Sandbox and Test Flow Guide

This document describes the public sandbox API, the global ConfigManager under AgentENV/src/cfg.rs, and what AgentENV/tests/integration/fc.rs, AgentENV/tests/integration/process.rs, AgentENV/tests/integration/snapshot.rs, and AgentENV/tests/integration/orchestrator.rs validate.

1) Sandbox Public Interface

Types and responsibilities

FirecrackerSandboxConfig (AgentENV/src/sandbox/firecracker/config.rs)
- Purpose: describes a fresh VM boot (kernel + tools drive + user image).
- Key fields
  - firecracker_binary: path to the firecracker executable.
  - kernel_image: path to vmlinux.bin.
  - tools_drive_version: immutable tools drive identity. The node resolves it to <deps_path>/tools/<version>/tools.ext4 before mounting it as /dev/vda.
  - user_image_config: overlaybd config for the writable user image (/dev/vdb).
  - boot_args: kernel boot args (e.g. console=ttyS0 ... init=/init).
  - vcpu_count, mem_size_mib: VM size.
  - runtime_policy: socket/envd timeouts and poll intervals.
  - common.stdout_path, common.stderr_path: explicit capture directories. Otherwise stdout/stderr capture is enabled only when common.firecracker_log_level is non-empty, using the serial directory or work_dir/logs fallback.
  - envd_version: expected envd version for the guest image.
  - env_vars: optional default environment variables injected after envd init.
  - ublk_config: optional ublk-backed rootfs configuration.
- Convenience
  - from_global_config() builds a config from ConfigManager using the internal default template rootfs image. The image must already be resolved under <image-cache-root>/configs.
  - from_global_config_with_user_image(...) builds a config from an already resolved overlaybd image config.
  - Runtime launches from committed snapshots are built through FirecrackerSandbox::from_snapshot(&RunnableSnapshot, &SandboxLaunchConfig).
  - uid/gid: File ownership (default 0).
ProcessOpts (AgentENV/src/sandbox/process.rs)
- Purpose: Options for starting a process inside the sandbox.
- Key fields
  - envs: HashMap<String, String> for process environment variables.
  - cwd: Optional working directory.
  - timeout: Optional max time to wait for completion.
- Builder methods: with_envs(), with_cwd(), with_timeout().
ProcessOutput (AgentENV/src/sandbox/process.rs)
- Purpose: Result of a completed process execution.
- Fields: stdout, stderr, exit_code.
ProcessHandle (AgentENV/src/sandbox/process.rs)
- Purpose: Handle to a running process inside the sandbox.
- Key methods
  - pid(): Returns the PID inside the guest VM.
  - wait().await: Waits for exit and collects all output.
  - send_stdin(data).await: Sends bytes to stdin.
  - send_signal(signal).await: Sends a signal (e.g. SIGTERM).
  - kill().await: Kills the process with SIGKILL.
FirecrackerSnapshotConfig (AgentENV/src/sandbox/firecracker/config.rs)
- Purpose: describes how to resume a VM from snapshot + memory + base disk.
- Key fields
  - vm_state_path: Firecracker VM state file.
  - mem_overlaybd_config: runtime memory overlay image config.
  - base_rootfs_path: writable disk image or overlaybd rootfs image config paired with that snapshot.
  - firecracker_binary: FC binary to use for resuming.
  - runtime_policy: socket/envd timeouts and poll intervals.
  - envd_version: guest envd version metadata.
  - env_vars: optional default environment variables restored on start.
- Notes
  - FirecrackerSnapshotConfig returned by pause() owns a tempdir (keeps files alive).
  - get_rootfs_path() resolves the snapshot rootfs path used by runtime tests.
SandboxBackend and SandboxExecutor (AgentENV/src/sandbox/backend.rs)
- Purpose: public traits for sandbox lifecycle and process execution.
- Key methods
  - SandboxBackend: start, start_nowait, wait_for_ready, pause, resume, stop.
  - SandboxExecutor: run_command, run_command_with_opts, start_process.
FirecrackerSandbox (AgentENV/src/sandbox/firecracker/sandbox.rs)
- Purpose: lifecycle controller for a single Firecracker instance.
- Main methods (detailed)
  - FirecrackerSandbox::new(FirecrackerSandboxConfig)
    - Creates a sandbox handle for a fresh boot.
    - Does not start Firecracker; it only prepares the object.
    - Each sandbox gets its own temporary work directory.
  - FirecrackerSandbox::resume_from_snapshot_config(&FirecrackerSnapshotConfig)
    - Creates a new sandbox handle and immediately starts it from a snapshot config.
    - Uses vm_state_path, the memory overlay image config, and base_rootfs_path from the config.
    - Returns a running sandbox after Firecracker and envd are ready.
  - FirecrackerSandbox::from_snapshot(&RunnableSnapshot, &SandboxLaunchConfig)
    - Creates a sandbox handle from committed snapshot state resolved for a node.
    - Uses snapshot vm_state.bin plus node-local materialized memory/image.json and rootfs/image.json.
    - Does not start Firecracker; call start().await afterward.
  - start().await
    - Spawns the Firecracker process, waits for the API socket, starts the VM, then waits for guest-level envd readiness.
  - pause().await
    - Sends PATCH /vm to pause the VM.
    - Creates snapshot artifacts including vm_state.bin, mem_image.json, mem_overlaybd/overlaybd.commit, and rootfs snapshot state.
    - Returns a FirecrackerSnapshotConfig that owns the tempdir holding these files.
  - resume().await
    - Resumes a paused VM in-place by sending PATCH /vm with Resumed.
    - Use this when you want to keep the same sandbox instance.
  - stop().await
    - Stops Firecracker, releases network resources, and cleans up optional ublk state.
  - run_command(cmd, args).await
    - Runs a command inside the sandbox via envd gRPC and waits for it to complete. Returns ProcessOutput with stdout, stderr, and exit code.
  - run_command_with_opts(cmd, args, opts).await
    - Same as run_command but accepts ProcessOpts for setting env vars, working directory, and execution timeout.
  - start_process(cmd, args, opts).await
    - Starts a long-running process inside the sandbox. Returns a ProcessHandle that can stream output, send stdin, or kill the process.
  - firecracker_stdout_path() / firecracker_stderr_path()
    - Resolved log paths used by this sandbox (including defaults).
  - work_rootfs_path()
    - Returns the path to the per-instance writable rootfs inside the work directory. Useful for inspecting or copying disk state after a run.

What happens under the hood (important for correct usage)

Each FirecrackerSandbox uses its own temp work directory.
Kernel is used via the configured host path.
When resuming, the writable rootfs and memory image are materialized into the work dir so each instance can mutate independently. For overlaybd-backed snapshot-backed template launches, this means recreating runtime image configs and fresh writable uppers from the committed layer stack.
Rootfs copies use copy-on-write helpers where available.
Firecracker runs with current_dir = work_dir, so relative paths work.
Firecracker logging is disabled by default. Set firecracker.log_level to enable stdout/stderr capture and firecracker.log under the configured serial directory. Direct Rust callers can also opt individual streams in using explicit destinations.
start() waits for both the API socket and envd readiness.
Although the Firecracker process will be killed on drop, explicit stop() is recommended.

Minimal usage example

#![allow(unused)]
fn main() {
use agentenv::sandbox::{FirecrackerSandbox, FirecrackerSandboxConfig, SandboxExecutor};

let mut cfg = FirecrackerSandboxConfig::new(
    "/path/to/firecracker".into(),
    "/path/to/vmlinux.bin".into(),
    "/path/to/rootfs.ext4".into(),
);
cfg.boot_args = Some("console=ttyS0 reboot=k panic=1 pci=off init=/init".into());

let mut sandbox = FirecrackerSandbox::new(cfg)?;
sandbox.start().await?;

// Run a command inside the VM
let output = sandbox.run_command("echo", &["hello"]).await?;
assert_eq!(output.exit_code, 0);
println!("{}", output.stdout);

let snapshot = sandbox.pause().await?;
sandbox.stop().await?;

let mut resumed_sandbox = FirecrackerSandbox::resume_from_snapshot_config(&snapshot).await?;
resumed_sandbox.stop().await?;
}

Runtime prerequisites

Linux host
/dev/kvm accessible by the current user, with the host modules matching virtualization_mode (KVM by default; PVM requires x86_64 and kvm_pvm)
debugfs (e2fsprogs) if you use init injection or disk-inspection helpers

2) Global Config Manager (`src/cfg.rs`)

The tests and benchmarks use a shared global config manager based on config/default.toml.

Config file format

AgentENV/config/default.toml (override via AENV_CONFIG_PATH or --config)

[firecracker]
boot_args = "your-kernel-boot-args"
binary_path = "/path/to/firecracker"
socket_timeout_secs = 3
socket_poll_ms = 20
# Optional override; defaults to $AENV_HOME/firecracker-work.
# work_dir = "/path/to/firecracker-work"
# Optional override; defaults to $AENV_HOME/logs/serial.
# serial_dir = "/path/to/serial"
# Optional; enables stdout/stderr capture and firecracker.log when non-empty.
# log_level = "Info"

[kernel]
image_path = "/path/to/vmlinux.bin"

[tools]
version = "0.1.0-custom.1"
drive_path = "/path/to/tools.ext4"

[machine]
mem_size_mib = 128
vcpu_count = 1

[envd]
version = "0.5.15"
init_timeout_secs = 30
poll_ms = 10

[orchestrator]
auto_evict_interval_ms = 1000
default_sandbox_timeout_secs = 15

[snapshot]
repository_backend = "posix_fs"
local_cache_path = "$AENV_HOME/snapshot-local-cache"

[backend.posix_fs]
snapshot_store = "$AENV_HOME/snapshot-store"

[ublk]
enabled = false

TOML parameter reference

[firecracker]
- boot_args: Kernel command line. The runtime appends init=/init if not already present (tools drive provides the init script).
- binary_path: Absolute or relative path to firecracker.
- socket_timeout_secs: Max time to wait for the Firecracker API socket.
- socket_poll_ms: Poll interval for checking socket existence.
- work_dir: Optional override for the per-sandbox Firecracker working directories. These directories include runtime sockets, symlinks, local logs, and writable OverlayBD upper layer data (overlaybd/upper.data and overlaybd/upper.index). Defaults to $AENV_HOME/firecracker-work.
- serial_dir: Optional override for the persistent output directory. When logging is enabled, serial output is written under a per-sandbox subdirectory and not removed by the sandbox. Defaults to $AENV_HOME/logs/serial. Setting this path alone does not enable logging.
- log_level: Optional Firecracker log level (Error, Warning, Info, Debug, Trace, case-insensitive). When set to a non-empty value, stdout/stderr capture and firecracker.log are enabled in the sandbox’s log directory. If omitted or empty, stdout/stderr are discarded and no log directories are created, unless explicit Rust capture destinations are set.
[kernel]
- image_path: Optional local kernel image path (vmlinux.bin). If omitted, AgentENV derives it from deps_path and the bundled dependency manifest.
[tools]
- drive_path: Optional local tools drive source. When set, an explicit version is required and setup imports it into the immutable version directory.
Template rootfs image
- User-visible rootfs images are selected when starting template builds through the optional fromImage field.
- If fromImage is omitted, AgentENV uses [image.resolver].default_image.
- Standard Docker Hub shortnames such as ubuntu:24.04 and node:20 are normalized before image resolution.
- Resolved image configs are cached under <image-cache-root>/configs/<slug>-<hash>-image.json; local layer products are cached under <image-cache-root>/layers/; overlaybd-native remote block reads are cached under <image-cache-root>/remote-blocks/.
- Registry authentication uses docker credentials from ~/.docker/config.json or $DOCKER_CONFIG/config.json.
[machine]
- mem_size_mib: Guest RAM size (default: 128).
- vcpu_count: Number of vCPUs (default: 1).
[envd]
- version: Expected envd version baked into the runtime image.
- init_timeout_secs: Max time to wait for the in-guest envd daemon to become ready after VM start (default: 30).
- poll_ms: Poll interval for envd health-check retries (default: 10).
[orchestrator]
- auto_evict_interval_ms: Poll interval for background timeout eviction.
- default_sandbox_timeout_secs: Default keep-alive timeout used by the orchestrator.
- auto_resume_min_sandbox_timeout_secs: When a data-plane request targets a non-running sandbox, automatically resume it (if auto-resume is enabled) and refresh its timeout for no-less than this duration.
[pool]
- low_watermark: Shared lower bound for warm resources. Defaults to 2.
- high_watermark: Shared upper bound target for warm resources. Defaults to 64.
- [pool.network].maintenance_enabled: Enables the background network-slot maintenance worker. Defaults to true.
- [pool.block].enabled: Enables the ublk overlaybd warm-device pool.
- [pool.firecracker].enabled: Enables pre-spawned Firecracker processes for snapshot resume.
[snapshot]
- local_cache_path: Manager-owned node-local snapshot artifact/cache root. Defaults to $AENV_HOME/snapshot-local-cache.
- repository_backend: Snapshot repository backend. Defaults to posix_fs.
[backend.posix_fs]
- snapshot_store: Committed snapshot store root. Defaults to $AENV_HOME/snapshot-store.
[ublk]
- daemon_binary_path: Optional path to uvm-ublk-daemon.
- daemon_socket_path: Optional unix socket path for daemon RPCs.
- daemon_log_path: Optional log file path for the daemon process.
[ublk.overlaybd]
- global_config_path: path to the generated overlaybd runtime config. Required for ublk. Per-image configs are derived from template build fromImage values and live under <image.cache.root_dir>/configs.

Defaults and optional fields

[firecracker].binary_path: Optional. If omitted, AgentENV derives it from deps_path and the bundled dependency manifest.
[firecracker].boot_args: Optional. Defaults to console=ttyS0 reboot=k panic=1 pci=off.
[firecracker].socket_timeout_secs: Optional. Defaults to 3 seconds.
[firecracker].socket_poll_ms: Optional. Defaults to 20 ms.
[firecracker].work_dir: Optional override. Defaults to $AENV_HOME/firecracker-work.
[firecracker].serial_dir: Optional override. Defaults to $AENV_HOME/logs/serial.
[kernel].image_path: Optional. If omitted, AgentENV derives it from deps_path and the bundled dependency manifest.
[tools].drive_path: Optional local source override; requires an explicit [tools].version when set.
[tools].url: Optional registry override; requires an explicit [tools].version when set.
[machine]: Optional. If omitted, defaults are 128 MiB RAM and 1 vCPU.
[envd]: Optional, but template and sandbox flows expect a valid version.
[envd].init_timeout_secs: Optional. Defaults to 30 seconds.
[envd].poll_ms: Optional. Defaults to 10 ms.
[orchestrator]: Optional. If omitted, orchestrator runtime defaults are used.
[pool]: Optional. If omitted, network pool maintenance is enabled with low watermark 2 and high watermark 64; block and Firecracker process pools are disabled unless enabled in their component subsections.
[snapshot]: Optional. If omitted, repository_backend defaults to posix_fs and the local cache root derives from AENV_HOME.
[backend.posix_fs]: Optional. If omitted, the POSIX snapshot store defaults to $AENV_HOME/snapshot-store.
[ublk]: Optional. If omitted, ublk is disabled.

Environment-based configuration

The config loader reads the full TOML file from the following sources:

AENV_CONFIG_PATH
--config <path>
config/default.toml

Runtime image and binary paths are configured in the TOML file itself.

Required fields

[kernel].image_path
[tools].version when [tools].drive_path or [tools].url is overridden
[envd].version

If the [orchestrator] section is present, it must include default_sandbox_timeout_secs.

ConfigManager (recommended entry point)

The helper is strict. If requirements are not met (missing files, invalid config), it returns an error and the test fails.

Path resolution order

AENV_CONFIG_PATH environment variable
--config CLI flag
config/default.toml (default)

Helper methods

ConfigManager::global() -> Result<&'static ConfigManager>
- Lazily initializes and returns a process-wide singleton.
ConfigManager::new() -> Result<ConfigManager>
- Loads config using the default resolution order.
ConfigManager::new_from_path(path) -> Result<ConfigManager>
- Loads a specific TOML file.
ConfigManager::config() -> &AppConfig
- Returns the parsed application config.
ConfigManager::global_config() -> Result<&'static AppConfig>
- Returns the global parsed config directly.
ConfigManager::get_orchestrator_config() -> Option<OrchestratorConfig>
- Returns orchestrator settings when configured.
AppConfig::resolved_snapshot_store() -> PathBuf
- Resolves the effective committed snapshot store root.
AppConfig::resolved_snapshot_local_cache_path() -> PathBuf
- Resolves the effective manager-owned node-local snapshot cache root.

Typical test setup flow

#![allow(unused)]
fn main() {
use agentenv::sandbox::{FirecrackerSandbox, FirecrackerSandboxConfig, SandboxExecutor};

let sandbox_config = FirecrackerSandboxConfig::from_global_config()?;

let mut sandbox = FirecrackerSandbox::new(sandbox_config)?;
sandbox.start().await?;

// Run commands via envd gRPC
let output = sandbox.run_command("echo", &["hello"]).await?;
assert_eq!(output.exit_code, 0);

let snapshot = sandbox.pause().await?;
sandbox.stop().await?;

let mut resumed_sandbox = FirecrackerSandbox::resume_from_snapshot_config(&snapshot).await?;
resumed_sandbox.stop().await?;
}

3) What the Integration Tests Validate

`integration/fc.rs`

microvm_startup_moves_to_running Boots a fresh VM and verifies that:
- Firecracker starts and accepts API calls.
- Kernel + rootfs can be configured without error.
- The VM can be cleanly shut down afterward.
microvm_pause_transitions_to_paused Validates pause + snapshot creation by:
- Pausing a running VM via FC API.
- Creating vm_state.bin, mem_image.json, and mem_overlaybd/overlaybd.commit.
- Ensuring those files exist on disk.
microvm_resume_from_pause_transitions_to_running Validates in-place resume by:
- Pausing a VM.
- Resuming it in the same sandbox instance.
- Confirming the control flow completes without error.
microvm_resume_transitions_to_running Validates resume into a new sandbox by:
- Pausing and snapshotting a VM.
- Shutting down the original sandbox.
- Resuming from the snapshot in a new sandbox instance.
snapshot_preserves_disk_state_on_resume Confirms disk state is preserved by:
- Writing a marker file via run_command.
- Pausing and resuming from the snapshot.
- Reading the marker file after resume and asserting it persists.
snapshot_chain_survives_after_parent_snapshot_handle_is_dropped Validates multi-level snapshot chains by:
- Creating a snapshot, resuming, creating a second snapshot.
- Dropping the first snapshot handle.
- Resuming from the second snapshot and verifying disk state.
- Also tests pause_to_dir with snapshot-owned inherited layer adoption.
microvm_can_access_internet Confirms guest networking works before and after snapshot resume.
multiple_resumes_have_independent_disk_state Confirms multiple resumptions are isolated by:
- Resuming multiple VMs from a single snapshot sequentially.
- Each resumed VM verifies the shared marker and writes its own file.
- Verifying each snapshot has independent state.

`integration/process.rs`

run_command_captures_stdout Runs echo hello world and verifies stdout contains the expected string.
run_command_captures_stderr Runs a shell command that writes to stderr and verifies stderr capture.
run_command_reports_exit_code Runs exit 42 and verifies the exit code is reported correctly.
run_command_with_opts_sets_env_and_cwd Verifies that environment variables and working directory options are honoured.
run_command_handles_timeout Verifies long-running commands fail when a timeout is configured.
run_command_with_unbounded_output Verifies excessive output is rejected instead of buffering forever.
start_process_interactive_stdin Starts cat, sends data via stdin, kills the process, and verifies the echoed output.
start_process_send_signal Starts sleep 300, sends SIGTERM, and verifies the process terminates with a non-zero exit code.
run_command_after_snapshot_resume Pauses a sandbox, resumes from a snapshot config, and verifies run_command works on the resumed instance.

`integration/snapshot.rs`

built_snapshot_can_be_loaded_by_alias_and_launched Builds a template-backed snapshot, loads it by alias, launches a sandbox from the resolved runnable snapshot, and verifies captured runtime state.
listed_committed_snapshot_can_be_loaded_and_resolved Verifies SnapshotManager::list_committed() returns usable committed snapshots that can be loaded and resolved into runnable paths.
loaded_runnable_snapshot_by_alias_can_be_resolved Verifies SnapshotManager::load_runnable() resolves alias-based lookups into runnable snapshot state in one step.
delete_committed_snapshot_by_alias_removes_it_from_repository Verifies deleting a committed snapshot by alias removes the durable record.
snapshot_rebuild_from_committed_snapshot_preserves_base_state Verifies rebuilding from a committed snapshot preserves the base filesystem state while adding new changes.
rebuild_merges_env_metadata_and_list_filter_can_select_by_id Verifies rebuilds merge environment metadata correctly and that committed snapshot listing can filter by snapshot id.

`integration/orchestrator.rs`

orchestrator_lifecycle Verifies the orchestrator can create, fetch, list, filter, keep alive, and delete template-backed sandboxes while keeping proxy lookup state in sync.

4) End-to-End Execution Checklist

Provide Firecracker binary, kernel, tools drive, overlaybd runtime, and ublk daemon. These are automatically downloaded when the server starts, or you can run cargo run --bin server -- --setup-only to provision runtime dependencies independently.
Provision host access once as root with server --setup-host --runtime-user <user> --runtime-group <group>. The group is the runtime service group: it owns AgentENV state and receives ublk device access. Normal server startup performs validation only and never invokes sudo.
Host setup installs a udev rule for /dev/ublk-control, /dev/ublkc*, and /dev/ublkb*, so the runtime group can access the control and dynamic device nodes.
Update config/default.toml paths, or point AENV_CONFIG_PATH to a custom config file.
Ensure /dev/kvm is accessible by the runtime user and the configured virtualization mode matches the host modules.
If you run template tests, ensure the host can run regctl (server setup installs it automatically from the [regclient] manifest entry) and access the registry for template fromImage resolution.
Run scripts/tests/e2e/run_e2e.sh for API-level E2E coverage. The runner exports E2E_TEMPLATE_USER_IMAGE. Suite 05_template_lifecycle.sh also creates a template build with E2E_SHORT_USER_IMAGE to verify short-name image resolution.
Run make test-agent-integration to run the agentenv integration test modules in tests/integration/ as a non-root user with the required capabilities, plus the Docker/MinIO-backed OSS snapshot repository test (crates/e2e-tests/tests/snapshot_oss_e2e_test.rs).
Run make test-ublk to run the uvm-ublk and overlaybd storage tests, including the Docker/MinIO-backed OSS backend test (storage/overlaybd/tests/oss_backend_minio.rs).

4.1) Firecracker Client and Runtime Upgrade Checklist

When bumping AgentENV to a patched Firecracker runtime, update both the generated client API and the binary that setup downloads:

Replace thirdparty/firecracker-client/firecracker.yaml with the target Firecracker OpenAPI spec.
Run make firecracker-client from the repository root and review generated changes under thirdparty/firecracker-client/.
Build the patched Firecracker release binary on a Linux host. Package it as a gzip tar archive named firecracker-{version}-{arch}.tgz containing a firecracker executable.
Update the [firecracker] entry in config/deps_manifest.toml with the new version and a download URL template that supports {version} and {arch}.
Before relying on the URL in tests, verify it is a direct download:
```
curl -L "<url>" -o /tmp/firecracker.tgz
tar -tzf /tmp/firecracker.tgz
```
The archive listing should include firecracker.
Run setup against a clean or temporary Firecracker dependency directory so it really downloads the archive:
```
cargo run --bin server -- --config /path/to/config.toml --setup-only
```

Validate the Rust side after code generation:

cargo fmt --check -p firecracker_client -p agentenv
cargo check -p firecracker_client
cargo check -p agentenv
cargo test -p agentenv --lib

5) Running Custom Scripts

Run commands via envd

Once the sandbox is started (with a rootfs that includes the envd daemon), you can execute commands inside the VM using the process API:

#![allow(unused)]
fn main() {
// Simple command
let output = sandbox.run_command("ls", &["-la", "/tmp"]).await?;
println!("exit={} stdout={}", output.exit_code, output.stdout);

// Command with options
use agentenv::sandbox::ProcessOpts;
let opts = ProcessOpts::new()
    .with_cwd("/tmp")
    .with_envs([("KEY".into(), "value".into())].into());
let output = sandbox.run_command_with_opts("env", &[], opts).await?;

// Interactive / long-running process
let mut handle = sandbox.start_process("cat", &[], ProcessOpts::default()).await?;
handle.send_stdin(b"hello\n").await?;
handle.kill().await?;
let output = handle.wait().await?;
}

The process API uses envd’s gRPC ProcessClient under the hood. It requires the envd daemon to be running inside the guest VM.

Run different scripts for multiple snapshot resumes

/init is not re-executed after resume, so per-instance behavior must come from per-instance disk contents.

A typical pattern:

Start a base VM and pause() to create a snapshot.
For each instance, copy the snapshot rootfs and write a per-instance file into it (for example using debugfs).
Create a FirecrackerSnapshotConfig that points to that per-instance rootfs and resume.

The guest must keep a long-running dispatcher (started during the initial boot) that watches for these per-instance files and executes them after resume.

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