A refusal for want of a provider names the modules another node cannot run in this composition, and a refusal naming no placed module is answered at once (issue 1126, probe #532)

ace and g14 answered 'nothing in this mesh provides artifact-store … assign
distribution to a node' while novox runs it: a module another node cannot run
in this composition (catalogue Resolution.Unhostable) offers nothing, silently.
The refusal now names each such module and what it lacks, and each node left
out. And the culprit search composed the whole mesh once per placed module for a
refusal no stub changes (ace's answer took 7m46s): only the modules the refusal
names are tried, at most 8.

go/compose/frombus/answer.go

@@ -212,6 +212,9 @@ func Answer(ctx context.Context, r Reader, mesh Mesh, s Scope, fps map[string]st
 	return a
 }
 
+// maxCulprits bounds the modules one refusal is tried on, each a whole composition.
+const maxCulprits = 8
+
 // MaxShown bounds the bytes of shown resource bodies in one answer (delivery.ModuleDiffers.Bodies).
 const MaxShown = 256 << 10
 
@@ -251,15 +254,12 @@ func culprit(in compose.Input, node string, fps map[string]string, failed map[st
 			assigned = n.Assigned
 		}
 	}
-	// The modules the error names first: the words of a module's refusal open with its name.
-	order := make([]string, 0, len(assigned))
+	// Only the modules the error names: the words of a module's refusal name it. Each try is a whole composition, so a
+	// refusal naming none is no module's, answered at once rather than after a composition per module (issue 1126: ace's
+	// answer took 7m46s).
+	var order []string
 	for _, m := range assigned {
-		if strings.Contains(err.Error(), m) {
-			order = append(order, m)
-		}
-	}
-	for _, m := range assigned {
-		if !strings.Contains(err.Error(), m) {
+		if strings.Contains(err.Error(), m) && len(order) < maxCulprits {
 			order = append(order, m)
 		}
 	}

go/compose/frombus/answer_rows_test.go

@@ -3,6 +3,7 @@ package frombus
 import (
 	"context"
 	"encoding/json"
+	"fmt"
 	"reflect"
 	"slices"
 	"strings"
@@ -107,6 +108,11 @@ func answerNamesEveryRefusalAndLeavesAnOldBuildToTheNode(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
+	// A refusal naming no module placed here is no module's: no compose is spent stubbing each in turn (issue 1126: ace's
+	// answer took 7m46s, one compose per module), though here a stub would compose.
+	if m := culprit(in, "n1", fps, map[string]string{}, fmt.Errorf("nothing in this mesh provides \"artifact-store\"")); m != "" {
+		t.Errorf("a refusal naming no placed module was laid on %s", m)
+	}
 	ownKey, _ := delivery.ComposedNodeKey("n1")
 	own := base[ownKey]
 	carriedRes := []json.RawMessage{
@@ -161,6 +167,32 @@ func answerNamesEveryRefusalAndLeavesAnOldBuildToTheNode(t *testing.T) {
 	}
 }
 
+// A module another node cannot run in this composition offers nothing in it: the refusal names it and why
+// (issue 1126, probe #532: ace and g14 answered "nothing in this mesh provides artifact-store … assign distribution to a
+// node" while novox runs it). And a refusal no module's stub changes is answered at once, not after a compose per module
+// (ace took 7m46s).
+func otherNodeNotResolvedIsNamed(t *testing.T) {
+	ctx := context.Background()
+	prov := []byte(`{"module":"prov","version":"1","capabilities":["container-runtime"],"provides":[{"name":"thing","scope":"mesh"}]}`)
+	cons := []byte(`{"module":"cons","version":"1","requires":["thing"]}`)
+	// n2 runs prov, which needs a container runtime n2's record does not say it has: n2's set does not resolve here.
+	b := &bus{placements: map[string]delivery.Placement{
+		"n1": {Node: "n1", Decided: delivery.NodeDecided{Hub: true, Address: "10.77.0.1", Endpoint: "n1.example:51820"}, Modules: []delivery.PlacedModule{{Module: "cons"}, {Module: overlay.Name}}},
+		"n2": {Node: "n2", Decided: delivery.NodeDecided{Address: "10.77.0.2"}, Modules: []delivery.PlacedModule{{Module: "prov"}, {Module: overlay.Name}}}},
+		records:   map[string]delivery.NodeRecord{"n1": {Node: "n1", TunnelKey: publicKey(t)}, "n2": {Node: "n2", TunnelKey: publicKey(t)}},
+		modules:   map[string]delivery.Module{"prov": {Manifest: delivery.ManifestDigest(prov)}, "cons": {Manifest: delivery.ManifestDigest(cons)}},
+		manifests: map[string][]byte{delivery.ManifestDigest(prov): prov, delivery.ManifestDigest(cons): cons}}
+	given := map[string]*delivery.Placement{}
+	for n, p := range b.placements {
+		p := p
+		given[n] = &p
+	}
+	a := Answer(ctx, b, Mesh{}, Scope{Node: "n1", Placements: given}, map[string]string{"cons": "src2:" + strings.Repeat("1", 64)}, nil, "h", 1)
+	if a.Verdict != delivery.CandidateRefused || !strings.Contains(a.Refused, "prov on n2 lacks container-runtime") {
+		t.Errorf("a refusal for want of n2's provider = %+v", a)
+	}
+}
+
 // A node that reaches no artifact store says what its composition holds of one (issue 1126: ace and g14 answered
 // "reaches no artifact store", and nothing said why).
 func noStoreReachedSaysWhy(t *testing.T) {

go/compose/frombus/frombus_test.go

@@ -123,6 +123,7 @@ func TestInput(t *testing.T) {
 	// saying why (answer_rows_test.go).
 	t.Run("every refusal, and an old build left to the node", answerNamesEveryRefusalAndLeavesAnOldBuildToTheNode)
 	t.Run("no store reached says why", noStoreReachedSaysWhy)
+	t.Run("another node not resolved is named", otherNodeNotResolvedIsNamed)
 	ctx := context.Background()
 	xterm, err := os.ReadFile("../../catalogue/testdata/catalogue-835bf22b-xterm.module.json")
 	if err != nil {

go/compose/network.go

@@ -24,6 +24,9 @@ type composer struct {
 	in   Input
 	said []string
 	seen map[string]bool
+	// unhostable is, from the last look at the rest of the mesh, each module assigned to another node that this
+	// composition finds that node cannot run: what it would have offered is missing here (issue 1126, probe #532).
+	unhostable []string
 }
 
 func newComposer(in Input) *composer { return &composer{in: in, seen: map[string]bool{}} }

go/compose/resolve.go

@@ -3,6 +3,7 @@ package compose
 import (
 	"fmt"
 	"slices"
+	"strings"
 
 	"git.novox.be/novox/mesh-sdk/go/catalogue"
 )
@@ -203,7 +204,7 @@ func (c *composer) resolution(nodeName string) (catalogue.Resolution, error) {
 		return catalogue.Resolution{}, err
 	}
 	shelf := c.shelfFor(node)
-	world, _, _, err := c.theRestOfTheMesh(nodeName)
+	world, _, left, err := c.theRestOfTheMesh(nodeName)
 	if err != nil {
 		return catalogue.Resolution{}, err
 	}
@@ -226,6 +227,19 @@ func (c *composer) resolution(nodeName string) (catalogue.Resolution, error) {
 			Account: node.Account, AccountHome: node.AccountHome,
 			AgentAccount: node.AgentAccount, AgentAccountHome: node.AgentAccountHome}, world)
 	if err != nil {
+		// What another node's set would have offered is missing when that set does not resolve here: said with the
+		// refusal, so "nothing provides" is read beside why (issue 1126, probe #532).
+		if len(left) > 0 {
+			var why []string
+			for _, l := range left {
+				why = append(why, l.String())
+			}
+			err = fmt.Errorf("%w\n\nand %d node(s) offer nothing here, as they could not be resolved: %s", err, len(left), strings.Join(why, "; "))
+		}
+		if len(c.unhostable) > 0 {
+			err = fmt.Errorf("%w\n\nand these modules offer nothing here, as their node cannot run them in this composition: %s",
+				err, strings.Join(c.unhostable, "; "))
+		}
 		// The node's own set does not compose: the only failure a mesh-wide gatherer may pass over.
 		return catalogue.Resolution{}, notResolvable{err}
 	}

go/compose/rest.go

@@ -4,6 +4,7 @@ import (
 	"errors"
 	"fmt"
 	"sort"
+	"strings"
 
 	"git.novox.be/novox/mesh-sdk/go/catalogue"
 )
@@ -100,6 +101,7 @@ func (c *composer) theRestOfTheMesh(exclude string) (catalogue.World, Passes, []
 	var left []LeftOut
 	offered := map[string][]catalogue.Provider{}
 	var firstHeld []catalogue.Held
+	c.unhostable = nil
 	for _, o := range others {
 		got, err := catalogue.Resolve(shelf, o.assigned, o.node, catalogue.World{Unchecked: true, Holdings: holdings})
 		if err != nil {
@@ -107,6 +109,9 @@ func (c *composer) theRestOfTheMesh(exclude string) (catalogue.World, Passes, []
 			left = append(left, LeftOut{Node: o.node.Name, Err: err})
 			continue
 		}
+		for _, u := range got.Unhostable {
+			c.unhostable = append(c.unhostable, fmt.Sprintf("%s on %s lacks %s", u.Module, o.node.Name, strings.Join(u.Missing, ", ")))
+		}
 		firstHeld = append(firstHeld, got.Claims...)
 		for _, m := range got.Modules {
 			for _, name := range m.OffersAt(catalogue.ScopeMesh) {