Hold the delivery planner to its recorded rules with a table and a property

A table of merges (two repositories, every edge kind, a diamond, a cycle, files no build reads) and a seeded property over 500 random catalogues pin what a merge moves and in which tiers, per ADR 0162 and ADR 0238 §3. The shared-repository rows document today's behaviour that issue 338 would change, once with hand edges and once with edges derived from the store. The cycle check fails on main: hasCycle reads a packages edge between two modules of the last tier as a cycle, so a plan with none is said to have one. Left failing, marked BUG, for the planner's fix.
2026-10-08 22:15:20 +02:00
parent 8170fc58a3
commit 8ca4b04321
2 changed files with 584 additions and 0 deletions
@@ -0,0 +1,131 @@
+package main
+
+import (
+	"reflect"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/novox/mesh-controller/internal/catalogue"
+	"github.com/novox/mesh-controller/internal/inventory"
+)
+
+// **The planner over edges the store derives**, not edges written by hand: modules registered, builds
+// recorded with what they stood on and which repositories they read, the relation answered by
+// inventory.Dependencies (dependenciesOf over the records), and the merge planned by reachOfMerge — the
+// path a real merge takes, short of the bus.
+//
+// **The repository rows are CURRENT BEHAVIOUR, documented — not the rule the operator states**
+// (novox/hq issue 338, and the decision pending on it): a build that read a repository gives its module a
+// packages edge to every module built from that repository, and mergeCandidates moves it on any merge to
+// that repository, whatever the files. So a change to C alone, or to a README, moves the module that
+// packages C's repository. ADR 0238 §3 records exactly that today ("a repository a recipe names"); the
+// expectations marked 338 change with that decision.
+func TestASharedRepositoryIsPlannedFromTheRecordsAsItIsToday(t *testing.T) {
+	inv := inventory.ForTest(t)
+	ctx := t.Context()
+	asked := time.Now().Add(-time.Hour)
+	register := func(m catalogue.Manifest, repository, path string, against []string, read []inventory.ReadRepository) {
+		t.Helper()
+		if err := inv.RegisterModule(ctx, m, inventory.Source{Repository: repository, Seat: "git", Path: path,
+			Ref: "main", BuiltFrom: "old", Asked: asked}); err != nil {
+			t.Fatal(err)
+		}
+		if err := inv.RecordBuild(ctx, inventory.Build{ID: "build-" + m.Module, Repository: repository, Ref: "main",
+			Module: m.Module, Commit: "old", On: "builder", Path: path, Against: against, Read: read, Asked: asked}); err != nil {
+			t.Fatal(err)
+		}
+	}
+	controllerRead := []inventory.ReadRepository{{Repository: "novox/mesh-controller", Ref: "main"}}
+	agent := catalogue.Manifest{Module: "build-agent", Version: "1",
+		Claims: []catalogue.Claim{{Name: "node-build-agent", Scope: catalogue.ScopeNode}}}
+
+	// The shape of issue 338.
+	register(catalogue.Manifest{Module: "mesh-controller", Version: "1"}, "novox/mesh-controller", "", nil, nil)
+	register(agent, "novox/mesh-catalog", "modules/build-agent", nil, controllerRead)
+	register(catalogue.Manifest{Module: "route-proxy", Version: "1"}, "novox/mesh-catalog", "modules/route-proxy", nil, controllerRead)
+	register(catalogue.Manifest{Module: "gitea", Version: "1"}, "novox/mesh-catalog", "modules/gitea", nil, nil)
+	// A, B and C in one repository; D built against A's artifact, E declaring B, P packaging the repository.
+	for _, n := range []string{"a", "b", "c"} {
+		register(catalogue.Manifest{Module: n, Version: "1"}, "novox/one", "modules/"+n, nil, nil)
+	}
+	register(catalogue.Manifest{Module: "d", Version: "1"}, "novox/two", "d",
+		[]string{catalogue.ArtifactStoreScheme + "a/runtime@sha256:" + strings.Repeat("0", 64)}, nil)
+	register(catalogue.Manifest{Module: "e", Version: "1", Build: &catalogue.Build{
+		On: []catalogue.BuildsOn{{Arg: "BASE", Module: "b", Artifact: "runtime"}}}}, "novox/two", "e", nil, nil)
+	register(catalogue.Manifest{Module: "p", Version: "1"}, "novox/two", "p", nil,
+		[]inventory.ReadRepository{{Repository: "novox/one", Ref: "main"}})
+
+	entries, err := inv.Catalogued(ctx)
+	if err != nil {
+		t.Fatal(err)
+	}
+	read, err := inv.ReadRepositories(ctx)
+	if err != nil {
+		t.Fatal(err)
+	}
+	edges, err := inv.Dependencies(ctx)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// The edges the hand-written rows of TestASharedRepositoryMovesWhatPackagesItAsItDoesToday use are
+	// the ones derived here.
+	var shared []inventory.Edge
+	in338 := map[string]bool{"mesh-controller": true, "build-agent": true, "route-proxy": true, "gitea": true}
+	for _, e := range edges {
+		if in338[e.From] && in338[e.To] {
+			shared = append(shared, e)
+		}
+	}
+	if !reflect.DeepEqual(shared, sharedRepositoryEdges) {
+		t.Errorf("derived %v\nthe hand-written rows use %v", shared, sharedRepositoryEdges)
+	}
+	// Each kind derived from its record: built against (stands-on), build.on (declared), read (packages).
+	for _, want := range []inventory.Edge{
+		dep("d", inventory.EdgeStandsOn, "a"),
+		dep("e", inventory.EdgeDeclared, "b"),
+		dep("p", inventory.EdgePackages, "a"),
+		dep("p", inventory.EdgePackages, "b"),
+		dep("p", inventory.EdgePackages, "c"),
+		dep("d", inventory.EdgeBuiltBy, "build-agent"),
+	} {
+		found := false
+		for _, e := range edges {
+			found = found || e == want
+		}
+		if !found {
+			t.Errorf("no %s %s %s derived: %v", want.From, want.Kind, want.To, edges)
+		}
+	}
+
+	for _, c := range []struct {
+		what, repo string
+		paths      []string
+		want       string
+		issue338   bool
+	}{
+		{"A and B changed, C untouched: D after A, E after B; P packages their repository", "one",
+			[]string{"modules/a/x.go", "modules/b/x.go"}, "a,b,p | d,e", false},
+		{"C alone: C, and P, which packages C's repository", "one",
+			[]string{"modules/c/x.go"}, "c,p", true},
+		{"a README of the repository P packages: P moves, nothing built from it does", "one",
+			[]string{"README.md"}, "p", true},
+		{"the dependent's repository: D alone", "two", []string{"d/main.go"}, "d", false},
+		{"a README of the controller's repository: all three, three tiers", "mesh-controller",
+			[]string{"README.md"}, "mesh-controller | build-agent | route-proxy", true},
+		{"the route proxy's directory in the catalogue: it alone", "mesh-catalog",
+			[]string{"modules/route-proxy/module.json"}, "route-proxy", false},
+		{"the build agent's directory: it alone, nothing it builds", "mesh-catalog",
+			[]string{"modules/build-agent/module.json"}, "build-agent", false},
+	} {
+		_, got := planMerge(t, c.repo, c.paths, entries, read, edges)
+		if got != c.want {
+			tag := ""
+			if c.issue338 {
+				tag = " (current behaviour, issue 338)"
+			}
+			t.Errorf("%s: planned %q, wanted %q%s", c.what, got, c.want, tag)
+		}
+	}
+}
@@ -0,0 +1,453 @@
+package main
+
+import (
+	"fmt"
+	"math/rand/v2"
+	"sort"
+	"strings"
+	"testing"
+
+	"github.com/novox/mesh-controller/internal/inventory"
+	"github.com/novox/mesh-controller/internal/link"
+)
+
+// The delivery planner's rules, as recorded (novox/hq ADR 0162 §1, ADR 0238 §3), held by one table and
+// one property over reachOfMerge — the planner's one answer to "what does this merge move, and in which
+// order". A row that fails here on main is a planner that breaks a recorded rule: the row stays, the
+// expectation is not bent to the code.
+//
+// The kinds of edge, as the code reads them (release_plan.go):
+//
+//	kind        widens the plan   orders the tiers
+//	stands-on   yes               yes, after its base is built
+//	declared    yes               yes, after its base is built
+//	packages    yes               no, the same tier (a code dependency)
+//	built-by    no                yes, after the build machine — except for what the build machine stands
+//	                              on, and for the controller whose worker it binds
+//	worker-of   no                yes, the build seat's holder after the controller (hq issue 206)
+
+const (
+	repoOne = "http://forge.internal:20000/novox/one.git"
+	repoTwo = "http://forge.internal:20000/novox/two.git"
+)
+
+// dep is one edge of the catalogue's relation: from depends on to, in the way kind says.
+func dep(from, kind, to string) inventory.Edge {
+	return inventory.Edge{From: from, To: to, Kind: kind}
+}
+
+// tiered is a plan's tiers as one line: a tier's modules by comma, tiers by " | ". Empty for no plan.
+func tiered(tiers [][]string) string {
+	var out []string
+	for _, t := range tiers {
+		out = append(out, strings.Join(t, ","))
+	}
+	return strings.Join(out, " | ")
+}
+
+// planMerge is what reachOfMerge plans for a merge of these files into a repository's main: its tiers as
+// one line, and the files no build reads. It also holds the plan to its own shape: every module it builds
+// is in exactly one tier.
+func planMerge(t *testing.T, repo string, paths []string, entries []inventory.Entry,
+	read map[string][]inventory.ReadRepository, edges []inventory.Edge) (mergeReach, string) {
+	t.Helper()
+	m := link.SourceMoved{Owner: "novox", Repo: repo, Base: "main", Commit: "head", Paths: paths}
+	r := reachOfMerge(m, entries, read, edges)
+	seen := map[string]int{}
+	for _, tier := range r.Plan.Tiers {
+		for _, name := range tier {
+			seen[name]++
+		}
+	}
+	for name := range r.Plan.Modules {
+		if seen[name] != 1 {
+			t.Errorf("%s/%v: %s is in %d tiers of %v", repo, paths, name, seen[name], r.Plan.Tiers)
+		}
+	}
+	if len(seen) != len(r.Plan.Modules) {
+		t.Errorf("%s/%v: the tiers %v hold modules the plan does not (%d)", repo, paths, r.Plan.Tiers, len(r.Plan.Modules))
+	}
+	return r, tiered(r.Plan.Tiers)
+}
+
+// **A merge moves the modules whose directory it changed, and everything built on them; nothing else.**
+// Each row is a catalogue (modules in directories of one or two repositories), its dependencies with
+// their kinds, the files one commit changed, and the exact plan: the moved set and its tier order.
+func TestAPlanIsWhatTheChangeTouchedAndWhatIsBuiltOnIt(t *testing.T) {
+	in := func(repo, dir string, names ...string) []inventory.Entry {
+		var out []inventory.Entry
+		for _, n := range names {
+			d := dir + "/" + n
+			if dir == "" {
+				d = n
+			}
+			out = append(out, fromRepo(n, repo, d))
+		}
+		return out
+	}
+	// Modules a to f, x and z in novox/one under modules/; g in novox/two at g/.
+	entries := append(in(repoOne, "modules", "a", "b", "c", "d", "e", "f", "x", "z"), in(repoTwo, "", "g")...)
+	const (
+		standsOn = inventory.EdgeStandsOn
+		declared = inventory.EdgeDeclared
+		packages = inventory.EdgePackages
+		builtBy  = inventory.EdgeBuiltBy
+		workerOf = inventory.EdgeWorkerOf
+	)
+	// The two real cycles the kinds resolve themselves (ADR 0162 §1, hq issue 206).
+	runtime := append(in(repoOne, "modules", "runtime", "builder"), fromRepo("controller", repoTwo, ""))
+	for _, c := range []struct {
+		what    string
+		entries []inventory.Entry
+		edges   []inventory.Edge
+		repo    string
+		paths   []string
+		want    string // the tiers, " | " between them
+		unread  string
+		cycle   bool
+	}{
+		// The operator's case: two modules changed, a third beside them in the same repository untouched,
+		// each changed one with a dependent.
+		{what: "A and B changed, C untouched beside them, D on A and E on B",
+			edges: []inventory.Edge{dep("d", standsOn, "a"), dep("e", declared, "b")},
+			repo:  "one", paths: []string{"modules/a/main.go", "modules/b/module.json"}, want: "a,b | d,e"},
+		{what: "only A's directory: A and what stands on it",
+			edges: []inventory.Edge{dep("d", standsOn, "a"), dep("e", declared, "b")},
+			repo:  "one", paths: []string{"modules/a/main.go"}, want: "a | d"},
+		{what: "only C's directory: C alone, nothing is built on it",
+			edges: []inventory.Edge{dep("d", standsOn, "a"), dep("e", declared, "b")},
+			repo:  "one", paths: []string{"modules/c/Dockerfile"}, want: "c"},
+		{what: "only the dependent changed: its base does not move",
+			edges: []inventory.Edge{dep("d", standsOn, "a")},
+			repo:  "one", paths: []string{"modules/d/main.go"}, want: "d"},
+		{what: "transitive: F on D on A, A changed",
+			edges: []inventory.Edge{dep("f", standsOn, "d"), dep("d", standsOn, "a")},
+			repo:  "one", paths: []string{"modules/a/x"}, want: "a | d | f"},
+		{what: "transitive across kinds: F declared on D, D packages A",
+			edges: []inventory.Edge{dep("f", declared, "d"), dep("d", packages, "a")},
+			repo:  "one", paths: []string{"modules/a/x"}, want: "a,d | f"},
+
+		// Each kind alone: X depends on A, A changed (widening), then both changed (ordering).
+		{what: "stands-on (built against A's artifact) widens", edges: []inventory.Edge{dep("x", standsOn, "a")},
+			repo: "one", paths: []string{"modules/a/x"}, want: "a | x"},
+		{what: "stands-on orders", edges: []inventory.Edge{dep("x", standsOn, "a")},
+			repo: "one", paths: []string{"modules/a/x", "modules/x/y"}, want: "a | x"},
+		{what: "declared (build.on) widens", edges: []inventory.Edge{dep("x", declared, "a")},
+			repo: "one", paths: []string{"modules/a/x"}, want: "a | x"},
+		{what: "declared orders", edges: []inventory.Edge{dep("x", declared, "a")},
+			repo: "one", paths: []string{"modules/a/x", "modules/x/y"}, want: "a | x"},
+		{what: "packages widens, into the same tier", edges: []inventory.Edge{dep("x", packages, "a")},
+			repo: "one", paths: []string{"modules/a/x"}, want: "a,x"},
+		{what: "packages does not order", edges: []inventory.Edge{dep("x", packages, "a")},
+			repo: "one", paths: []string{"modules/a/x", "modules/x/y"}, want: "a,x"},
+		{what: "built-by never widens", edges: []inventory.Edge{dep("x", builtBy, "a")},
+			repo: "one", paths: []string{"modules/a/x"}, want: "a"},
+		{what: "built-by orders", edges: []inventory.Edge{dep("x", builtBy, "a")},
+			repo: "one", paths: []string{"modules/a/x", "modules/x/y"}, want: "a | x"},
+		{what: "worker-of never widens", edges: []inventory.Edge{dep("x", workerOf, "a")},
+			repo: "one", paths: []string{"modules/a/x"}, want: "a"},
+		{what: "worker-of orders", edges: []inventory.Edge{dep("x", workerOf, "a")},
+			repo: "one", paths: []string{"modules/a/x", "modules/x/y"}, want: "a | x"},
+		{what: "a change to the base alone does not move what it builds", edges: []inventory.Edge{dep("x", builtBy, "a"),
+			dep("d", builtBy, "a"), dep("e", standsOn, "a")},
+			repo: "one", paths: []string{"modules/a/module.json"}, want: "a | e"},
+
+		// The cycles the kinds resolve: the build machine stands on the runtime image the runtime image is
+		// built by; the build seat's holder follows the controller that is built by it.
+		{what: "the build machine's base comes first, built by the build machine that runs", entries: runtime,
+			edges: []inventory.Edge{dep("runtime", builtBy, "builder"), dep("builder", standsOn, "runtime")},
+			repo:  "one", paths: []string{"modules/runtime/Dockerfile", "modules/builder/main.go"}, want: "runtime | builder"},
+		{what: "the runtime image alone takes the build machine on it along", entries: runtime,
+			edges: []inventory.Edge{dep("runtime", builtBy, "builder"), dep("builder", standsOn, "runtime")},
+			repo:  "one", paths: []string{"modules/runtime/Dockerfile"}, want: "runtime | builder"},
+		{what: "the build machine alone moves alone", entries: runtime,
+			edges: []inventory.Edge{dep("runtime", builtBy, "builder"), dep("builder", standsOn, "runtime")},
+			repo:  "one", paths: []string{"modules/builder/main.go"}, want: "builder"},
+		{what: "the build seat's holder follows the controller it binds the worker of", entries: runtime,
+			edges: []inventory.Edge{dep("controller", builtBy, "builder"), dep("builder", workerOf, "controller")},
+			repo:  "two", paths: []string{"cmd/main.go"}, want: "controller"},
+
+		// Two repositories.
+		{what: "a dependent in another repository follows its base",
+			edges: []inventory.Edge{dep("g", standsOn, "a")},
+			repo:  "one", paths: []string{"modules/a/x"}, want: "a | g"},
+		{what: "a directory of the same name in another repository is not this one's",
+			edges: []inventory.Edge{dep("g", standsOn, "a")},
+			repo:  "two", paths: []string{"modules/a/x"}, want: "", unread: "modules/a/x"},
+		{what: "the dependent's own repository moves the dependent alone",
+			edges: []inventory.Edge{dep("g", standsOn, "a")},
+			repo:  "two", paths: []string{"g/main.go"}, want: "g"},
+
+		// A diamond.
+		{what: "a diamond, one side changed", edges: []inventory.Edge{dep("d", standsOn, "a"), dep("d", standsOn, "b")},
+			repo: "one", paths: []string{"modules/a/x"}, want: "a | d"},
+		{what: "a diamond, both sides changed", edges: []inventory.Edge{dep("d", standsOn, "a"), dep("d", standsOn, "b")},
+			repo: "one", paths: []string{"modules/a/x", "modules/b/x"}, want: "a,b | d"},
+		{what: "a diamond on one base", edges: []inventory.Edge{dep("d", standsOn, "a"), dep("d", declared, "b"),
+			dep("a", standsOn, "z"), dep("b", standsOn, "z")},
+			repo: "one", paths: []string{"modules/z/x"}, want: "z | a,b | d"},
+		{what: "a diamond of mixed kinds orders on the ordering side only",
+			edges: []inventory.Edge{dep("d", standsOn, "a"), dep("d", packages, "b")},
+			repo:  "one", paths: []string{"modules/b/x"}, want: "b,d"},
+
+		// A cycle the catalogue should never produce: what remains is one last tier, and said.
+		{what: "a cycle is one last tier, not lost", edges: []inventory.Edge{dep("a", standsOn, "b"), dep("b", standsOn, "a"),
+			dep("f", standsOn, "c")},
+			repo: "one", paths: []string{"modules/a/x", "modules/c/x"}, want: "c | f | a,b", cycle: true},
+
+		// Files no build reads.
+		{what: "a README at the root of a repository whose modules all live below it",
+			edges: []inventory.Edge{dep("d", standsOn, "a")},
+			repo:  "one", paths: []string{"README.md"}, want: "", unread: "README.md"},
+		{what: "a directory no module lives in", edges: []inventory.Edge{dep("d", standsOn, "a")},
+			repo: "one", paths: []string{"modules/lib/x.go", "modules/README.md"}, want: "",
+			unread: "modules/lib/x.go,modules/README.md"},
+		{what: "a module's directory beside a root file", edges: []inventory.Edge{dep("d", standsOn, "a")},
+			repo: "one", paths: []string{"merge-check.sh", "modules/a/x"}, want: "a | d", unread: "merge-check.sh"},
+		{what: "a directory whose name begins with a module's", edges: []inventory.Edge{dep("d", standsOn, "a")},
+			repo: "one", paths: []string{"modules/ab/x"}, want: "", unread: "modules/ab/x"},
+	} {
+		e := entries
+		if c.entries != nil {
+			e = c.entries
+		}
+		r, got := planMerge(t, c.repo, c.paths, e, nil, c.edges)
+		if got != c.want {
+			t.Errorf("%s: planned %q, wanted %q", c.what, got, c.want)
+		}
+		if u := strings.Join(r.Unread, ","); u != c.unread {
+			t.Errorf("%s: unread %q, wanted %q", c.what, u, c.unread)
+		}
+		// BUG found by this test on main (8170fc5): hasCycle reads any edge between two modules of the last
+		// tier as a cycle — a packages edge too, which puts both there by rule — so the merge handler logs
+		// "the last tier depends on itself … built together, in no order" for a plan with no cycle (the
+		// packages rows and the mixed diamond fail here). ADR 0162: a cycle is one last tier, *and said*;
+		// one said where there is none is the rule broken. The expectation stays.
+		if hasCycle(r.Plan.Tiers, c.edges) != c.cycle {
+			t.Errorf("%s: a cycle said %v, wanted %v (%v) [BUG: hasCycle counts a packages edge]",
+				c.what, !c.cycle, c.cycle, r.Plan.Tiers)
+		}
+	}
+}
+
+// **CURRENT BEHAVIOUR, documented — not the rule the operator states.** novox/hq issue 338 (a module
+// built from a shared repository moves on every merge to it) and the decision pending on it would change
+// every row here. Today:
+//
+//   - mesh-controller is built from its repository's root, so every file of that repository touches it;
+//   - route-proxy and build-agent package the whole of that repository (a build context), so the build
+//     record's `read` makes them move on any merge to it, whatever the files, and dependenciesOf gives
+//     each a packages edge to every module built from it;
+//   - built-by (route-proxy on build-agent) and worker-of (build-agent on the controller) make it three
+//     tiers.
+//
+// These follow ADR 0238 §3 as written ("the whole repository for a module built from its root, and a
+// repository a recipe names"), so they are not failures; when the decision on issue 338 lands, these
+// expectations change with it. The edges are the ones dependenciesOf derives from this catalogue — held
+// to that by TestASharedRepositoryIsPlannedFromTheRecordsAsItIsToday, which derives them from the store.
+func TestASharedRepositoryMovesWhatPackagesItAsItDoesToday(t *testing.T) {
+	const catalogueRepo = "http://forge.internal:20000/novox/mesh-catalog.git"
+	const controllerRepo = "http://forge.internal:20000/novox/mesh-controller.git"
+	entries := []inventory.Entry{
+		fromRepo("mesh-controller", controllerRepo, ""),
+		fromRepo("build-agent", catalogueRepo, "modules/build-agent"),
+		fromRepo("route-proxy", catalogueRepo, "modules/route-proxy"),
+		fromRepo("gitea", catalogueRepo, "modules/gitea"),
+	}
+	read := map[string][]inventory.ReadRepository{
+		"build-agent": {{Repository: "novox/mesh-controller", Ref: "main"}},
+		"route-proxy": {{Repository: "novox/mesh-controller", Ref: "main"}},
+	}
+	edges := sharedRepositoryEdges
+	for _, c := range []struct {
+		what, repo string
+		paths      []string
+		want       string
+	}{
+		// The live three-tier plan of 2026-10-08 (issue 338). It is also the worker-of order (issue 206)
+		// that TestAMergeIsPlannedInTiersAlongTheThreeKindsOfDependency's controller case predates: that
+		// fixture has no worker-of edge and expects the builder first, then the controller and the proxy.
+		{"a README of the controller's repository moves all three, in three tiers", "mesh-controller",
+			[]string{"README.md"}, "mesh-controller | build-agent | route-proxy"},
+		{"the controller's own code: the same", "mesh-controller",
+			[]string{"cmd/mesh-controller/main.go"}, "mesh-controller | build-agent | route-proxy"},
+		{"the route proxy's program alone: the same, the controller with it", "mesh-controller",
+			[]string{"examples/route-proxy/main.go"}, "mesh-controller | build-agent | route-proxy"},
+		// In the catalogue, where they live, the rule is path-precise.
+		{"the route proxy's directory in the catalogue: it alone", "mesh-catalog",
+			[]string{"modules/route-proxy/module.json"}, "route-proxy"},
+		{"the build agent's directory: it alone, nothing it builds", "mesh-catalog",
+			[]string{"modules/build-agent/module.json"}, "build-agent"},
+		{"another module of the catalogue: neither", "mesh-catalog",
+			[]string{"modules/gitea/index.ts"}, "gitea"},
+	} {
+		r, got := planMerge(t, c.repo, c.paths, entries, read, edges)
+		if got != c.want {
+			t.Errorf("%s: planned %q, wanted %q (as today; issue 338)", c.what, got, c.want)
+		}
+		if c.repo == "mesh-controller" && strings.Join(r.Unread, ",") != "" {
+			t.Errorf("%s: a root-built module reads every file, and %v were said unread", c.what, r.Unread)
+		}
+	}
+}
+
+// sharedRepositoryEdges is what dependenciesOf derives for the catalogue of the test above, sorted as it
+// sorts them.
+var sharedRepositoryEdges = []inventory.Edge{
+	dep("build-agent", inventory.EdgePackages, "mesh-controller"),
+	dep("build-agent", inventory.EdgeWorkerOf, "mesh-controller"),
+	dep("gitea", inventory.EdgeBuiltBy, "build-agent"),
+	dep("mesh-controller", inventory.EdgeBuiltBy, "build-agent"),
+	dep("route-proxy", inventory.EdgeBuiltBy, "build-agent"),
+	dep("route-proxy", inventory.EdgePackages, "mesh-controller"),
+}
+
+// **The planner's invariant, over random catalogues.** For any catalogue whose dependencies form no cycle
+// and any set of changed files in one repository:
+//
+//   - the plan is exactly the modules of that repository whose directory holds a changed file (every file,
+//     for a module built from the root), and everything reachable from them along stands-on, declared and
+//     packages — never along built-by or worker-of;
+//   - every stands-on, declared, built-by and worker-of edge with both ends in the plan has the module
+//     depended on in an earlier tier;
+//   - no cycle is said (this one fails on main: see the BUG note below).
+//
+// Seeded, so a failure is replayed by its seed and case.
+func TestAPlanIsTheTouchedModulesAndWhatIsReachableAlongTheWideningEdges(t *testing.T) {
+	kinds := []string{inventory.EdgeStandsOn, inventory.EdgeDeclared, inventory.EdgePackages,
+		inventory.EdgeBuiltBy, inventory.EdgeWorkerOf}
+	widens := map[string]bool{inventory.EdgeStandsOn: true, inventory.EdgeDeclared: true, inventory.EdgePackages: true}
+	orders := map[string]bool{inventory.EdgeStandsOn: true, inventory.EdgeDeclared: true,
+		inventory.EdgeBuiltBy: true, inventory.EdgeWorkerOf: true}
+	// Directory names drawn from one pool, so two repositories hold directories of the same name, and one
+	// is a prefix of another.
+	dirs := []string{"a", "ab", "b", "c", "lib/x", "lib/y", "modules/a", "modules/a/sub"}
+	files := []string{"README.md", "merge-check.sh", "lib/z.go", "docs/x.md", "modules/README.md"}
+
+	falseCycles, firstFalseCycle := 0, ""
+	for _, seed := range []uint64{1, 2, 3, 0x338, 0x162} {
+		rng := rand.New(rand.NewPCG(seed, seed^0x9e3779b97f4a7c15))
+		for n := 0; n < 100; n++ {
+			repos := 1 + rng.IntN(3)
+			repoName := func(i int) string { return fmt.Sprintf("r%d", i) }
+			count := 1 + rng.IntN(12)
+			var entries []inventory.Entry
+			repoOf, dirOf := map[string]int{}, map[string]string{}
+			for i := 0; i < count; i++ {
+				name := fmt.Sprintf("m%02d", i)
+				repo := rng.IntN(repos)
+				dir := dirs[rng.IntN(len(dirs))]
+				if rng.IntN(12) == 0 {
+					dir = "" // built from the repository's root
+				}
+				repoOf[name], dirOf[name] = repo, dir
+				entries = append(entries, fromRepo(name, "http://forge.internal:20000/novox/"+repoName(repo)+".git", dir))
+			}
+			// A graph with no cycle: a module depends only on modules made before it.
+			var edges []inventory.Edge
+			for i := 1; i < count; i++ {
+				for j := 0; j < i; j++ {
+					if rng.IntN(4) == 0 {
+						edges = append(edges, dep(fmt.Sprintf("m%02d", i), kinds[rng.IntN(len(kinds))], fmt.Sprintf("m%02d", j)))
+					}
+				}
+			}
+			merged := rng.IntN(repos)
+			var paths []string
+			for k := 1 + rng.IntN(4); k > 0; k-- {
+				if rng.IntN(3) == 0 {
+					paths = append(paths, files[rng.IntN(len(files))])
+				} else {
+					paths = append(paths, dirs[rng.IntN(len(dirs))]+"/f.go")
+				}
+			}
+
+			// What the rules say.
+			want := map[string]bool{}
+			for _, e := range entries {
+				name := e.Manifest.Module
+				if repoOf[name] != merged {
+					continue
+				}
+				for _, p := range paths {
+					if d := dirOf[name]; d == "" || p == d || strings.HasPrefix(p, d+"/") {
+						want[name] = true
+					}
+				}
+			}
+			for grew := true; grew; {
+				grew = false
+				for _, e := range edges {
+					if widens[e.Kind] && want[e.To] && !want[e.From] {
+						want[e.From], grew = true, true
+					}
+				}
+			}
+
+			r, _ := planMerge(t, repoName(merged), paths, entries, nil, edges)
+			got := map[string]bool{}
+			tierOf := map[string]int{}
+			for i, tier := range r.Plan.Tiers {
+				for _, name := range tier {
+					got[name], tierOf[name] = true, i
+				}
+			}
+			replay := func() string {
+				return fmt.Sprintf("seed %#x case %d: repository %s, files %v\n  modules %v\n  edges %v\n  tiers %v",
+					seed, n, repoName(merged), paths, describe(entries), edges, r.Plan.Tiers)
+			}
+			if !sameSet(got, want) {
+				t.Fatalf("planned %v, wanted %v\n%s", keys(got), keys(want), replay())
+			}
+			for _, e := range edges {
+				if orders[e.Kind] && got[e.From] && got[e.To] && tierOf[e.To] >= tierOf[e.From] {
+					t.Fatalf("%s %s %s, and %s is in tier %d, not before %s's %d\n%s", e.From, e.Kind, e.To,
+						e.To, tierOf[e.To], e.From, tierOf[e.From], replay())
+				}
+			}
+			if hasCycle(r.Plan.Tiers, edges) {
+				falseCycles++
+				if firstFalseCycle == "" {
+					firstFalseCycle = replay()
+				}
+			}
+		}
+	}
+	// BUG found by this property on main (8170fc5); see the table's cycle check. Said once, after the
+	// other invariants have run over every case, so it hides none of them.
+	if falseCycles > 0 {
+		t.Errorf("a cycle said of a graph with none in %d of 500 cases [BUG: hasCycle counts a packages edge]; "+
+			"the first:\n%s", falseCycles, firstFalseCycle)
+	}
+}
+
+func sameSet(a, b map[string]bool) bool {
+	if len(a) != len(b) {
+		return false
+	}
+	for k := range a {
+		if !b[k] {
+			return false
+		}
+	}
+	return true
+}
+
+func keys(m map[string]bool) []string {
+	out := make([]string, 0, len(m))
+	for k := range m {
+		out = append(out, k)
+	}
+	sort.Strings(out)
+	return out
+}
+
+func describe(entries []inventory.Entry) []string {
+	var out []string
+	for _, e := range entries {
+		out = append(out, fmt.Sprintf("%s@%s:%q", e.Manifest.Module,
+			strings.TrimSuffix(strings.TrimPrefix(e.Source.Repository, "http://forge.internal:20000/novox/"), ".git"),
+			e.Source.Path))
+	}
+	return out
+}