libsnapshot: sync header metadata

After we write emit a label, we need to update the number of resume
points + sequence data and op_count. Realistically we could just call
Finalize, but maybe synching these specific fields could prevent
unexpected outcomes.

Test: cow_api_test
Change-Id: I1585601a134221689ce8d5675a2a3e32f1e8a0e6

fs_mgr/libsnapshot/libsnapshot_cow/test_v3.cpp

@@ -482,5 +482,43 @@ TEST_F(CowTestV3, ResumePointTest) {
     header = reader.header_v3();
     ASSERT_EQ(header.op_count, 15);
 }
+
+TEST_F(CowTestV3, BufferMetadataSyncTest) {
+    CowOptions options;
+    options.op_count_max = 100;
+    auto writer = CreateCowWriter(3, options, GetCowFd());
+    /*
+    Header metadafields
+    sequence_data_count = 0;
+    resume_point_count = 0;
+    resume_point_max = 4;
+    */
+    ASSERT_TRUE(writer->Finalize());
+
+    CowReader reader;
+    ASSERT_TRUE(reader.Parse(cow_->fd));
+
+    auto header = reader.header_v3();
+    ASSERT_EQ(header.sequence_data_count, 0);
+    ASSERT_EQ(header.resume_point_count, 0);
+    ASSERT_EQ(header.resume_point_max, 4);
+
+    writer->AddLabel(0);
+    ASSERT_TRUE(reader.Parse(cow_->fd));
+    header = reader.header_v3();
+    ASSERT_EQ(header.sequence_data_count, 0);
+    ASSERT_EQ(header.resume_point_count, 1);
+    ASSERT_EQ(header.resume_point_max, 4);
+
+    ASSERT_TRUE(reader.Parse(cow_->fd));
+    header = reader.header_v3();
+
+    /*
+    Header metadafields
+    sequence_data_count = 1;
+    resume_point_count = 0;
+    resume_point_max = 4;
+    */
+}
 }  // namespace snapshot
 }  // namespace android

fs_mgr/libsnapshot/libsnapshot_cow/writer_v3.cpp

@@ -311,7 +311,7 @@ bool CowWriterV3::EmitLabel(uint64_t label) {
         PLOG(ERROR) << "writing resume buffer failed";
         return false;
     }
-    return Sync();
+    return Finalize();
 }
 
 bool CowWriterV3::EmitSequenceData(size_t num_ops, const uint32_t* data) {