type Clock interface {
    // After waits for the duration to elapse and then sends the
    // current time on the returned channel.
    After(time.Duration) <-chan time.Time
}

type Config struct {
    // UpgradeComplete is a lock used to synchronise workers that must start
    // after database upgrades are verified as completed.
    UpgradeComplete gate.Lock

    // Tag is the current machine tag.
    Tag names.Tag

    // agent is the running machine agent.
    Agent agent.Agent

    // Logger is the logger for this worker.
    Logger Logger

    // Open state is a function pointer for returning a state pool indirection.
    OpenState func() (Pool, error)

    // PerformUpgrade is a function pointer for executing the DB upgrade steps.
    // Context retrieval is lazy because because it requires a real
    // state.StatePool that we cast our Pool indirection back to.
    // We need the concrete type, because we are unable to indirect all the
    // state methods that upgrade steps might require.
    // This is OK for in-theatre operation, but is not suitable for testing.
    PerformUpgrade func(version.Number, []upgrades.Target, func() upgrades.Context) error

    // RetryStrategy is the strategy to use for re-attempting failed upgrades.
    RetryStrategy utils.AttemptStrategy

    // Clock is used to enforce time-out logic for controllers waiting for the
    // master MongoDB upgrades to execute.
    Clock Clock
}

type Logger interface {
    Debugf(message string, args ...interface{})
    Infof(message string, args ...interface{})
    Errorf(message string, args ...interface{})
}

type ManifoldConfig struct {
    AgentName         string
    UpgradeDBGateName string
    Logger            Logger
    OpenState         func() (*state.StatePool, error)
    Clock             Clock
}

type Pool interface {
    // IsPrimary returns true if the Mongo primary is
    // running on the machine with the input ID.
    IsPrimary(string) (bool, error)

    // SetStatus updates the status of the machine with the input ID.
    SetStatus(string, status.Status, string) error

    // EnsureUpgradeInfo ensures that a document exists in the "upgradeInfo"
    // collection for coordinating the current upgrade.
    EnsureUpgradeInfo(string, version.Number, version.Number) (UpgradeInfo, error)

    // Close closes the state pool.
    Close() error
}

type UpgradeInfo interface {
    // Status returns the current status of the upgrade.
    Status() state.UpgradeStatus

    // SetStatus sets the current status of the upgrade.
    SetStatus(status state.UpgradeStatus) error

    // Watch returns a watcher that notifies of changes to the UpgradeInfo.
    Watch() state.NotifyWatcher

    // Refresh refreshes the UpgradeInfo from state.
    Refresh() error
}