Hyper-V API: Snapshots feature#4722
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Base automatically changed from
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May 5, 2026 20:36
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Pull request overview
Adds a virtdisk-based snapshot implementation for the Hyper-V API backend, with a focus on “forward-merge on erase” semantics so deleting a snapshot preserves descendant state by consolidating differencing disks.
Changes:
- Introduce
VirtDiskSnapshot(capture/apply/erase) backed by VHDX/AVHDX differencing disks. - Wire snapshot creation/loading into
HCSVirtualMachine, and make VM startup preferhead.avhdxwhen present. - Add Hyper-V API integration tests covering snapshot erase/merge scenarios and update CMake sources.
Reviewed changes
Copilot reviewed 7 out of 7 changed files in this pull request and generated 5 comments.
Show a summary per file
| File | Description |
|---|---|
| tests/unit/hyperv_api/test_it_hyperv_virtdisk_snapshot.cpp | New integration tests validating erase/merge behavior across several snapshot topologies |
| tests/unit/hyperv_api/CMakeLists.txt | Adds the new snapshot integration test source to the Hyper-V API test target |
| tests/unit/CMakeLists.txt | Ensures the integration test executable has baseline sources |
| src/platform/backends/hyperv_api/virtdisk/virtdisk_snapshot.h | Declares the virtdisk snapshot implementation and helpers (head name, paths) |
| src/platform/backends/hyperv_api/virtdisk/virtdisk_snapshot.cpp | Implements capture/apply/erase using differencing disks + forward-merge on erase |
| src/platform/backends/hyperv_api/hcs_virtual_machine.cpp | Enables snapshot support and prefers head.avhdx as the primary disk when present; adds resize guard |
| src/platform/backends/hyperv_api/CMakeLists.txt | Builds the new virtdisk_snapshot.cpp into the backend library |
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| VMSpecs specs{.num_cores = 1, | ||
| .mem_size = MemorySize{"1G"}, | ||
| .disk_space = MemorySize{"5G"}, | ||
| .default_mac_address = "00:00:00:00:00:00", | ||
| .extra_interfaces = {}, | ||
| .ssh_username = "ubuntu", | ||
| .state = VirtualMachine::State::off, | ||
| .mounts = {}, | ||
| .deleted = false, | ||
| .metadata = {}}; | ||
| ++counter; | ||
| auto ss = std::make_shared<VirtDiskSnapshot>(name, | ||
| "", | ||
| "test-id", | ||
| std::move(parent), | ||
| specs, | ||
| vm, | ||
| desc); | ||
| snapshots.push_back(ss); | ||
| ss->capture(); | ||
| return ss; | ||
| } |
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| std::string VirtDiskSnapshot::make_snapshot_filename(const Snapshot& ss) | ||
| { | ||
| return fmt::format("{}.avhdx", ss.get_name()); | ||
| } |
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| // Check if head disk already exists. The head disk may not exist for a VM | ||
| // that has no snapshots yet. | ||
| if (!MP_FILEOPS.exists(head_path)) | ||
| { | ||
| const auto parent = get_parent(); | ||
| const auto target = parent ? make_snapshot_path(*parent) : base_vhdx_path; | ||
| create_new_child_disk(target, head_path); | ||
| } | ||
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| // Step 1: Rename current head to snapshot name | ||
| MP_FILEOPS.rename(head_path, snapshot_path); | ||
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| // Step 2: Create a new head from the snapshot | ||
| create_new_child_disk(snapshot_path, head_path); | ||
| } |
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| auto self_temp_path = self_path; | ||
| self_temp_path += ".tmp"; | ||
| MP_FILEOPS.rename(self_path, self_temp_path); | ||
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| // Pass 1: merge each child into a copy, stash the result as child.new | ||
| // No original files are modified. If anything fails, we clean up and bail. | ||
| std::vector<std::pair<std::filesystem::path, std::filesystem::path>> | ||
| staged; // {child, child.new} | ||
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| auto rollback = [&] { | ||
| for (const auto& [original, stashed] : staged) | ||
| { | ||
| std::error_code ec; | ||
| MP_FILEOPS.remove(stashed, ec); | ||
| } | ||
| // Restore the snapshot file | ||
| MP_FILEOPS.remove(self_path); | ||
| MP_FILEOPS.rename(self_temp_path, self_path); | ||
| }; |
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| if (get_num_snapshots() > 0) | ||
| { | ||
| throw ResizeDiskException{"Cannot resize the primary disk while there are " | ||
| "snapshots. To resize, delete the snapshots first."}; | ||
| } | ||
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| if (const auto result = | ||
| VirtDisk().resize_virtual_disk(description.image.image_path, new_size.in_bytes()); | ||
| !result) |
This reverts commit 421d99c.
The old merge behavior was backward, meaning the deleted disk would be merged to its parent. The new implementation now correctly merges forward -- each child disk would have the deleted disk's contents. Added integration tests for the snapshot erase.
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Implement forward-merge on erase behavior for the virtdisk snapshot feature.