While a node fails a version alone, no further node gets a want for it (ADR 0557 §2, issue 1496)

ADR 0557 version 9: a failure that is one node's holds nothing and sets nothing
back, but the version reaches no node that does not want it yet until it is
over. A node whose want does not name F gets no new want naming it (rule 2, in
words naming the failing nodes); wants already written stay.

modules/mesh-delivery-assigner/internal/assign/wants.go

@@ -2,6 +2,7 @@ package assign
 
 import (
 	"fmt"
+	"maps"
 	"slices"
 	"sort"
 	"strings"
@@ -315,6 +316,11 @@ func Wants(m Module, n Nodes, wants map[string]delivery.Want, lastHealthy map[st
 				ok, target = true, have.FP
 			}
 		}
+		if ok && target == m.FP && len(out.NodeFailed) > 0 && !(had && !before.Removed && before.FP == m.FP) {
+			// While a node fails F alone, F reaches no further node: a node whose want does not name F yet gets no new want
+			// naming it, and wants already written stay (ADR 0557 §2).
+			ok, rule, words = false, delivery.RuleNotHeld, FailedAloneWords(m.Name, m.FP, slices.Sorted(maps.Keys(out.NodeFailed)))
+		}
 		switch {
 		case ok:
 			d, _ := m.composition(node, target)
@@ -460,6 +466,12 @@ func failedElsewhere(m Module, n Nodes, node string) bool {
 	return false
 }
 
+// FailedAloneWords say why a node gets no new want naming fp: fp fails its health check on the nodes named, alone.
+func FailedAloneWords(module, fp string, failing []string) string {
+	return fmt.Sprintf("%s at %s fails its health check on %s, so no further node moves to it until that is over", module, short(fp),
+		strings.Join(failing, ", "))
+}
+
 // NodeFailure is a failed health check of a module's newest fingerprint on one node while it runs healthy on Others: the have's
 // health, phase and words (HeldWords without a time), and the healthy witnesses by name.
 type NodeFailure struct {

modules/mesh-delivery-assigner/internal/assign/wants_test.go

@@ -164,10 +164,29 @@ func TestAFailedHealthHoldsAndSetsBack(t *testing.T) {
 		t.Errorf("node failed = %+v", out.NodeFailed)
 	}
 
-	// ADR 0557 §2: a witness that is offline keeps the have it last wrote and witnesses nothing (D1); F failing on a second node is
-	// not one node's failure (D3). Either way F is held and ace is set back.
+	// ADR 0557 §2 (version 9): while one node fails F alone, novox, whose want names fOld, gets no new want naming F; the wants
+	// at F stay. The canary ace runs F healthy here and g14 fails it, so the canary rule (3) would let novox move: only §2 holds it.
 	three := m
 	three.Nodes = []string{"ace", "g14", "novox"}
+	failedHave := n.Haves["ace"]["mail"]
+	n.Haves["ace"]["mail"], n.Haves["g14"]["mail"] = n.Haves["g14"]["mail"], failedHave
+	n.Online["novox"] = true
+	n.Haves["novox"] = map[string]delivery.Have{"mail": {FP: fOld, Phase: delivery.PhaseRunning, Health: delivery.HealthOK}}
+	before["novox"] = delivery.Want{FP: fOld, Composed: dig(fOld)}
+	out = Wants(three, n, before, map[string]string{"ace": fOld, "g14": fOld, "novox": fOld})
+	if _, alone := out.NodeFailed["g14"]; !alone {
+		t.Errorf("g14 does not fail alone: %+v", out.NodeFailed)
+	}
+	if w, written := out.Wants["novox"]; out.Held != nil || len(out.NodeFailed) != 1 || (written && w.FP == fNew) || out.Rules["novox"].Rule != delivery.RuleNotHeld {
+		t.Errorf("held %+v, node failed %+v, novox's want %+v (%+v): want novox kept off F", out.Held, out.NodeFailed, out.Wants["novox"], out.Rules["novox"])
+	}
+	if _, written := out.Wants["g14"]; written {
+		t.Errorf("g14's want at F was rewritten: %+v", out.Wants["g14"])
+	}
+	n.Haves["ace"]["mail"], n.Haves["g14"]["mail"] = failedHave, n.Haves["ace"]["mail"]
+
+	// ADR 0557 §2: a witness that is offline keeps the have it last wrote and witnesses nothing (D1); F failing on a second node is
+	// not one node's failure (D3). Either way F is held and ace is set back.
 	for _, c := range []struct {
 		name   string
 		online map[string]bool