libsnapshot: Fix CowWriter::Finalize() not restoring footer correctly.

Bug: 173432386
Test: cow_api_test
Change-Id: If1b856fcdcd6b6ded5e00342037308c6b35d6fc0

fs_mgr/libsnapshot/cow_api_test.cpp

@@ -757,6 +757,30 @@ TEST_F(CowTest, ClusterAppendTest) {
     ASSERT_TRUE(iter->Done());
 }
 
+TEST_F(CowTest, AppendAfterFinalize) {
+    CowOptions options;
+    options.cluster_ops = 0;
+    auto writer = std::make_unique<CowWriter>(options);
+    ASSERT_TRUE(writer->Initialize(cow_->fd));
+
+    std::string data = "This is some data, believe it";
+    data.resize(options.block_size, '\0');
+    ASSERT_TRUE(writer->AddRawBlocks(50, data.data(), data.size()));
+    ASSERT_TRUE(writer->AddLabel(3));
+    ASSERT_TRUE(writer->Finalize());
+
+    std::string data2 = "More data!";
+    data2.resize(options.block_size, '\0');
+    ASSERT_TRUE(writer->AddRawBlocks(51, data2.data(), data2.size()));
+    ASSERT_TRUE(writer->Finalize());
+
+    ASSERT_EQ(lseek(cow_->fd, 0, SEEK_SET), 0);
+
+    // COW should be valid.
+    CowReader reader;
+    ASSERT_TRUE(reader.Parse(cow_->fd));
+}
+
 }  // namespace snapshot
 }  // namespace android
 

fs_mgr/libsnapshot/cow_reader.cpp

@@ -229,7 +229,8 @@ bool CowReader::ParseOps(std::optional<uint64_t> label) {
 
     if (footer_) {
         if (ops_buffer->size() != footer_->op.num_ops) {
-            LOG(ERROR) << "num ops does not match";
+            LOG(ERROR) << "num ops does not match, expected " << footer_->op.num_ops << ", found "
+                       << ops_buffer->size();
             return false;
         }
         if (ops_buffer->size() * sizeof(CowOperation) != footer_->op.ops_size) {

fs_mgr/libsnapshot/cow_writer.cpp

@@ -376,6 +376,7 @@ bool CowWriter::Finalize() {
     auto continue_data_pos = next_data_pos_;
     auto continue_op_pos = next_op_pos_;
     auto continue_size = ops_.size();
+    auto continue_num_ops = footer_.op.num_ops;
     bool extra_cluster = false;
 
     // Footer should be at the end of a file, so if there is data after the current block, end it
@@ -408,9 +409,9 @@ bool CowWriter::Finalize() {
         current_data_size_ = continue_data_size;
         next_data_pos_ = continue_data_pos;
         next_op_pos_ = continue_op_pos;
+        footer_.op.num_ops = continue_num_ops;
         ops_.resize(continue_size);
     }
-
     return Sync();
 }