A holder by derivation is recorded before an assignment can unsettle it (hq 170)

`assign ace postgres` made the control plane's own store unresolvable:
postgres's manifest claims mesh-store, nobody was ever recorded as its
holder, and with two eligible assignments and nothing on record both
claimed and both were refused — novox's included. ADR 0110 says the
assignment holds, by a deliberate act; ADR 0131 gave the record its force
but left a seat nobody handed over held by whichever assignment happened
to be alone.

Before acting on an assignment the controller now writes the derived
answer down: every mesh-scoped seat the store knows, resolved to exactly
one holder with nothing on record, gets that holder recorded — the same
record `seat <name> --to <node>/<module>` makes by hand. The next
assignment able to hold the seat then stands beside the holder, eligible
and silent. A seat with two derived claimants is left for a person; a
seat on record is never rewritten; seats the store does not list stay
held by derivation as before. And the refusal, when it still happens,
names the handover that records the holder.

Verified: catalogue tests against the real catalogue; the cmd suite
against a store (the only failure, TestConvergingPreviewsThenChanges…,
fails identically on main).

cmd/mesh-controller/acts.go

@@ -46,6 +46,13 @@ func assign(ctx context.Context, open *stores, node, module string) (string, err
 		return "", err
 	}
 	defer release()
+	// **Before the new assignment can unsettle a seat somebody holds only by being alone**
+	// (novox/hq 04-ISSUES/170): what the mesh derived so far is written down, and then the
+	// assignment resolves against a record rather than against a coincidence.
+	settled, err := recordDerivedHolders(ctx, open)
+	if err != nil {
+		return "", err
+	}
 	fresh, err := open.inventory.Assign(ctx, node, module)
 	if err != nil {
 		return "", err
@@ -57,6 +64,9 @@ func assign(ctx context.Context, open *stores, node, module string) (string, err
 			node, module), nil
 	}
 	said := fmt.Sprintf("%s is assigned %s", node, module)
+	for _, line := range settled {
+		said += "\n  " + line
+	}
 	plan, _, err := planFor(ctx, open, node)
 	if err != nil {
 		// Kept, and still refused. Both halves are the answer, and the rest of the mesh is still

cmd/mesh-controller/holdings.go

@@ -0,0 +1,92 @@
+package main
+
+import (
+	"context"
+	"fmt"
+	"sort"
+
+	"github.com/novox/mesh-controller/internal/catalogue"
+)
+
+// recordDerivedHolders writes down who holds each mesh-scoped seat that nobody was ever recorded
+// as holding.
+//
+// **A seat held by derivation is a seat held by accident of being alone** (novox/hq
+// 04-ISSUES/170). ADR 0131 lets a holder on record settle a seat, and lets any other assignment
+// whose module could hold it stand beside the holder, eligible and silent. But a seat nobody
+// ever handed over has no record, so its holder is whichever assignment happened to be the sole
+// claimant — and the day a second one is assigned, both claim, both are refused, and the first
+// one's whole machine stops resolving. That is what assigning a second postgres did to the
+// control plane's own store.
+//
+// So the mesh writes the derived answer down before it acts on an assignment: for every
+// mesh-scoped seat with exactly one resolved holder and nothing on record, that holder is
+// recorded as the standing one — the same record `seat <name> --to <node>/<module>` makes by
+// hand, made from what the mesh already resolved. A seat with two derived claimants is left
+// alone: that is the ambiguity a person settles, and recording either would be guessing.
+//
+// Node-scoped seats are untouched: a record is one holder per seat, and a node-scoped seat has
+// one holder per machine (ADR 0121), so there is nothing for a record to settle there.
+func recordDerivedHolders(ctx context.Context, open *stores) ([]string, error) {
+	inv := open.inventory
+	shelf, err := inv.Catalogue(ctx)
+	if err != nil {
+		return nil, err
+	}
+	// exclude nobody: every node's claims, resolved with the holdings on record.
+	world, err := theRestOfTheMesh(ctx, inv, shelf, "")
+	if err != nil {
+		return nil, err
+	}
+	recorded, err := inv.Holdings(ctx)
+	if err != nil {
+		return nil, err
+	}
+	// A record is a row against a seat the store knows. A seat it does not — a mesh whose seats
+	// were never seeded, a seat a module declares for itself — stays held by derivation, as it
+	// always was; a missing row is not a reason an assignment fails.
+	known, err := inv.Seats(ctx)
+	if err != nil {
+		return nil, err
+	}
+	recordable := map[string]bool{}
+	for _, s := range known {
+		recordable[s.Name] = true
+	}
+	onRecord := map[string]bool{}
+	for _, h := range recorded {
+		if s, ok := catalogue.SeatNamed(h.Claim); ok {
+			onRecord[s.Name] = true
+		}
+	}
+	holders := map[string][]catalogue.Held{}
+	for _, h := range world.Held {
+		if h.Scope != catalogue.ScopeMesh {
+			continue
+		}
+		s, ok := catalogue.SeatNamed(h.Claim)
+		if !ok || onRecord[s.Name] || !recordable[s.Name] {
+			continue
+		}
+		holders[s.Name] = append(holders[s.Name], h)
+	}
+	names := make([]string, 0, len(holders))
+	for name := range holders {
+		names = append(names, name)
+	}
+	sort.Strings(names)
+	var said []string
+	for _, name := range names {
+		if len(holders[name]) != 1 {
+			continue
+		}
+		h := holders[name][0]
+		if err := inv.HoldSeat(ctx, name, catalogue.ScopeMesh, h.Node, h.Module); err != nil {
+			return said, err
+		}
+		said = append(said, fmt.Sprintf(
+			"recorded %s on %s as the standing holder of %s, which it held only by being alone",
+			h.Module, h.Node, name))
+	}
+	return said, nil
+}

cmd/mesh-controller/holdings_test.go

@@ -0,0 +1,110 @@
+package main
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/novox/mesh-controller/internal/catalogue"
+)
+
+// A seat nobody ever handed over is held by whichever assignment happened to be alone — and the
+// day a second module able to hold it is assigned, both claimed, both were refused, and the first
+// one's machine stopped resolving (novox/hq 04-ISSUES/170). The mesh now writes the derived holder
+// down before it acts, so the second assignment stands beside the holder on record.
+
+func aSeatedStore() catalogue.Manifest {
+	return catalogue.Manifest{Module: "store", Version: "1",
+		Provides: []catalogue.Offer{{Name: "postgres-database", Scope: catalogue.ScopeMesh}},
+		Serves:   map[string]map[string]any{"postgres-database": {"port": 5432}},
+		Claims:   []catalogue.Claim{{Name: "mesh-store", Scope: catalogue.ScopeMesh}}}
+}
+
+func TestASecondEligibleHolderStandsBesideTheOneHeldByBeingAlone(t *testing.T) {
+	open := aMesh(t)
+	ctx := t.Context()
+	// Every deploy seeds the mesh's own seats; a record is a row against one of them.
+	if _, err := open.inventory.SeedSeats(ctx, catalogue.DefaultSeats()); err != nil {
+		t.Fatal(err)
+	}
+	register(t, open, aSeatedStore())
+
+	if _, err := assign(ctx, open, "anchor", "store"); err != nil {
+		t.Fatal(err)
+	}
+	said, err := assign(ctx, open, "laptop", "store")
+	if err != nil {
+		t.Fatalf("a second store, eligible for the seat, was refused:\n%s\n%v", said, err)
+	}
+	if strings.Contains(said, "cannot be worked out") {
+		t.Fatalf("assigning a second store unsettled the first one's machine:\n%s", said)
+	}
+	if !strings.Contains(said, "recorded store on anchor as the standing holder of mesh-store") {
+		t.Fatalf("the holder by derivation was not written down:\n%s", said)
+	}
+
+	holdings, err := open.inventory.Holdings(ctx)
+	if err != nil {
+		t.Fatal(err)
+	}
+	var found bool
+	for _, h := range holdings {
+		if h.Claim == "mesh-store" {
+			found = true
+			if h.Node != "anchor" || h.Module != "store" {
+				t.Fatalf("mesh-store is recorded on %s/%s, not on the one that held it", h.Node, h.Module)
+			}
+		}
+	}
+	if !found {
+		t.Fatalf("mesh-store has no holder on record after assigning: %v", holdings)
+	}
+
+	// And the record decides from here: the anchor's plan holds the seat, the laptop's does not.
+	for node, holds := range map[string]bool{"anchor": true, "laptop": false} {
+		plan, _, err := planFor(ctx, open, node)
+		if err != nil {
+			t.Fatalf("%s no longer resolves: %v", node, err)
+		}
+		var claimed bool
+		for _, c := range plan.Claims {
+			if c.Claim == "mesh-store" {
+				claimed = true
+			}
+		}
+		if claimed != holds {
+			t.Fatalf("%s holds mesh-store: %v, want %v", node, claimed, holds)
+		}
+	}
+}
+
+func TestAHolderOnRecordIsNotRewrittenByDerivation(t *testing.T) {
+	open := aMesh(t)
+	ctx := t.Context()
+	if _, err := open.inventory.SeedSeats(ctx, catalogue.DefaultSeats()); err != nil {
+		t.Fatal(err)
+	}
+	register(t, open, aSeatedStore())
+	for _, node := range []string{"anchor", "laptop"} {
+		if _, err := assign(ctx, open, node, "store"); err != nil {
+			t.Fatal(err)
+		}
+	}
+	// A person hands the seat to the laptop. From here the record decides, and what the mesh
+	// derives must never write over it.
+	if err := open.inventory.HoldSeat(ctx, "mesh-store", catalogue.ScopeMesh, "laptop", "store"); err != nil {
+		t.Fatal(err)
+	}
+	said, err := recordDerivedHolders(ctx, open)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(said) != 0 {
+		t.Fatalf("a seat on record was written again from derivation: %v", said)
+	}
+	holdings, _ := open.inventory.Holdings(ctx)
+	for _, h := range holdings {
+		if h.Claim == "mesh-store" && h.Node != "laptop" {
+			t.Fatalf("the record moved to %s", h.Node)
+		}
+	}
+}