ignore pad bits when checking for bit list equality

c++/src/capnp/any-test.c++

@@ -354,6 +354,29 @@ TEST(Any, Equals) {
   EXPECT_EQ(Equality::EQUAL, anyA.equals(anyB));
 }
 
+KJ_TEST("Bit list with nonzero pad bits") {
+  AlignedData<2> segment1 = {{
+      0x01, 0x00, 0x00, 0x00, 0x59, 0x00, 0x00, 0x00, // eleven bit-sized elements
+      0xee, 0x0f, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, // twelfth bit is set!
+  }};
+  kj::ArrayPtr<const word> segments1[1] = {
+    kj::arrayPtr(segment1.words, 2)
+  };
+  SegmentArrayMessageReader message1(kj::arrayPtr(segments1, 1));
+
+  AlignedData<2> segment2 = {{
+      0x01, 0x00, 0x00, 0x00, 0x59, 0x00, 0x00, 0x00, // eleven bit-sized elements
+      0xee, 0x07, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, // twelfth bit is not set
+  }};
+  kj::ArrayPtr<const word> segments2[1] = {
+    kj::arrayPtr(segment2.words, 2)
+  };
+  SegmentArrayMessageReader message2(kj::arrayPtr(segments2, 1));
+
+  // Should be equal, despite nonzero padding.
+  KJ_ASSERT(message1.getRoot<AnyList>() == message2.getRoot<AnyList>());
+}
+
 }  // namespace
 }  // namespace _ (private)
 }  // namespace capnp

c++/src/capnp/any.c++

@@ -148,16 +148,30 @@ Equality AnyList::Reader::equals(AnyList::Reader right) {
     case ElementSize::BYTE:
     case ElementSize::TWO_BYTES:
     case ElementSize::FOUR_BYTES:
-    case ElementSize::EIGHT_BYTES:
-      if(getElementSize() == right.getElementSize()) {
-        if(memcmp(getRawBytes().begin(), right.getRawBytes().begin(), getRawBytes().size()) == 0) {
-          return Equality::EQUAL;
-        } else {
+    case ElementSize::EIGHT_BYTES: {
+      if (getElementSize() != right.getElementSize()) {
+        return Equality::NOT_EQUAL;
+      }
+
+      size_t cmpSize = getRawBytes().size();
+
+      if (getElementSize() == ElementSize::BIT && size() % 8 != 0) {
+        // The list does not end on a byte boundary. We need special handling for the final
+        // byte because we only care about the bits that are actually elements of the list.
+
+        uint8_t mask = (1 << (size() % 8)) - 1; // lowest size() bits set
+        if ((getRawBytes()[cmpSize - 1] & mask) != (right.getRawBytes()[cmpSize - 1] & mask)) {
           return Equality::NOT_EQUAL;
         }
+        cmpSize -= 1;
+      }
+
+      if (memcmp(getRawBytes().begin(), right.getRawBytes().begin(), cmpSize) == 0) {
+        return Equality::EQUAL;
       } else {
         return Equality::NOT_EQUAL;
       }
+    }
     case ElementSize::POINTER:
     case ElementSize::INLINE_COMPOSITE: {
       auto llist = as<List<AnyStruct>>();