Leave a module out of a walk when its check built it the same as what runs (hq issue 721)

A one-sentence change in a package mesh-builder imports but never reaches moved
build-agent. A merge check now compares each module it builds with the build the
mesh holds, the whole image without file times, labels or creation time, and a
walk is cut without a module every check of its pull requests found the same,
while the mesh still holds that build. A comparison that fails moves the module
as before.

cmd/mesh-builder/main.go

@@ -525,7 +525,7 @@ func checkBuiltOf(built []builder.CheckBuilt) []link.CheckBuilt {
 	var out []link.CheckBuilt
 	for _, b := range built {
 		c := link.CheckBuilt{Module: b.Module, Key: b.Key, NotBuilt: b.NotBuilt, Took: b.Took.Round(time.Second).String(),
-			Kept: b.Kept}
+			Kept: b.Kept, Same: b.Same, SameAs: b.SameAs}
 		for _, m := range b.Made {
 			c.Made = append(c.Made, link.MadeArtifact{Name: m.Name, Kind: m.Kind, Reference: m.Reference})
 		}

cmd/mesh-controller/batches.go

@@ -1000,10 +1000,15 @@ func planBatch(ctx context.Context, open *stores, batch *inventory.Plan, carry [
 	plan.Delivery.Merges = named
 	plan.Delivery.Alone = batch.Delivery != nil && batch.Delivery.Alone
 	plan.Delivery.Withheld = withheld
+	// **Left where it is, a module whose program did not change** (novox/hq issue 721): its check found it the same as
+	// the build the mesh holds.
+	for n, why := range leaveSameAsRunning(ctx, inv, &plan) {
+		unchanged[n] = why
+	}
 	plan.Delivery.Unchanged = unchangedLeft(unchanged, plan.Modules)
 	// **Its moments and its class** (novox/hq ADR 0282 decision 6): measured, never acted on.
 	plan.Times = walkTimesAtCut(*batch, plan, entries, now)
-	if len(moved) == 0 {
+	if len(moved) == 0 || len(plan.Modules) == 0 {
 		plan.State = inventory.PlanDone
 		plan.Tiers = [][]string{}
 		plan.Note = "moved nothing the mesh holds: nothing to walk"

cmd/mesh-controller/same_program.go

@@ -0,0 +1,126 @@
+package main
+
+import (
+	"context"
+	"fmt"
+	"sort"
+	"strings"
+
+	"github.com/novox/mesh-controller/internal/inventory"
+	"github.com/novox/mesh-controller/internal/link"
+)
+
+// **A module moves only when its program changes** (novox/hq issue 721). A merge check builds every module the
+// delivery plan moves and compares each with the build the mesh holds of it; a walk is cut without a module whose
+// check found its whole image the same as that build, while the mesh still holds it, and says so among the modules
+// it left unchanged. A merge of one sentence in a package a program imports but never reaches moves no build of it.
+
+// sameAsRunning are the modules of a walk its checks found the same as the build the mesh holds, each with what its
+// check said. **Only when every pull request the walk answers was checked at the head it merged, and every one of
+// those checks built the module and found it the same as the build the mesh still holds**: a merge the walk answers
+// without a pull request, a check of another head, a check that built it differently, or a held build that moved
+// since, leaves the module in the walk.
+func sameAsRunning(plan *inventory.Plan, builds []link.CheckedBuild, held map[string]string) map[string]string {
+	if plan.Delivery == nil || len(plan.Delivery.Merges) == 0 {
+		return nil
+	}
+	for _, m := range plan.Delivery.Merges {
+		if _, ok := pullOf(m); !ok {
+			return nil
+		}
+	}
+	pulls := reuseAskOf(plan).Pulls
+	heads := mergedHeadsOf(plan)
+	out := map[string]string{}
+	for name := range plan.Modules {
+		said := ""
+		for _, pull := range pulls {
+			b, ok := checkedBuildOf(builds, pull, heads[pull], name)
+			if !ok || !stillHeld(b.Built, name, held) {
+				said = ""
+				break
+			}
+			if said == "" {
+				said = b.Built.Same
+			}
+		}
+		if said != "" {
+			out[name] = said
+		}
+	}
+	if len(out) == 0 {
+		return nil
+	}
+	return out
+}
+
+// checkedBuildOf is the newest build a check of the pull request at head made of the module; false when none did.
+func checkedBuildOf(builds []link.CheckedBuild, pull link.PullRef, head, module string) (link.CheckedBuild, bool) {
+	if head == "" {
+		return link.CheckedBuild{}, false
+	}
+	for _, b := range builds {
+		if b.Pull == pull && b.Built.Module == module && link.SameHead(b.Head, head) {
+			return b, true
+		}
+	}
+	return link.CheckedBuild{}, false
+}
+
+// stillHeld is whether a check found every artifact of the module the same as a build the mesh still holds.
+func stillHeld(b link.CheckBuilt, module string, held map[string]string) bool {
+	if b.Same == "" || b.NotBuilt != "" || len(b.Made) == 0 || len(b.SameAs) != len(b.Made) {
+		return false
+	}
+	for _, m := range b.Made {
+		was := pinnedDigest(b.SameAs[m.Name])
+		if was == "" || was != pinnedDigest(held[module+"/"+m.Name]) {
+			return false
+		}
+	}
+	return true
+}
+
+// pinnedDigest is a reference's pinned digest, empty when it is not pinned.
+func pinnedDigest(ref string) string {
+	if i := strings.LastIndex(ref, "@"); i >= 0 {
+		return ref[i+1:]
+	}
+	return ""
+}
+
+// leaveSameAsRunning takes from a walk being cut every module its checks found the same as the build the mesh holds
+// (sameAsRunning): what was taken, with why. **Anything not read leaves the walk as it was**, said.
+func leaveSameAsRunning(ctx context.Context, inv *inventory.Inventory, plan *inventory.Plan) map[string]string {
+	if plan.Delivery == nil || len(plan.Modules) == 0 {
+		return nil
+	}
+	pulls := reuseAskOf(plan).Pulls
+	if len(pulls) == 0 {
+		return nil
+	}
+	builds, err := checkedBuildsOf(ctx, pulls)
+	if err != nil {
+		fmt.Printf("  the checks' builds could not be read, so every module moves: %v\n", err)
+		return nil
+	}
+	held, err := inv.Held(ctx)
+	if err != nil {
+		fmt.Printf("  what the mesh holds could not be read, so every module moves: %v\n", err)
+		return nil
+	}
+	same := sameAsRunning(plan, builds, held)
+	if len(same) == 0 {
+		return nil
+	}
+	names := make([]string, 0, len(same))
+	for n := range same {
+		names = append(names, n)
+	}
+	sort.Strings(names)
+	withhold(plan, names, nil)
+	for _, n := range names {
+		fmt.Printf("  %s left out: %s\n", n, same[n])
+	}
+	return same
+}

cmd/mesh-controller/same_program_test.go

@@ -0,0 +1,67 @@
+package main
+
+import (
+	"testing"
+
+	"github.com/novox/mesh-controller/internal/inventory"
+	"github.com/novox/mesh-controller/internal/link"
+)
+
+// TestAWalkLeavesOutAModuleWhoseProgramDidNotChange is novox/hq issue 721: mesh-controller #335 changed one sentence
+// that mesh-builder never reaches; its check found build-agent the same as the build the mesh holds, so the walk moves
+// mesh-controller alone and says build-agent is unchanged.
+func TestAWalkLeavesOutAModuleWhoseProgramDidNotChange(t *testing.T) {
+	pull := link.PullRef{Owner: "novox", Repo: "mesh-controller", Number: 335}
+	head := "34050dc6aaaabbbbccccdddd"
+	aPlan := func() *inventory.Plan {
+		return &inventory.Plan{Tiers: [][]string{{"mesh-controller", "build-agent"}},
+			Modules: map[string]*inventory.PlanModule{"mesh-controller": {}, "build-agent": {}},
+			Delivery: &inventory.PlanDelivery{Merges: []inventory.PlanMerge{{Repository: "novox/mesh-controller",
+				Commit: "c2ec0a60", Number: 335, Head: head}}}}
+	}
+	held := map[string]string{"build-agent/agent": "store/build-agent@sha256:aaaa",
+		"mesh-controller/controller": "store/mesh-controller@sha256:cccc"}
+	builds := []link.CheckedBuild{
+		{Pull: pull, Head: head[:12], Built: link.CheckBuilt{Module: "build-agent", Key: "k1",
+			Made:   []link.MadeArtifact{{Name: "agent", Kind: "image", Reference: "scratch/build-agent@sha256:bbbb"}},
+			Same:   "unchanged: its program is the same as running build agent aaaa",
+			SameAs: map[string]string{"agent": "10.0.0.1:5000/build-agent@sha256:aaaa"}}},
+		{Pull: pull, Head: head[:12], Built: link.CheckBuilt{Module: "mesh-controller", Key: "k2",
+			Made: []link.MadeArtifact{{Name: "controller", Kind: "image", Reference: "scratch/mesh-controller@sha256:dddd"}}}},
+	}
+
+	p := aPlan()
+	same := sameAsRunning(p, builds, held)
+	if len(same) != 1 || same["build-agent"] == "" {
+		t.Fatalf("the walk leaves out %v, want build-agent alone", same)
+	}
+	withhold(p, []string{"build-agent"}, nil)
+	if len(p.Tiers) != 1 || len(p.Tiers[0]) != 1 || p.Tiers[0][0] != "mesh-controller" {
+		t.Fatalf("the walk moves %v, want mesh-controller alone", p.Tiers)
+	}
+
+	// **The held build moved since the check**: what it was compared with no longer runs.
+	moved := map[string]string{"build-agent/agent": "store/build-agent@sha256:eeee"}
+	if same := sameAsRunning(aPlan(), builds, moved); len(same) != 0 {
+		t.Fatalf("a module compared with a build the mesh no longer holds is left out: %v", same)
+	}
+	// **A check of another head** says nothing of this merge.
+	other := aPlan()
+	other.Delivery.Merges[0].Head = "ffffffffffffffff"
+	if same := sameAsRunning(other, builds, held); len(same) != 0 {
+		t.Fatalf("a check of another head left out %v", same)
+	}
+	// **Another pull request the walk answers** that did not find it the same leaves it in.
+	two := aPlan()
+	two.Delivery.Merges = append(two.Delivery.Merges, inventory.PlanMerge{Repository: "novox/mesh-catalog",
+		Commit: "1234", Number: 9, Head: "abcdefabcdefabcdef"})
+	if same := sameAsRunning(two, builds, held); len(same) != 0 {
+		t.Fatalf("a module another merge moves is left out: %v", same)
+	}
+	// **A merge with no pull request** leaves the walk as it is.
+	direct := aPlan()
+	direct.Delivery.Merges[0].Number = 0
+	if same := sameAsRunning(direct, builds, held); len(same) != 0 {
+		t.Fatalf("a walk answering a merge without a pull request left out %v", same)
+	}
+}