Make a seat pass no repair, read every machine before passing, keep the gate's pass in memory (issue 1317, review round 1)

internal/inventory/chain.go

@@ -85,6 +85,7 @@ func (i *Inventory) UseChain(c *chain.Chain) {
 // 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.MarkThrowaway()
 	i.UseChain(chain.InMemory(nil))
 	i.markLaidOut()
 }
@@ -129,3 +130,9 @@ func (i *Inventory) theChain(ctx context.Context) (*chain.Chain, error) {
 	}
 	return c, nil
 }
+
+// MarkThrowaway says this inventory is a mesh raised for a moment: a composition on it writes nothing outside it.
+func (i *Inventory) MarkThrowaway() { i.throwaway = true }
+
+// Throwaway says whether this inventory is a mesh raised for a moment (UseThrowawayChain).
+func (i *Inventory) Throwaway() bool { return i != nil && i.throwaway }

internal/inventory/nodes.go

@@ -39,6 +39,9 @@ type Inventory struct {
 	state *chainState
 	// recorded, when set, takes the durations a save of the chain measures in place of the store: a test with no store.
 	recorded func(context.Context, Duration) error
+	// throwaway is set on an inventory raised for a moment beside the real mesh (the merge gate's): nothing it
+	// composes is to be written to the store or the bus.
+	throwaway bool
 }
 
 // Open connects to the inventory store.