExisting(T* other) {
     refs->incWeakRequireWeak(other);
 
     wp<T> ret;
+    ret.m_ptr = other;
     ret.m_refs = refs;
     return ret;
 }