Add wire format parsing unit tests for implicit presence fields.

This gist of the new test case coverage can be summarized as:
- The wire does not distinguish between explicit and implicit fields.
- For implicit presence fields, zeros on the wire can overwrite nonzero values
  (i.e. they are treated as a 'clear' operation).

It's TBD whether we want to accept this behaviour going forward.
Right now we are leaning towards keeping this behaviour, because:
- If we receive zeros on the wire for implicit-presence fields, the protobuf
  wire format is "wrong" anyway. Well-behaved code should never generate zeros
  on the wire for implicit presence fields or serialize the same field multiple
  times.
- There might be some value to enforce that "anything on the wire is accepted".
  This can make handling of wire format simpler.

There are some drawbacks with this approach:
- It might be somewhat surprising for users that zeros on the wire are always
  read, even for implicit-presence fields.
- It might make the transition from implicit-presence to explicit-presence
  harder (or more unsafe) if user wants to migrate.
- It leads to an inconsistency between what it means to "Merge".
  - Merging from a constructed object, with implicit presence and with field
    set to zero, will not overwrite.
  - Merging from the wire, with implicit presence and with zero explicitly
    present on the wire, WILL overwrite.

I still need to add conformance tests to ensure that this is a consistent
behavior across all languages, but for now let's at least add some coverage in
C++ to ensure that this is a tested behaviour.

PiperOrigin-RevId: 657724599

src/google/protobuf/BUILD.bazel

@@ -1657,6 +1657,8 @@ cc_test(
     deps = [
         ":cc_test_protos",
         ":protobuf",
+        "@com_google_absl//absl/log:absl_check",
+        "@com_google_absl//absl/strings:string_view",
         "@com_google_googletest//:gtest",
         "@com_google_googletest//:gtest_main",
     ],

src/google/protobuf/no_field_presence_test.cc

@@ -8,7 +8,10 @@
 #include <string>
 
 #include "google/protobuf/descriptor.pb.h"
+#include <gmock/gmock.h>
 #include <gtest/gtest.h>
+#include "absl/log/absl_check.h"
+#include "absl/strings/string_view.h"
 #include "google/protobuf/descriptor.h"
 #include "google/protobuf/unittest.pb.h"
 #include "google/protobuf/unittest_no_field_presence.pb.h"
@@ -17,6 +20,8 @@ namespace google {
 namespace protobuf {
 namespace {
 
+using ::testing::StrEq;
+
 // Helper: checks that all fields have default (zero/empty) values.
 void CheckDefaultValues(
     const proto2_nofieldpresence_unittest::TestAllTypes& m) {
@@ -457,6 +462,74 @@ TEST(NoFieldPresenceTest, MergeFromIfNonzeroTest) {
   EXPECT_EQ("test2", dest.optional_string());
 }
 
+TEST(NoFieldPresenceTest, ExtraZeroesInWireParseTest) {
+  // check extra serialized zeroes on the wire are parsed into the object.
+  proto2_nofieldpresence_unittest::ForeignMessage dest;
+  dest.set_c(42);
+  ASSERT_EQ(42, dest.c());
+
+  // ExplicitForeignMessage has the same fields as ForeignMessage, but with
+  // explicit presence instead of implicit presence.
+  proto2_nofieldpresence_unittest::ExplicitForeignMessage source;
+  source.set_c(0);
+  std::string wire = source.SerializeAsString();
+  ASSERT_THAT(wire, StrEq(absl::string_view{"\x08\x00", 2}));
+
+  // The "parse" operation clears all fields before merging from wire.
+  ASSERT_TRUE(dest.ParseFromString(wire));
+  EXPECT_EQ(0, dest.c());
+  std::string dest_data;
+  EXPECT_TRUE(dest.SerializeToString(&dest_data));
+  EXPECT_TRUE(dest_data.empty());
+}
+
+TEST(NoFieldPresenceTest, ExtraZeroesInWireMergeTest) {
+  // check explicit zeros on the wire are merged into an implicit one.
+  proto2_nofieldpresence_unittest::ForeignMessage dest;
+  dest.set_c(42);
+  ASSERT_EQ(42, dest.c());
+
+  // ExplicitForeignMessage has the same fields as ForeignMessage, but with
+  // explicit presence instead of implicit presence.
+  proto2_nofieldpresence_unittest::ExplicitForeignMessage source;
+  source.set_c(0);
+  std::string wire = source.SerializeAsString();
+  ASSERT_THAT(wire, StrEq(absl::string_view{"\x08\x00", 2}));
+
+  // TODO: b/356132170 -- Add conformance tests to ensure this behaviour is
+  //                      well-defined.
+  // As implemented, the C++ "merge" operation does not distinguish between
+  // implicit and explicit fields when reading from the wire.
+  ASSERT_TRUE(dest.MergeFromString(wire));
+  // If zero is present on the wire, the original value is overwritten, even
+  // though this is specified as an "implicit presence" field.
+  EXPECT_EQ(0, dest.c());
+  std::string dest_data;
+  EXPECT_TRUE(dest.SerializeToString(&dest_data));
+  EXPECT_TRUE(dest_data.empty());
+}
+
+TEST(NoFieldPresenceTest, ExtraZeroesInWireLastWins) {
+  // check that, when the same field is present multiple times on the wire, we
+  // always take the last one -- even if it is a zero.
+
+  absl::string_view wire{"\x08\x01\x08\x00", /*len=*/4};  // note the null-byte.
+  proto2_nofieldpresence_unittest::ForeignMessage dest;
+
+  // TODO: b/356132170 -- Add conformance tests to ensure this behaviour is
+  //                      well-defined.
+  // As implemented, the C++ "merge" operation does not distinguish between
+  // implicit and explicit fields when reading from the wire.
+  ASSERT_TRUE(dest.MergeFromString(wire));
+  // If the same field is present multiple times on the wire, "last one wins".
+  // i.e. -- the last seen field content will always overwrite, even if it's
+  // zero and the field is implicit presence.
+  EXPECT_EQ(0, dest.c());
+  std::string dest_data;
+  EXPECT_TRUE(dest.SerializeToString(&dest_data));
+  EXPECT_TRUE(dest_data.empty());
+}
+
 TEST(NoFieldPresenceTest, IsInitializedTest) {
   // Check that IsInitialized works properly.
   proto2_nofieldpresence_unittest::TestProto2Required message;

src/google/protobuf/unittest_no_field_presence.proto

@@ -92,7 +92,7 @@ message TestAllTypes {
   repeated string repeated_string_piece = 54 [ctype = STRING_PIECE];
   repeated string repeated_cord = 55 [ctype = CORD];
 
-  repeated NestedMessage repeated_lazy_message = 57 ;
+  repeated NestedMessage repeated_lazy_message = 57;
 
   oneof oneof_field {
     uint32 oneof_uint32 = 111;
@@ -112,6 +112,12 @@ message ForeignMessage {
   int32 c = 1;
 }
 
+// Same as ForeignMessage, but all fields have explicit presence.
+// It can be useful for testing explicit-implicit presence interop behaviour.
+message ExplicitForeignMessage {
+  int32 c = 1 [features.field_presence = EXPLICIT];
+}
+
 enum ForeignEnum {
   FOREIGN_FOO = 0;
   FOREIGN_BAR = 1;