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| - CreateSnapshotPartitions
| - SnapshotManager::NewForFirstStageMount "first stage mount"
\ - New(info) "SnapshotManager构造函数，初始化两个目录 gsid_dir_=ota metadata_dir_=/metadata/ota"
| - ForceLocalImageManager "构造ImageManager"
\ - ImageManager::Open(gsid_dir_) "初始化两个目录 metadata_dir = /metadata/gsi/ota data_dir=/data/gsi/ota"
| - sm->NeedSnapshotsInFirstStageMount() "调用上面SnapshotManager的实例的该函数，根据merge state确定是否进行后续步骤"
\ - GetCurrentSlot() != Slot::Target " 根据这个判断是不是ota后的切换slot过程，Target=2，Source=1 "
" =1说明未进行ota，未进行ota就不需要挂载snapshot，return false"
| - ReadUpdateState
\ - ReadSnapshotUpdateStatus " /metadata/ota/state文件中获取ota后的merge状态"
" Unverified Merging MergeFailed 返回true"
| - true - InitRequiredDevices({"userdata"}) "NeedSnapshotsInFirstStageMount 返回true时"
\ - InitDeviceMapper "初始化dm框架，创建uevent监听 /sys/devices/virtual/misc/device-mapper"
| - return UeventCallback(uevent, &devices); "如果下面有userdata设备创建了，才说明required Device初始化成功了，才返回true。 而lp_metadata_partition_是FirstStageMount早期阶段初始化的/devices/virtual/block/dm-<>/dm/name文件中读出的super partition名字，赋值给/dev/block/mapper/<partition>"
| - true - sm->CreateLogicalAndSnapshotPartitions(lp_metadata_partition_) "创建logical device和snapshot device"
\ - ReadMetadata(opener, super_device, slot) "读取super device对应slot的metadata描述"
| - CreateLogicalPartitionParams (for params.partition:metadata->partitions) "挨个挂接metadata描述的partitions及其对应的snapshot设备"
\ - MapPartitionWithSnapshot(std::move(params))
\ - GetSnapshotStatusFilePath(params.GetPartitionName() "/metadata/ota/snapshots/<partition_name>"
| - ReadSnapshotStatus(lock, params.GetPartitionName(), &*live_snapshot_status) # /metadata/ota/snapshots/<partition_name>,
\ - status->ParseFromFileDescriptor(fd.get()) "解析该文件，获得live_snapshot_status状态"
| - # MERGE_COMPLETED | NONE | "cow_partition_size + cow_file_size = 0 代表no live snapshot"
| - live_snapshot_status.has_value- "如果有live snapshot"
\ - params.device_name = GetBaseDeviceName(params.GetPartitionName()); "devicename变成了<partionname>—base"
| - created_devices.EmplaceBack<AutoUnmapDevice>(&dm, params.GetDeviceName())
| - CreateLogicalPartition(params, &base_path) "映射logical device 动态分区的映射，返回的path为/dev/block/dm-<num>"
| - live_snapshot_status.has_value- "如果有live snapshot"
\ - dm.GetDeviceString(params.GetDeviceName(), &base_device) "返回base_device major:minor设备号"
| - CreateLogicalPartitionParams cow_params = params
| - MapCowDevices(lock, cow_params, *live_snapshot_status, &created_devices, &cow_name) "映射cow_device写时复制"
\ - cow_image_name = GetCowImageDeviceName(partition_name) #<partion_name>-cow-img
| - *cow_name = GetCowName(partition_name) #<partition_name>-cow
| - EnsureImageManager()
| - "因为前面已经执行过ForceLocalImageManager，所以image_不为空，后面的步骤不走了"
| - MapCowImage(partition_name, remaining_time)
| - has_local_image_manager_- true "image_为之前ForceLocalImageManager时的实例，指向ImageManager"
\ - images_->MapImageWithDeviceMapper(opener, cow_image_name, &cow_dev) "cow_image_name为<partion_name>-cow-img"
| - MapWithDmLinear(opener, cow_image_name, {}, &ignore_path)
\ - metadata = OpenMetadata(metadata_dir_) "metadata_dir_=/metadata/gsi/ota"
\ - metadata_file = GetMetadataFile(metadata_dir_) "/metadata/gsi/ota/lp_metadata"
| | | - metadata = ReadFromImageFile(metadata_file) "从上述文件中解析出metadata描述"
| - super = GetMetadataSuperBlockDevice() "获得super封装"
| - block_device = GetBlockDevicePartitionName(super)
| - CreateLogicalPartition(params, path) "dm-linear 跟之前的动态分区的逻辑一样，映射设备，获得path /dev/block/dm-<num>"
| - WriteStringToFile(status_string, status_file) "/metadata/gsi/ota/<parition_name-cow-img>.status中写入dm:<partition_name-cow-img>"
| - dm.GetDeviceString(name, dev) "返回major：minor号给dev"
| - created_devices->EmplaceBack<AutoUnmapImage>(images_.get(), cow_image_name） "created_devices再push新的cow_img"
| - CreateLogicalPartitionParams cow_partition_params = params (.partition_name=*cow_name;);
"cow_name = <partition>_cow"
| - CreateDmTable(cow_partition_params, &table) "创建映射表"
| - cow_partition_sectors = snapshot_status.cow_partition_size() / kSectorSize "扇区"
| - cow_image_sectors = snapshot_status.cow_file_size() / kSectorSize;
| - table.Emplace<DmTargetLinear>(cow_partition_sectors, cow_image_sectors, cow_image_device, 0);
" 从cow-img的尾部开始算，长度为cow_image_sectors 个扇区，block_device还是用前面挂接的cow-img的device"
| - dm.CreateDevice(*cow_name, table, &cow_path, remaining_time) "为<partition_name>-cow创建dm设备"
| - created_devices->EmplaceBack<AutoUnmapDevice>(&dm, *cow_name); "push到created_devices中"
| - dm.GetDeviceString(cow_name, &cow_device) "<partion_name>-cow获取设备号major /minor"
| - MapSnapshot(lock, params.GetPartitionName(), base_device, cow_device, remaining_time, path)
"base_device 代表动态分区的base_device, cow_device代表映射partion_name-cow的设备，为这两个设备之间建立映射快照"
\ - ReadSnapshotStatus(name, &status) "/metadata/ota/snapshots/<partition_name>读取状态，如果是MERGE_COMPLETED|NONE就退出，不需要快照了"
| - snapshot_sectors = status.snapshot_size() / kSectorSize "snapshot快照的扇区数"
| - linear_sectors = (status.device_size() - status.snapshot_size()) / kSectorSize "logical本身的扇区数"
| - ReadUpdateState() "/metadata/ota/state中读取ota的状态 MergeCompleted| MergeNeedsReboot return false，退出
Merging|MergeFailed mode=merge或者Persistent"
| - snap_name = GetSnapshotExtraDeviceName(partition_name) "<parition_name>-inner"
| - table.Emplace<DmTargetSnapshot>(0, snapshot_sectors, base_device, cow_device, mode, kSnapshotChunkSize)
"创建映射表，单位是4k，即8*sector_size DmTargetSnapshot start=0. length = snapshot_sectors,mode=merge|Persistent"
"DmTargetSnapshot::name() mode=merge时返回"snapshot-merge",其他状态下返回"snapshot""
| - dm.CreateDevice(snap_name, table, dev_path, timeout_ms) "创建dm设备"
| - linear_sectors>0 - "动态分区里的img不为空 "
\ - dm.GetDeviceString(snap_name, &snap_dev) "获取snap_dev的major/minor"
| - table.Emplace<DmTargetLinear>(0, snapshot_sectors, snap_dev, 0); "先是snap_device"
| - table.Emplace<DmTargetLinear>(snapshot_sectors, linear_sectors, base_device, snapshot_sectors);
"再是linear_device追加"
| - dm.CreateDevice(name, table, dev_path, timeout_ms) "最终创建<partition_name>的dm设备映射"
|