mesh-controller/internal/inventory/chain.go

package inventory

import (
	"context"
	"errors"
	"fmt"
	"sync"

	"github.com/novox/mesh-controller/internal/chain"
)

// **The chain lives on the bus** (novox/hq ADR 0408, design 47 milestone 5): builds, rollouts and assignments are keys of the
// bus's key-value store, the controller's store keeps none of them, and every method of a kind reads and writes the chain this
// inventory is given (theChain).

// testChains are the chains in memory a test made for its store, by the store's address: every inventory opened on that
// store is given the same chain, as every process of a mesh is on the same bus. Empty outside a test.
var testChains sync.Map

// ErrNoChain is a read or write of the chain by an inventory that has none attached: a process that did not open the bus.
// Refused loudly.
var ErrNoChain = errors.New("this kind of record lives on the bus, and this process has not opened the bus")

// chainState is what an inventory keeps of the chain it is given.
type chainState struct {
	mu   sync.Mutex
	open ChainOpener
	// view is the rollouts bucket kept in memory by one watch (issue 1092); pending are the merges added to a plan that
	// is not saved yet.
	view    *rolloutView
	pending map[string]pendingMerge
	// requireLayout is set for a chain on the bus: its builds are read only once the controller's start has laid them out
	// under their modules (issue 1121), and laid is that seen.
	requireLayout, laid bool
}

// RequireLayout makes every read of the chain refuse (ErrNotLaidOut) until the builds are laid out under their modules.
func (i *Inventory) RequireLayout() {
	i.state.mu.Lock()
	i.state.requireLayout = true
	i.state.mu.Unlock()
}

func (i *Inventory) markLaidOut() {
	i.state.mu.Lock()
	i.state.laid = true
	i.state.mu.Unlock()
}

// ChainOpener opens the chain on the bus when a process first needs it: a command that reads and never moved anything has
// no bus connection until a kind it reads turns out to be moved.
type ChainOpener func(ctx context.Context) (*chain.Chain, error)

// UseChainOpener gives the inventory the means to open the chain when it needs it and none is attached.
func (i *Inventory) UseChainOpener(open ChainOpener) {
	i.state.mu.Lock()
	i.state.open = open
	i.state.requireLayout = true
	i.state.mu.Unlock()
}

// UseChain attaches the chain the moved kinds are read and written through. The serving controller attaches the bus's,
// holding the lease for its writes; a test attaches one in memory.
func (i *Inventory) UseChain(c *chain.Chain) {
	i.state.mu.Lock()
	defer i.state.mu.Unlock()
	i.chain = c
	if i.state.view != nil {
		i.state.view.stop()
		i.state.view = nil
	}
}

// UseThrowawayChain gives the inventory a chain in memory that is its own, laid out already: for a mesh raised for a moment
// beside the real one, as the merge gate raises the facts snapshot's mesh in a throwaway store (issue 1121). Nothing of it
// reaches the bus.
func (i *Inventory) UseThrowawayChain() {
	i.UseChain(chain.InMemory(nil))
	i.markLaidOut()
}

// attached is the chain this inventory has, nil when none.
func (i *Inventory) attached() *chain.Chain {
	i.state.mu.Lock()
	defer i.state.mu.Unlock()
	return i.chain
}

// theChain is the chain this inventory writes and reads, opened if a command has not yet. On the bus it is refused until the
// builds are laid out under their modules.
func (i *Inventory) theChain(ctx context.Context) (*chain.Chain, error) {
	i.state.mu.Lock()
	if i.chain == nil && i.state.open != nil {
		c, err := i.state.open(ctx)
		if err != nil {
			i.state.mu.Unlock()
			return nil, fmt.Errorf("%w: %v", ErrNoChain, err)
		}
		i.chain = c
	}
	c, check := i.chain, i.state.requireLayout && !i.state.laid
	i.state.mu.Unlock()
	if c == nil {
		return nil, ErrNoChain
	}
	if check {
		if _, found, err := c.Builds.GetRaw(ctx, chain.LayoutMarkerKey); err != nil {
			return nil, err
		} else if !found {
			return nil, ErrNotLaidOut
		}
		i.markLaidOut()
	}
	return c, nil
}