fix: Temporary fix for loading plain lists

**The Issue**

We do not serialize lists in PLAIN format well today. We're working on
getting it right, but existing wire format simply doesn't work.

**The (temporary) fix**

This PR attempts to load lists as usual, but upon failure it reverts to
loading a plain list format. This is not a perfect solution, and in some
extreme edge cases it could fail. But.. it's temporary.

src/server/list_family_test.cc

@@ -372,6 +372,15 @@ TEST_F(ListFamilyTest, LRem) {
   ASSERT_THAT(Run({"lrem", "nexists", "0", "elem"}), IntArg(0));
 }
 
+TEST_F(ListFamilyTest, DumpRestorePlain) {
+  const string kValue(10'000, '#');
+  EXPECT_EQ(CheckedInt({"LPUSH", kKey1, kValue}), 1);
+  auto buffer = Run({"DUMP", kKey1}).GetBuf();
+  EXPECT_EQ(Run({"RESTORE", kKey2, "0", ToSV(buffer)}), "OK");
+  EXPECT_EQ(CheckedInt({"LLEN", kKey2}), 1);
+  EXPECT_EQ(Run({"LRANGE", kKey2, "0", "1"}), kValue);
+}
+
 TEST_F(ListFamilyTest, LTrim) {
   Run({"rpush", kKey1, "a", "b", "c", "d"});
   ASSERT_EQ(Run({"ltrim", kKey1, "-2", "-1"}), "OK");

src/server/rdb_load.cc

@@ -722,13 +722,19 @@ void RdbLoaderBase::OpaqueObjLoader::CreateList(const LoadTrace* ltrace) {
     if (ec_)
       return false;
 
+    uint8_t* lp = nullptr;
+
+    auto load_plain = [&]() {
+      lp = (uint8_t*)zmalloc(sv.size());
+      ::memcpy(lp, (uint8_t*)sv.data(), sv.size());
+      quicklistAppendPlainNode(ql, lp, sv.size());
+    };
+
     if (container == QUICKLIST_NODE_CONTAINER_PLAIN) {
-      quicklistAppendPlainNode(ql, (uint8_t*)sv.data(), sv.size());
+      load_plain();
       return true;
     }
 
-    uint8_t* lp = nullptr;
-
     if (rdb_type_ == RDB_TYPE_LIST_QUICKLIST_2) {
       uint8_t* src = (uint8_t*)sv.data();
       if (!lpValidateIntegrity(src, sv.size(), 0, nullptr, nullptr)) {
@@ -747,10 +753,13 @@ void RdbLoaderBase::OpaqueObjLoader::CreateList(const LoadTrace* ltrace) {
       lp = lpNew(sv.size());
       if (!ziplistValidateIntegrity((uint8_t*)sv.data(), sv.size(), 1,
                                     ziplistEntryConvertAndValidate, &lp)) {
-        LOG(ERROR) << "Ziplist integrity check failed: " << sv.size();
         zfree(lp);
-        ec_ = RdbError(errc::rdb_file_corrupted);
-        return false;
+        // TODO: Revert loading plain format here once we fix serialization size.
+        // LOG(ERROR) << "Ziplist integrity check failed: " << sv.size();
+        // ec_ = RdbError(errc::rdb_file_corrupted);
+        // return false;
+        load_plain();
+        return true;
       }
 
       /* Silently skip empty ziplists, if we'll end up with empty quicklist we'll fail later. */