Fail a candidate's build when its fetch answers files in conflict or no merge, so the trunk's build is not stored under the candidate's fingerprint (review D4)

modules/mesh-delivery-builder/cmd/mesh-delivery-builder/integration_test.go

@@ -109,7 +109,11 @@ func (f *fakeBuildModules) serve(t *testing.T, nc *nats.Conn, module string) {
 			step.Ended = time.Now().UTC()
 			switch {
 			case strings.HasSuffix(tool, "_fetch"):
-				step.State, step.Result = delivery.StepPassed, json.RawMessage(`{"workspace":"w`+id+`","commit":"`+commit+`"}`)
+				merge := ""
+				if args["base"] != nil { // a fetch with a base answers the merge commit (mesh-build-go FetchResult)
+					merge = `,"merge":"` + strings.Repeat("9", 40) + `"`
+				}
+				step.State, step.Result = delivery.StepPassed, json.RawMessage(`{"workspace":"w`+id+`","commit":"`+commit+`"`+merge+`}`)
 			case f.failBuild.Load() && node == "n1":
 				step.State, step.Reason = delivery.StepFailed, fmt.Sprintf("go build of %v: boom", args["artifact"])
 			default:

modules/mesh-delivery-builder/internal/build/candidates_test.go

@@ -2,6 +2,7 @@ package build
 
 import (
 	"context"
+	"strings"
 	"testing"
 
 	"git.novox.be/novox/mesh-sdk/go/delivery"
@@ -94,3 +95,20 @@ func TestAFingerprintTheTrunkCarriesIsBuiltFromTheTrunksCommit(t *testing.T) { /
 		t.Errorf("build = %+v", v)
 	}
 }
+
+func TestACandidateWhoseHeadDoesNotMergeIsFailedAndNoTrunkBuildIsStoredUnderIt(t *testing.T) { // review D4
+	for name, set := range map[string]func(*rig){
+		"conflict": func(r *rig) { r.bm.conflicts = "go.mod" },
+		"no merge": func(r *rig) { r.bm.noMerge = true },
+	} {
+		r := newRig(t, goBundle)
+		trunkModule(t, r)
+		set(r)
+		r.b.Candidate(context.Background(), "feat%2Fx", candidateValue(t, "feat/x", 7, headX))
+		r.b.Wait()
+		v := valueOf(t, r.st, r.key)
+		if v.State != delivery.BuildFailed || (name == "conflict" && !strings.Contains(v.Error, "go.mod")) || len(r.bm.callsOf("build_go_build")) != 0 {
+			t.Errorf("%s: build = %+v, builds %d, want failed and nothing built", name, v, len(r.bm.callsOf("build_go_build")))
+		}
+	}
+}

modules/mesh-delivery-builder/internal/build/run.go

@@ -524,12 +524,26 @@ func (b *Builder) runLane(ctx context.Context, l *lane, repository string, m del
 		return
 	}
 	var fetched struct {
-		Workspace string `json:"workspace"`
+		Workspace string   `json:"workspace"`
+		Merge     string   `json:"merge"`
+		Conflicts []string `json:"conflicts"`
 	}
 	if err := json.Unmarshal(raw, &fetched); err != nil || fetched.Workspace == "" {
 		l.err = fmt.Errorf("the fetch on %s answered no workspace: %s", l.node, raw)
 		return
 	}
+	// A candidate is built from its head merged onto the trunk. A head that does not merge ends the fetch passed with the tree at
+	// the trunk, and building that would store the trunk's build under the candidate's fingerprint.
+	if _, candidate := fetchArgs["base"]; candidate {
+		if len(fetched.Conflicts) > 0 {
+			l.err = fmt.Errorf("the head does not merge into the trunk on %s: files in conflict: %s", l.node, strings.Join(fetched.Conflicts, ", "))
+			return
+		}
+		if fetched.Merge == "" {
+			l.err = fmt.Errorf("the fetch with a base on %s answered no merge: %s", l.node, raw)
+			return
+		}
+	}
 	for _, t := range l.tasks {
 		raw, err := b.step(ctx, l.module, l.node, "build", map[string]any{"workspace": fetched.Workspace, "module": m.Path, "artifact": t.Artifact})
 		if err != nil {

modules/mesh-delivery-builder/internal/build/run_test.go

@@ -24,8 +24,11 @@ type buildModules struct {
 	digests map[string]string // `<node>/<artifact>` -> digest; the default is `digest`
 	fails   map[string]string // `<node>/<tool>` -> the reason the step fails with
 	// hold, when set before the builds start, blocks every call until it is closed; entered is sent to as a call arrives.
-	hold    chan struct{}
-	entered chan struct{}
+	// conflicts, when set, is a file a fetch with a base reports in conflict; noMerge makes it answer neither a merge nor a conflict.
+	conflicts string
+	noMerge   bool
+	hold      chan struct{}
+	entered   chan struct{}
 }
 
 func newBuildModules() *buildModules {
@@ -52,6 +55,13 @@ func (b *buildModules) CallOn(node, key string, body any) (json.RawMessage, erro
 		step.State, step.Reason = delivery.StepFailed, reason
 	} else if strings.HasSuffix(tool, "_fetch") {
 		step.Result = json.RawMessage(`{"workspace":"w-` + node + `"}`)
+		if args["base"] != nil { // a fetch with a base answers the merge, or the files in conflict (mesh-build-go FetchResult)
+			if b.conflicts != "" {
+				step.Result = json.RawMessage(`{"workspace":"w-` + node + `","base":"` + fmt.Sprint(args["base"]) + `","conflicts":["` + b.conflicts + `"]}`)
+			} else if !b.noMerge {
+				step.Result = json.RawMessage(`{"workspace":"w-` + node + `","base":"` + fmt.Sprint(args["base"]) + `","merge":"` + strings.Repeat("9", 40) + `"}`)
+			}
+		}
 	} else {
 		d := digest
 		if v, ok := b.digests[node+"/"+fmt.Sprint(args["artifact"])]; ok {