banger/internal/daemon/ARCHITECTURE.md

# `internal/daemon` architecture

This document describes the current daemon package layout: the `Daemon`
composition root, the subpackages that own stateless helpers and shared
primitives, and the lock ordering every caller must respect.

## Composition

`Daemon` is the composition root. Subsystem state and locks live on their
owning types:

- Layout, config, store, runner, logger, pid — infrastructure handles.
- `vmLocks vmLockSet` — per-VM `*sync.Mutex`, one per VM ID. Held only
  across short, synchronous state validation and DB mutations so slow
  guest I/O does not block lifecycle ops on the same VM.
- `workspaceLocks vmLockSet` — per-VM mutex scoped to
  `workspace.prepare` / `workspace.export`. Serialises concurrent
  workspace operations on a single VM (two simultaneous tar imports
  would clobber each other) without touching `vmLocks`, so
  `vm stop` / `delete` / `restart` never queue behind a slow import.
- `handles *handleCache` — in-memory map of per-VM transient kernel/
  process handles (PID, tap device, loop devices, DM target). The
  cache is rebuildable: each VM directory holds a small
  `handles.json` scratch file that the daemon reads at startup to
  reconstruct the cache and verify processes against `/proc` via
  pgrep. Nothing in the durable `vms` SQLite row describes transient
  kernel state. See `internal/daemon/vm_handles.go`.
- `createVMMu sync.Mutex` — serialises `CreateVM` (guards name uniqueness
  + guest IP allocation window).
- `imageOpsMu sync.Mutex` — serialises image-registry mutations
  (`PullImage`, `RegisterImage`, `PromoteImage`, `DeleteImage`).
- `createOps opstate.Registry[*vmCreateOperationState]` — in-flight VM
  create operations; owns its own lock.
- `tapPool tapPool` — TAP interface pool; owns its own lock.
- `sessions sessionRegistry` — active guest session controllers; owns
  its own lock.
- `listener`, `vmDNS` — networking.
- `vmCaps` — registered VM capability hooks.
- `pullAndFlatten`, `finalizePulledRootfs`, `bundleFetch`,
  `requestHandler`, `guestWaitForSSH`, `guestDial`,
  `waitForGuestSessionReady` — injectable seams used by tests.

## Subpackages

Pure helpers have moved into subpackages so the daemon package itself stays
focused on orchestration. Each subpackage takes explicit dependencies
(typically a `system.Runner`-compatible interface) and holds no global
state beyond small test seams.

| Subpackage                        | Purpose                                                                |
| --------------------------------- | ---------------------------------------------------------------------- |
| `internal/daemon/opstate`         | Generic `Registry[T AsyncOp]` for async-operation bookkeeping.         |
| `internal/daemon/dmsnap`          | Device-mapper COW snapshot create/cleanup/remove.                      |
| `internal/daemon/fcproc`          | Firecracker process primitives (bridge, tap, binary, PID, kill, wait). |
| `internal/daemon/imagemgr`        | Image subsystem pure helpers: validators, staging, build script gen.   |
| `internal/daemon/session`         | Guest-session helpers: state paths, scripts, parsing, utilities.       |
| `internal/daemon/workspace`       | Workspace helpers: git inspection, copy prep, guest import script.     |

`workspace` imports `session` for `ShellQuote` and `FormatStepError`; all
other subpackages are leaves (no other intra-daemon subpackage imports).

## Lock ordering

Acquire in this order, release in reverse. Never acquire in the opposite
direction.

```
vmLocks[id]  →  workspaceLocks[id]  →  {createVMMu, imageOpsMu}  →  subsystem-local locks
```

`vmLocks[id]` and `workspaceLocks[id]` are NEVER held at the same
time. `workspace.prepare` acquires `vmLocks[id]` just long enough to
validate VM state, releases it, then acquires `workspaceLocks[id]`
for the guest I/O phase.

Subsystem-local locks (`tapPool.mu`, `sessionRegistry.mu`,
`opstate.Registry` mu, `guestSessionController.attachMu` /
`writeMu`) are leaves. They do not contend with each other.

Notes:

- `vmLocks[id]` is the outer lock for any operation scoped to a single VM.
  Acquired via `withVMLockByID` / `withVMLockByRef`.
- `createVMMu` and `imageOpsMu` are narrow: each guards one family of
  mutations and is released before any blocking guest I/O.
- Holding a subsystem-local lock while calling into guest SSH is
  discouraged; copy needed state out under the lock and release before
  blocking I/O.

## External API

Only `internal/cli` imports this package. The surface is:

- `daemon.Open(ctx) (*Daemon, error)`
- `(*Daemon).Serve(ctx) error`
- `(*Daemon).Close() error`
- `daemon.Doctor(...)` — host diagnostics (no receiver).

All other `*Daemon` methods are reached only through the RPC `dispatch`
switch in `daemon.go` and are free to move/rename during refactoring.