Say what a build was made from, and hand an image compiling Go only that (hq ADR 0267, issue 363)

A merge to the controller's repository moved the route proxy and the build
seat's holder whatever it changed, because nothing said which files their
builds read. A build of a trunk commit now says its build source per
repository: a Go program's import closure, an archive's directory, an
image's recipe and the package it names in the new 'compiles' field. That
image is built from its build source alone, so a recipe reading past it
fails by name, and its fingerprint is over what it was handed.

internal/builder/build_source_test.go

@@ -0,0 +1,176 @@
+package builder
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"slices"
+	"strings"
+	"testing"
+)
+
+// What a build says it was made from, as files (novox/hq ADR 0267), and what a build compiling a Go
+// program is handed.
+
+// onTrunk answers the trunk's questions as a clone of a commit on main would, or off it.
+type onTrunk struct {
+	*recorded
+	off bool
+}
+
+func (o onTrunk) run(ctx context.Context, dir, name string, args ...string) (string, error) {
+	if name == "git" && len(args) > 0 && args[0] == "symbolic-ref" {
+		return "origin/main\n", nil
+	}
+	if name == "git" && len(args) > 0 && args[0] == "merge-base" && o.off {
+		return "", os.ErrNotExist
+	}
+	return compiling{o.recorded}.run(ctx, dir, name, args...)
+}
+
+// aSharedRepository is a repository holding two programs that share a package, as the controller's does.
+func aSharedRepository(readme, shared string) map[string]string {
+	return map[string]string{
+		"go.mod":               "module example.com/ctl\n\ngo 1.22\n",
+		"go.sum":               "",
+		"README.md":            readme,
+		"cmd/ctl/main.go":      "package main\n\nimport _ \"example.com/ctl/internal/shared\"\n\nfunc main() {}\n",
+		"proxy/main.go":        "package main\n\nimport _ \"example.com/ctl/internal/shared\"\n\nfunc main() {}\n",
+		"internal/shared/s.go": "package shared\n\nconst S = " + shared + "\n",
+		"internal/only/o.go":   "package only\n",
+	}
+}
+
+const aProxy = `{"module":"route-proxy","version":"1",
+  "build":{"artifacts":[
+    {"name":"server","kind":"image","from":"Dockerfile","compiles":"proxy",
+     "context":{"repository":"https://forge.invalid/ctl.git","ref":"main"}},
+    {"name":"trust","kind":"upstream","from":"alpine@sha256:` + "3333333333333333333333333333333333333333333333333333333333333333" + `"}]}}`
+
+func buildTheProxy(t *testing.T, context_ map[string]string, off bool) (Result, *recorded, string) {
+	t.Helper()
+	r := &recorded{
+		contents:  map[string]string{"modules/route-proxy/" + ManifestName: aProxy, "modules/route-proxy/Dockerfile": "FROM scratch\nCOPY . .\n", "modules/route-proxy/README.md": "x"},
+		secondary: map[string]map[string]string{"https://forge.invalid/ctl.git": context_},
+	}
+	workspace := t.TempDir()
+	got, err := Build(context.Background(), onTrunk{r, off}.run, r,
+		"https://forge.invalid/catalogue.git", "modules/route-proxy", "", workspace, nil, Npmrc{}, GitCredential{}, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return got, r, workspace
+}
+
+func TestAnImageCompilingGoIsHandedItsBuildSourceAndSaysIt(t *testing.T) {
+	got, r, workspace := buildTheProxy(t, aSharedRepository("one", "1"), false)
+
+	// Built in the narrowed tree, which holds the program's closure and nothing else.
+	at := ""
+	for i, line := range r.ran {
+		if strings.HasPrefix(line, "docker build ") {
+			at = r.dirs[i]
+		}
+	}
+	if filepath.Base(at) != "narrow-server" {
+		t.Fatalf("docker build ran in %q, not the narrowed build source", at)
+	}
+	for file, want := range map[string]bool{"proxy/main.go": true, "internal/shared/s.go": true, "go.mod": true,
+		"cmd/ctl/main.go": false, "internal/only/o.go": false, "README.md": false} {
+		_, err := os.Stat(filepath.Join(workspace, "narrow-server", filepath.FromSlash(file)))
+		if (err == nil) != want {
+			t.Errorf("%s handed to the recipe: %v, wanted %v", file, err == nil, want)
+		}
+	}
+
+	// Said per repository: its own, the manifest and the recipe; the context's, the closure.
+	if len(got.Sources) != 2 {
+		t.Fatalf("sources %+v", got.Sources)
+	}
+	own, ctx := got.Sources[0], got.Sources[1]
+	if own.Repository != "" || !slices.Equal(own.Paths, []string{"modules/route-proxy/Dockerfile", "modules/route-proxy/module.json"}) {
+		t.Errorf("its own build source: %+v", own)
+	}
+	if ctx.Repository != "https://forge.invalid/ctl.git" || ctx.Ref != "main" {
+		t.Errorf("the context's build source names %q at %q", ctx.Repository, ctx.Ref)
+	}
+	for file, want := range map[string]bool{"proxy/main.go": true, "internal/shared/s.go": true, "go.mod": true,
+		"cmd/ctl/main.go": false, "README.md": false} {
+		if SourceHolds(ctx.Paths, file) != want {
+			t.Errorf("the context's build source holds %s: %v, wanted %v (%v)", file, !want, want, ctx.Paths)
+		}
+	}
+
+	// **The fingerprint is over the build source** (rule 5): a change outside it is one build, inside it another.
+	readme, _, _ := buildTheProxy(t, aSharedRepository("two", "1"), false)
+	if readme.Source != got.Source {
+		t.Errorf("a README of the context changed the fingerprint: %s %s", got.Source, readme.Source)
+	}
+	shared, _, _ := buildTheProxy(t, aSharedRepository("one", "2"), false)
+	if shared.Source == got.Source {
+		t.Error("a change to the program's closure kept its fingerprint")
+	}
+}
+
+// A build off the trunk says no build source: the planner maps a merge onto the trunk's.
+func TestABuildOffTheTrunkSaysNoBuildSource(t *testing.T) {
+	got, _, _ := buildTheProxy(t, aSharedRepository("one", "1"), true)
+	if len(got.Sources) != 0 {
+		t.Fatalf("a build off the trunk said %+v", got.Sources)
+	}
+}
+
+// A recipe reading past its build source fails, naming what it could not find — in the real docker build;
+// here, the file is simply not in the tree it is handed, which is what makes that so.
+func TestAnImageCompilingANonexistentPackageFails(t *testing.T) {
+	r := &recorded{
+		contents: map[string]string{ManifestName: strings.Replace(aProxy, `"compiles":"proxy"`, `"compiles":"nowhere"`, 1),
+			"Dockerfile": "FROM scratch\n"},
+		secondary: map[string]map[string]string{"https://forge.invalid/ctl.git": aSharedRepository("one", "1")},
+	}
+	_, err := Build(context.Background(), onTrunk{r, false}.run, r,
+		"https://forge.invalid/catalogue.git", "", "", t.TempDir(), nil, Npmrc{}, GitCredential{}, nil)
+	if err == nil || !strings.Contains(err.Error(), "nowhere") {
+		t.Fatalf("a package that is not there built: %v", err)
+	}
+}
+
+// A Go bundle of a module built from its repository's root says its import closure, and the manifest;
+// an image that compiles nothing it was told of leaves the module's source whole, and says none.
+func TestAGoBundleSaysItsClosureAndAnUntoldImageNothing(t *testing.T) {
+	files := aSharedRepository("one", "1")
+	manifest := `{"module":"ctl","version":"1","build":{"artifacts":[
+	  {"name":"controller","kind":"bundle","language":"go","system":"arch","from":"cmd/ctl","binary":"ctl"}]},
+	  "resources":[{"id":"controller","type":"process","name":"ctl","artifact":"controller","run":["./ctl"]}]}`
+	r := &recorded{contents: map[string]string{ManifestName: manifest}}
+	for k, v := range files {
+		r.contents[k] = v
+	}
+	held := map[string]string{"mesh-tools-go/build": "registry.invalid/mesh-tools-go/build@sha256:" + strings.Repeat("b", 64)}
+	got, err := Build(context.Background(), onTrunk{r, false}.run, r,
+		"https://forge.invalid/ctl.git", "", "", t.TempDir(), held, Npmrc{}, GitCredential{}, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(got.Sources) != 1 || got.Sources[0].Repository != "" {
+		t.Fatalf("sources %+v", got.Sources)
+	}
+	for file, want := range map[string]bool{"cmd/ctl/main.go": true, "internal/shared/s.go": true, ManifestName: true,
+		"go.sum": true, "proxy/main.go": false, "README.md": false, "internal/only/o.go": false} {
+		if SourceHolds(got.Sources[0].Paths, file) != want {
+			t.Errorf("%s: held %v, wanted %v (%v)", file, !want, want, got.Sources[0].Paths)
+		}
+	}
+
+	untold := `{"module":"ctl","version":"1","build":{"artifacts":[
+	  {"name":"server","kind":"image","from":"Dockerfile"}]}}`
+	r = &recorded{contents: map[string]string{ManifestName: untold, "Dockerfile": "FROM scratch\n"}}
+	got, err = Build(context.Background(), onTrunk{r, false}.run, r,
+		"https://forge.invalid/ctl.git", "", "", t.TempDir(), nil, Npmrc{}, GitCredential{}, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(got.Sources) != 0 {
+		t.Fatalf("an image compiling nothing it was told of said a build source: %+v", got.Sources)
+	}
+}

internal/builder/builder.go

@@ -11,6 +11,7 @@ import (
 	"io"
 	"os"
 	"os/exec"
+	"path"
 	"path/filepath"
 	"regexp"
 	"sort"
@@ -87,6 +88,23 @@ type Result struct {
 	// pin the build. Two builds with one fingerprint are one build, whatever digests they made
 	// (novox/hq issue 280).
 	Source string
+
+	// Sources are what this build was made from, as files (novox/hq ADR 0267 rule 1): per repository, the
+	// entries a changed file is tested against (SourceHolds). The module's own repository has an empty
+	// Repository. Said only for a build of a commit on the trunk, and only for a repository whose every
+	// artifact's build source is known — a Go program's import closure, an archive's directory, an image's
+	// recipe and the package it compiles; for any other, nothing is said and the whole of what the build
+	// sees stays its source, as before.
+	Sources []BuildSource
+}
+
+// BuildSource is the build source a build read in one repository (novox/hq ADR 0267).
+type BuildSource struct {
+	// Repository and Ref are a context's, as the manifest names it; empty for the module's own.
+	Repository string
+	Ref        string
+	// Paths are the entries, relative to the repository's root (SourceHolds).
+	Paths []string
 }
 
 // GitCredential is the forge credential a clone may present when the server asks for one.
@@ -206,6 +224,10 @@ func build(ctx context.Context, run Runner, publish Publisher,
 
 	// What it is made from, for its source fingerprint: the module's own tree first.
 	src := newSourceInputs(manifest.Module)
+	src.prefix = strings.Trim(filepath.ToSlash(filepath.Clean(path)), "/")
+	if src.prefix == "." {
+		src.prefix = ""
+	}
 	if src.tree, err = gitTree(ctx, run, tree, path); err != nil {
 		src.notPinned("its tree could not be named: " + err.Error())
 	}
@@ -298,9 +320,22 @@ func build(ctx context.Context, run Runner, publish Publisher,
 	if fingerprint == "" {
 		say("source", "no source fingerprint: %s", orNoTree(src.unpinned))
 	}
+	// **Only a trunk build says its build source** (novox/hq ADR 0267): the planner maps the next merge onto
+	// the build source of the trunk's last build, and a branch's closure is not the trunk's.
+	var sources []BuildSource
+	if trunk != "" && onTrunk {
+		sources = src.buildSources()
+		for _, s := range sources {
+			where := "its own repository"
+			if s.Repository != "" {
+				where = s.Repository
+			}
+			say("source", "%d path(s) of %s", len(s.Paths), where)
+		}
+	}
 	return Result{Manifest: resolved, Commit: commit, Built: built,
 		Against: against(within, manifest, stoodOn), Read: readBy(manifest), Source: fingerprint,
-		Trunk: trunk, OnTrunk: onTrunk, Branches: branches}, nil
+		Trunk: trunk, OnTrunk: onTrunk, Branches: branches, Sources: sources}, nil
 }
 
 // branchesHolding is every branch of a fresh clone's origin the commit is on, without `origin/`.
@@ -648,15 +683,48 @@ func one(ctx context.Context, run Runner, publish Publisher,
 			} else if src != nil {
 				src.contexts[a.Name] = t
 			}
-			// docker build accepts -f outside the context it is given; the recipe stays exactly
-			// where it was read from and validated against, absolute so the working directory
-			// switching to the cloned context does not change which file that is.
+			buildDir = cloned
+		}
+		// docker build accepts -f outside the context it is given; the recipe stays exactly where it was
+		// read from and validated against, absolute so a context elsewhere does not change which file
+		// that is.
+		if buildDir != tree || a.Compiles != "" {
 			absRecipe, err := filepath.Abs(filepath.Join(tree, a.From))
 			if err != nil {
 				return catalogue.Built{}, fmt.Errorf("%s: %s's recipe: %w", module, a.Name, err)
 			}
 			recipePath = absRecipe
-			buildDir = cloned
+		}
+		// **An image that compiles a Go program is handed its build source and nothing else** (novox/hq
+		// ADR 0267 rules 1 and 3): the program's import closure, read from the context it is built in, so a
+		// merge elsewhere in that repository is no change to it — and a recipe that copies a file outside
+		// it fails here, naming the file, rather than building from something no merge is mapped onto.
+		if a.Compiles != "" {
+			paths, err := GoBuildSource(buildDir, a.Compiles)
+			if err != nil {
+				return catalogue.Built{}, fmt.Errorf("%s: %s compiles %s, whose build source cannot be read: %w",
+					module, a.Name, a.Compiles, err)
+			}
+			narrowed := filepath.Join(workspace, "narrow-"+a.Name)
+			sum, err := narrowTree(buildDir, narrowed, paths)
+			if err != nil {
+				return catalogue.Built{}, fmt.Errorf("%s: handing %s its build source: %w", module, a.Name, err)
+			}
+			say("image", "%s is handed its build source: %d path(s) of %s", a.Name, len(paths), a.Compiles)
+			if a.Context != nil {
+				src.contexts[a.Name] = sum
+				src.readIn(*a.Context, paths)
+			} else {
+				src.ownHas(paths...)
+			}
+			buildDir = narrowed
+		} else if a.Context != nil {
+			src.readWhole(*a.Context)
+		}
+		if a.Compiles != "" || a.Context != nil {
+			src.ownHas(a.From)
+		} else {
+			src.ownWhole()
 		}
 		invocation := append([]string{"build", "-f", recipePath, "-t", local}, args...)
 		if a.Target != "" {
@@ -708,6 +776,18 @@ func one(ctx context.Context, run Runner, publish Publisher,
 		if src != nil {
 			src.toolchains[a.Name] = toolchainOf(chain, base)
 		}
+		// A Go program's build source is its import closure (novox/hq ADR 0267 rule 1). Not read, it is the
+		// module's whole directory, as before — said, so the wider plan has a reason a person can find.
+		if chain.Language == "go" {
+			if paths, err := GoBuildSource(tree, a.From); err != nil {
+				say("bundle", "%s's build source is its whole directory: %v", a.Name, err)
+				src.ownWhole()
+			} else {
+				src.ownHas(paths...)
+			}
+		} else {
+			src.ownWhole()
+		}
 		if chain.Language == "typescript" {
 			if own, _ := ownDependencies(tree); len(own) > 0 {
 				src.notPinned(a.Name + " resolves packages of its own at build time")
@@ -754,6 +834,7 @@ func one(ctx context.Context, run Runner, publish Publisher,
 		// there is no Publisher call — the container itself publishes, with the credential the
 		// build was handed.
 		say("package", "building and publishing %s (%s)", a.Name, a.Language)
+		src.ownWhole()
 		src.notPinned(a.Name + " is a package, built from what the registry holds when it is built")
 		reference, err := publishPackage(ctx, run, module, tree, a, npmrc, say)
 		if err != nil {
@@ -763,6 +844,7 @@ func one(ctx context.Context, run Runner, publish Publisher,
 		return catalogue.Built{Name: a.Name, Kind: a.Kind, Reference: reference}, nil
 
 	case catalogue.ArtifactArchive:
+		src.ownHas(strings.Trim(path.Clean("/"+filepath.ToSlash(a.From)), "/") + "/**")
 		body, err := pack(filepath.Join(tree, a.From))
 		if err != nil {
 			return catalogue.Built{}, fmt.Errorf("%s: packing %s failed: %w", module, a.Name, err)

internal/builder/gosource.go

@@ -0,0 +1,485 @@
+package builder
+
+import (
+	"bufio"
+	"crypto/sha256"
+	"encoding/hex"
+	"errors"
+	"fmt"
+	"go/parser"
+	"go/token"
+	"io"
+	"io/fs"
+	"os"
+	"path"
+	"path/filepath"
+	"sort"
+	"strconv"
+	"strings"
+)
+
+// A Go program's build source (novox/hq ADR 0267 rule 1): the files it is built from, derived from its
+// import closure rather than listed by hand, because a list drifts from the imports it describes and a
+// path missing from it is a real change missed — worse than a needless rebuild.
+//
+// **The closure read here is never narrower than `go list -deps`.** Every .go file of a package that is
+// not a test is read, whatever its build constraint, so the imports are the union over every system and
+// tag; a directory is held whole (but for its tests), so a file added to a package is in it; an embed is
+// held by the directory its pattern starts in, everything below it. Read with the standard library's
+// parser and no toolchain: the build machine carries none, and a closure that needed the network to
+// read would be one a build could not say offline.
+//
+// A build source is a list of entries, relative to the root the build sees:
+//
+//	dir/      a Go package's directory: every file directly in it but its tests (`*_test.go`)
+//	dir/**    everything below a directory (an embed)
+//	**        the whole tree
+//	file      one file
+//
+// The root package's directory is `./`.
+
+// GoPackageDirEntry is the entry for a Go package's directory.
+func goPackageDirEntry(dir string) string {
+	if dir == "" || dir == "." {
+		return "./"
+	}
+	return dir + "/"
+}
+
+// SourceHolds is whether a changed file — a path relative to the root the build source was read in — is
+// in that build source.
+func SourceHolds(entries []string, file string) bool {
+	file = strings.TrimPrefix(path.Clean("/"+strings.TrimSpace(file)), "/")
+	for _, e := range entries {
+		switch {
+		case e == "**":
+			return true
+		case strings.HasSuffix(e, "/**"):
+			dir := strings.TrimSuffix(e, "/**")
+			if dir == "." || dir == "" || file == dir || strings.HasPrefix(file, dir+"/") {
+				return true
+			}
+		case strings.HasSuffix(e, "/"):
+			dir := strings.TrimSuffix(e, "/")
+			parent := path.Dir(file)
+			if (dir == "." && parent == ".") || parent == dir {
+				if !strings.HasSuffix(file, "_test.go") {
+					return true
+				}
+			}
+		case e == file:
+			return true
+		}
+	}
+	return false
+}
+
+// GoBuildSource is the build source of the Go program whose main package is pkg, a directory relative to
+// root: the directories of every package of its import closure inside root, the embeds those packages
+// name, its module's go.mod, go.sum and vendor/modules.txt, and a go.work wherever one would be read. An
+// entry for a file that does not exist is kept: creating it is a change to the build.
+//
+// Refused — so the build source is not narrowed, and nothing is missed — when the closure cannot be told
+// from the files: no go.mod holds the package, a go.work is present, a local replace leaves root, a file
+// does not parse, or a cgo preamble reaches outside its directory.
+func GoBuildSource(root, pkg string) ([]string, error) {
+	root, err := filepath.Abs(root)
+	if err != nil {
+		return nil, err
+	}
+	rel := path.Clean(strings.TrimPrefix(filepath.ToSlash(strings.TrimSpace(pkg)), "/"))
+	if rel == ".." || strings.HasPrefix(rel, "../") {
+		return nil, fmt.Errorf("the package %q leaves the tree it is built from", pkg)
+	}
+	if info, err := os.Stat(filepath.Join(root, filepath.FromSlash(rel))); err != nil || !info.IsDir() {
+		return nil, fmt.Errorf("%q is not a directory of the tree it is built from", pkg)
+	}
+	// The module holding the package: the nearest go.mod at or above it, within root.
+	modRoot := ""
+	for dir := rel; ; dir = path.Dir(dir) {
+		if _, err := os.Stat(filepath.Join(root, filepath.FromSlash(dir), "go.mod")); err == nil {
+			modRoot = dir
+			break
+		}
+		if dir == "." {
+			break
+		}
+	}
+	if modRoot == "" {
+		return nil, fmt.Errorf("no go.mod holds %q within the tree it is built from", pkg)
+	}
+	entries := map[string]bool{}
+	file := func(dir, name string) {
+		entries[strings.TrimPrefix(path.Join(dir, name), "./")] = true
+	}
+	file(modRoot, "go.mod")
+	file(modRoot, "go.sum")
+	file(modRoot, "vendor/modules.txt")
+	// A workspace changes how every import resolves; one present is not read past, one created later is a
+	// change to the build.
+	for dir := modRoot; ; dir = path.Dir(dir) {
+		file(dir, "go.work")
+		file(dir, "go.work.sum")
+		if _, err := os.Stat(filepath.Join(root, filepath.FromSlash(dir), "go.work")); err == nil {
+			return nil, fmt.Errorf("%s holds a go.work, and a workspace's imports are not read here", path.Join(dir, "go.work"))
+		}
+		if dir == "." {
+			break
+		}
+	}
+	modPath, replaces, err := readGoMod(filepath.Join(root, filepath.FromSlash(modRoot), "go.mod"))
+	if err != nil {
+		return nil, err
+	}
+	vendored := false
+	if _, err := os.Stat(filepath.Join(root, filepath.FromSlash(modRoot), "vendor", "modules.txt")); err == nil {
+		vendored = true
+	}
+	// resolve is the directories, relative to root, an import may be read from: none for the standard
+	// library and for a module outside the tree, which go.mod and go.sum pin. A vendored module replaced
+	// by a local directory is both — vendor/ under -mod=vendor, the directory under -mod=mod — and both
+	// are held, so neither way of building it is missed.
+	resolve := func(importPath string) ([]string, error) {
+		within := func(prefix, dir string) (string, bool) {
+			if importPath == prefix {
+				return dir, true
+			}
+			if rest, ok := strings.CutPrefix(importPath, prefix+"/"); ok {
+				return path.Join(dir, rest), true
+			}
+			return "", false
+		}
+		if dir, ok := within(modPath, modRoot); ok {
+			return []string{dir}, nil
+		}
+		var dirs []string
+		for _, r := range replaces {
+			if sub, ok := within(r.from, ""); ok {
+				target := path.Clean(path.Join(modRoot, r.to))
+				if target == ".." || strings.HasPrefix(target, "../") {
+					return nil, fmt.Errorf("go.mod replaces %s with %s, outside the tree it is built from", r.from, r.to)
+				}
+				dirs = append(dirs, path.Join(target, sub))
+				// Its go.mod states what it requires, read when it is built from there.
+				entries[path.Join(target, "go.mod")] = true
+				break
+			}
+		}
+		first, _, _ := strings.Cut(importPath, "/")
+		if len(dirs) == 0 && !strings.Contains(first, ".") {
+			return nil, nil // the standard library
+		}
+		if vendored {
+			dirs = append(dirs, path.Join(modRoot, "vendor", importPath))
+		}
+		return dirs, nil
+	}
+
+	seen := map[string]bool{}
+	queue := []string{rel}
+	for len(queue) > 0 {
+		dir := queue[0]
+		queue = queue[1:]
+		if seen[dir] {
+			continue
+		}
+		seen[dir] = true
+		entries[goPackageDirEntry(dir)] = true
+		listing, err := os.ReadDir(filepath.Join(root, filepath.FromSlash(dir)))
+		if err != nil {
+			// A package that is not there fails the build that imports it; its directory is held, so
+			// adding it is a change.
+			continue
+		}
+		for _, f := range listing {
+			name := f.Name()
+			if f.IsDir() || !strings.HasSuffix(name, ".go") || strings.HasSuffix(name, "_test.go") {
+				continue
+			}
+			full := filepath.Join(root, filepath.FromSlash(dir), name)
+			imports, embeds, err := goFileReads(full)
+			if err != nil {
+				return nil, fmt.Errorf("%s: %w", path.Join(dir, name), err)
+			}
+			for _, ip := range imports {
+				targets, err := resolve(ip)
+				if err != nil {
+					return nil, err
+				}
+				for _, target := range targets {
+					if !seen[target] {
+						queue = append(queue, target)
+					}
+				}
+			}
+			for _, e := range embeds {
+				entries[path.Join(dir, e)+"/**"] = true
+				if !strings.ContainsAny(e, "*?[\\") {
+					entries[path.Join(dir, e)] = true
+				}
+			}
+		}
+	}
+	out := make([]string, 0, len(entries))
+	for e := range entries {
+		out = append(out, e)
+	}
+	sort.Strings(out)
+	return out, nil
+}
+
+// goReplace is one local replacement in go.mod: an import path read from a directory.
+type goReplace struct{ from, to string }
+
+// readGoMod is a go.mod's module path and its replacements by a local directory. A replacement by another
+// module version is resolved by go.sum, which the build source holds.
+func readGoMod(file string) (string, []goReplace, error) {
+	f, err := os.Open(file)
+	if err != nil {
+		return "", nil, err
+	}
+	defer f.Close()
+	var module string
+	var replaces []goReplace
+	inReplace := false
+	scanner := bufio.NewScanner(f)
+	for scanner.Scan() {
+		line := scanner.Text()
+		if i := strings.Index(line, "//"); i >= 0 {
+			line = line[:i]
+		}
+		line = strings.TrimSpace(line)
+		switch {
+		case line == "":
+			continue
+		case inReplace && line == ")":
+			inReplace = false
+			continue
+		case strings.HasPrefix(line, "module "):
+			module = unquoteGoMod(strings.TrimSpace(strings.TrimPrefix(line, "module")))
+			continue
+		case line == "replace (":
+			inReplace = true
+			continue
+		case strings.HasPrefix(line, "replace "):
+			line = strings.TrimSpace(strings.TrimPrefix(line, "replace"))
+		case !inReplace:
+			continue
+		}
+		left, right, found := strings.Cut(line, "=>")
+		if !found {
+			continue
+		}
+		from := strings.Fields(left)
+		to := strings.Fields(right)
+		if len(from) == 0 || len(to) == 0 {
+			continue
+		}
+		target := unquoteGoMod(to[0])
+		// A local replacement is a path: ./, ../ or absolute. Anything else names a module version.
+		if strings.HasPrefix(target, "./") || strings.HasPrefix(target, "../") || target == "." || target == ".." {
+			replaces = append(replaces, goReplace{from: unquoteGoMod(from[0]), to: target})
+		} else if strings.HasPrefix(target, "/") {
+			return "", nil, fmt.Errorf("go.mod replaces %s with %s, outside the tree it is built from", from[0], target)
+		}
+	}
+	if err := scanner.Err(); err != nil {
+		return "", nil, err
+	}
+	if module == "" {
+		return "", nil, fmt.Errorf("%s names no module", file)
+	}
+	// The longest replacement first, so a replaced sub-path wins over its parent.
+	sort.SliceStable(replaces, func(i, j int) bool { return len(replaces[i].from) > len(replaces[j].from) })
+	return module, replaces, nil
+}
+
+func unquoteGoMod(s string) string {
+	if u, err := strconv.Unquote(s); err == nil {
+		return u
+	}
+	return s
+}
+
+// goFileReads is what one Go file makes its build read: the packages it imports, and the directories its
+// //go:embed patterns start in, relative to its own directory ("." for the directory itself).
+//
+// **A cgo preamble reaching outside its directory is refused**: a header included by a relative path, or
+// a flag naming ${SRCDIR}/.., is a file of the build no import names. Inside the directory it is held
+// already.
+func goFileReads(file string) (imports, embeds []string, err error) {
+	src, err := os.ReadFile(file)
+	if err != nil {
+		return nil, nil, err
+	}
+	fset := token.NewFileSet()
+	parsed, err := parser.ParseFile(fset, file, src, parser.ImportsOnly|parser.ParseComments)
+	if err != nil {
+		return nil, nil, err
+	}
+	for _, spec := range parsed.Imports {
+		ip, err := strconv.Unquote(spec.Path.Value)
+		if err != nil {
+			return nil, nil, err
+		}
+		if ip == "C" {
+			// The preamble is the comment before the import; only its #include and #cgo lines read files.
+			for _, cg := range parsed.Comments {
+				if cg.End() > spec.Pos() {
+					continue
+				}
+				for _, line := range strings.Split(cg.Text(), "\n") {
+					line = strings.TrimSpace(line)
+					if (strings.HasPrefix(line, "#include") || strings.HasPrefix(line, "#cgo")) && strings.Contains(line, "..") {
+						return nil, nil, errors.New("its cgo preamble names a path outside its directory: " + line)
+					}
+				}
+			}
+			continue
+		}
+		imports = append(imports, ip)
+	}
+	// //go:embed directives may stand anywhere in the file, so the whole text is read for them.
+	scanner := bufio.NewScanner(strings.NewReader(string(src)))
+	scanner.Buffer(make([]byte, 0, 64*1024), 4*1024*1024)
+	for scanner.Scan() {
+		line := strings.TrimSpace(scanner.Text())
+		rest, ok := strings.CutPrefix(line, "//go:embed")
+		if !ok || (rest != "" && rest[0] != ' ' && rest[0] != '\t') {
+			continue
+		}
+		patterns, err := embedPatterns(rest)
+		if err != nil {
+			return nil, nil, err
+		}
+		for _, p := range patterns {
+			embeds = append(embeds, embedRoot(p))
+		}
+	}
+	return imports, embeds, scanner.Err()
+}
+
+// embedPatterns splits a //go:embed line's patterns: separated by spaces, each possibly quoted.
+func embedPatterns(s string) ([]string, error) {
+	var out []string
+	s = strings.TrimSpace(s)
+	for s != "" {
+		var p string
+		switch s[0] {
+		case '"', '`':
+			q, err := strconv.QuotedPrefix(s)
+			if err != nil {
+				return nil, fmt.Errorf("an embed pattern does not parse: %w", err)
+			}
+			if p, err = strconv.Unquote(q); err != nil {
+				return nil, err
+			}
+			s = s[len(q):]
+		default:
+			end := strings.IndexAny(s, " \t")
+			if end < 0 {
+				end = len(s)
+			}
+			p, s = s[:end], s[end:]
+		}
+		out = append(out, p)
+		s = strings.TrimSpace(s)
+	}
+	return out, nil
+}
+
+// embedRoot is the directory an embed pattern starts in, relative to the package: its leading elements
+// without a wildcard. A pattern naming a file or a directory outright is held as itself.
+func embedRoot(pattern string) string {
+	pattern = strings.TrimPrefix(pattern, "all:")
+	var kept []string
+	for _, el := range strings.Split(pattern, "/") {
+		if strings.ContainsAny(el, "*?[\\") {
+			break
+		}
+		kept = append(kept, el)
+	}
+	if len(kept) == 0 {
+		return "."
+	}
+	return path.Clean(strings.Join(kept, "/"))
+}
+
+// narrowTree copies into dst the files of src a build source holds, and nothing else — never `.git` — and
+// returns a fingerprint of what it copied: each file's path, mode and content, hashed in path order. **A
+// build handed only its build source cannot read past it** (novox/hq ADR 0267 rule 3): a recipe that
+// copies a file outside it fails, naming the file, where it would have built and been missed.
+func narrowTree(src, dst string, entries []string) (string, error) {
+	if err := os.RemoveAll(dst); err != nil {
+		return "", err
+	}
+	if err := os.MkdirAll(dst, 0o755); err != nil {
+		return "", err
+	}
+	var lines []string
+	err := filepath.WalkDir(src, func(p string, d fs.DirEntry, err error) error {
+		if err != nil {
+			return err
+		}
+		rel, err := filepath.Rel(src, p)
+		if err != nil {
+			return err
+		}
+		rel = filepath.ToSlash(rel)
+		if d.IsDir() {
+			if d.Name() == ".git" {
+				return filepath.SkipDir
+			}
+			return nil
+		}
+		if !SourceHolds(entries, rel) {
+			return nil
+		}
+		out := filepath.Join(dst, filepath.FromSlash(rel))
+		if err := os.MkdirAll(filepath.Dir(out), 0o755); err != nil {
+			return err
+		}
+		info, err := d.Info()
+		if err != nil {
+			return err
+		}
+		if info.Mode()&fs.ModeSymlink != 0 {
+			// A link in the repository is copied as the link it is, and what it names said in the
+			// fingerprint.
+			target, err := os.Readlink(p)
+			if err != nil {
+				return err
+			}
+			lines = append(lines, fmt.Sprintf("%s link %s", rel, target))
+			return os.Symlink(target, out)
+		}
+		if !info.Mode().IsRegular() {
+			return nil
+		}
+		in, err := os.Open(p)
+		if err != nil {
+			return err
+		}
+		defer in.Close()
+		w, err := os.OpenFile(out, os.O_CREATE|os.O_WRONLY|os.O_TRUNC, info.Mode().Perm())
+		if err != nil {
+			return err
+		}
+		sum := sha256.New()
+		if _, err := io.Copy(io.MultiWriter(w, sum), in); err != nil {
+			w.Close()
+			return err
+		}
+		if err := w.Close(); err != nil {
+			return err
+		}
+		lines = append(lines, fmt.Sprintf("%s %o %s", rel, info.Mode().Perm()&0o111, hex.EncodeToString(sum.Sum(nil))))
+		return nil
+	})
+	if err != nil {
+		return "", err
+	}
+	sort.Strings(lines)
+	sum := sha256.Sum256([]byte(strings.Join(lines, "\n")))
+	return "narrow:" + hex.EncodeToString(sum[:]), nil
+}

internal/builder/gosource_test.go

@@ -0,0 +1,288 @@
+package builder
+
+import (
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+// A Go program's build source is its import closure (novox/hq ADR 0267 rule 1): never narrower than what
+// `go build` reads, whatever the system, the tags, the vendoring or the embeds.
+
+// aGoTree writes files under a fresh directory and returns it.
+func aGoTree(t *testing.T, files map[string]string) string {
+	t.Helper()
+	root := t.TempDir()
+	for name, body := range files {
+		full := filepath.Join(root, filepath.FromSlash(name))
+		if err := os.MkdirAll(filepath.Dir(full), 0o755); err != nil {
+			t.Fatal(err)
+		}
+		if err := os.WriteFile(full, []byte(body), 0o644); err != nil {
+			t.Fatal(err)
+		}
+	}
+	return root
+}
+
+// aProgram is a module whose program imports its own packages, a vendored module, a local replacement
+// that is vendored too, an embed and a package only one system builds; beside them, a package nothing
+// imports and one only a test imports.
+var aProgram = map[string]string{
+	"go.mod": "module example.com/fix\n\ngo 1.22\n\nrequire (\n\texample.org/dep v1.0.0\n\texample.net/local v0.0.0\n)\n\n" +
+		"replace example.net/local => ./third_party/local\n",
+	"go.sum": "",
+	"vendor/modules.txt": "# example.net/local v0.0.0 => ./third_party/local\n## explicit; go 1.22\nexample.net/local/pkg\n" +
+		"# example.org/dep v1.0.0\n## explicit; go 1.22\nexample.org/dep\nexample.org/dep/sub\n# example.net/local => ./third_party/local\n",
+	"vendor/example.org/dep/dep.go":     "package dep\n\nimport _ \"example.org/dep/sub\"\n",
+	"vendor/example.org/dep/sub/sub.go": "package sub\n",
+	"vendor/example.org/other/other.go": "package other\n",
+	"vendor/example.net/local/pkg/p.go": "package pkg\n",
+	"third_party/local/go.mod":          "module example.net/local\n\ngo 1.22\n",
+	"third_party/local/pkg/p.go":        "package pkg\n",
+	"cmd/prog/main.go": "package main\n\nimport (\n\t\"fmt\"\n\n\t_ \"example.com/fix/emb\"\n\t\"example.com/fix/lib\"\n" +
+		"\t_ \"example.net/local/pkg\"\n\t_ \"example.org/dep\"\n)\n\nfunc main() { fmt.Println(lib.X) }\n",
+	"lib/lib.go":       "package lib\n\nconst X = 1\n",
+	"lib/lib_plan9.go": "//go:build plan9\n\npackage lib\n\nimport _ \"example.com/fix/plan9only\"\n",
+	"lib/lib_test.go":  "package lib\n\nimport _ \"example.com/fix/testonly\"\n",
+	"lib/lib_amd64.s":  "",
+	"lib/sub/sub.go":   "package sub\n",
+	"plan9only/p.go":   "package plan9only\n",
+	"testonly/t.go":    "package testonly\n",
+	"unrelated/u.go":   "package unrelated\n",
+	"emb/emb.go": "package emb\n\nimport \"embed\"\n\n//go:embed static/*\nvar Static embed.FS\n\n" +
+		"//go:embed \"a b.txt\"\nvar Text string\n",
+	"emb/static/index.html":     "x",
+	"emb/static/deep/style.css": "x",
+	"emb/a b.txt":               "x",
+	"README.md":                 "x",
+}
+
+func TestAGoProgramsBuildSourceIsItsImportClosure(t *testing.T) {
+	root := aGoTree(t, aProgram)
+	got, err := GoBuildSource(root, "cmd/prog")
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, c := range []struct {
+		file string
+		held bool
+		why  string
+	}{
+		{"cmd/prog/main.go", true, "the program itself"},
+		{"cmd/prog/helper.go", true, "a file added to the program's package"},
+		{"lib/lib.go", true, "a package it imports"},
+		{"lib/lib_amd64.s", true, "assembly beside a package's Go"},
+		{"lib/lib_plan9.go", true, "a file one system builds"},
+		{"plan9only/p.go", true, "what a file one system builds imports"},
+		{"emb/static/index.html", true, "an embedded file"},
+		{"emb/static/deep/style.css", true, "an embedded file below the pattern's directory"},
+		{"emb/a b.txt", true, "an embed named outright, quoted"},
+		{"vendor/example.org/dep/dep.go", true, "a vendored package it imports"},
+		{"vendor/example.org/dep/sub/sub.go", true, "what a vendored package imports"},
+		{"vendor/example.net/local/pkg/p.go", true, "a replaced module, as vendored"},
+		{"third_party/local/pkg/p.go", true, "a replaced module, at its directory"},
+		{"third_party/local/go.mod", true, "a replaced module's requirements"},
+		{"go.mod", true, "the module's requirements"},
+		{"go.sum", true, "the module's sums"},
+		{"vendor/modules.txt", true, "the vendored modules"},
+		{"go.work", true, "a workspace, were one made"},
+		{"lib/lib_test.go", false, "a test is not built"},
+		{"testonly/t.go", false, "a package only a test imports"},
+		{"lib/sub/sub.go", false, "a package below an imported one, not imported"},
+		{"unrelated/u.go", false, "a package nothing imports"},
+		{"vendor/example.org/other/other.go", false, "a vendored package nothing imports"},
+		{"README.md", false, "a file no build reads"},
+	} {
+		if SourceHolds(got, c.file) != c.held {
+			t.Errorf("%s (%s): held %v, wanted %v\n  %v", c.file, c.why, !c.held, c.held, got)
+		}
+	}
+
+	// **Never narrower than go list -deps**: every package go names, and every file it compiles or embeds,
+	// is held. Where no go command is at hand, said, not passed.
+	goCmd, err := exec.LookPath("go")
+	if err != nil {
+		t.Skip("NOT COMPARED: no go command to list the closure with")
+	}
+	list := exec.Command(goCmd, "list", "-deps", "-f",
+		`{{if not .Standard}}{{$d := .Dir}}{{range .GoFiles}}{{$d}}/{{.}}
+{{end}}{{range .SFiles}}{{$d}}/{{.}}
+{{end}}{{range .EmbedFiles}}{{$d}}/{{.}}
+{{end}}{{end}}`, "./cmd/prog")
+	list.Dir = root
+	list.Env = append(os.Environ(), "GOFLAGS=-mod=vendor", "GOPROXY=off", "GOWORK=off", "GOOS=linux", "GOARCH=amd64")
+	out, err := list.CombinedOutput()
+	if err != nil {
+		t.Fatalf("go list: %v\n%s", err, out)
+	}
+	real, _ := filepath.EvalSymlinks(root)
+	for _, line := range strings.Split(strings.TrimSpace(string(out)), "\n") {
+		if line == "" {
+			continue
+		}
+		rel, err := filepath.Rel(real, line)
+		if err != nil || strings.HasPrefix(rel, "..") {
+			rel, _ = filepath.Rel(root, line)
+		}
+		if !SourceHolds(got, filepath.ToSlash(rel)) {
+			t.Errorf("go list builds %s, and the build source does not hold it", rel)
+		}
+	}
+}
+
+// A file added to the program's import closure moves its build source with it: the closure read again
+// grows by the package.
+func TestTheBuildSourceGrowsWithAnImport(t *testing.T) {
+	files := map[string]string{}
+	for k, v := range aProgram {
+		files[k] = v
+	}
+	before, err := GoBuildSource(aGoTree(t, files), "cmd/prog")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if SourceHolds(before, "unrelated/u.go") {
+		t.Fatal("held before it was imported")
+	}
+	files["cmd/prog/more.go"] = "package main\n\nimport _ \"example.com/fix/unrelated\"\n"
+	after, err := GoBuildSource(aGoTree(t, files), "cmd/prog")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !SourceHolds(after, "unrelated/u.go") {
+		t.Fatalf("an import added did not grow the build source: %v", after)
+	}
+}
+
+// What cannot be read is refused, so the build source is not narrowed and nothing is missed.
+func TestABuildSourceThatCannotBeReadIsRefused(t *testing.T) {
+	for _, c := range []struct {
+		what  string
+		files map[string]string
+		pkg   string
+		says  string
+	}{
+		{"no go.mod", map[string]string{"cmd/p/main.go": "package main\n"}, "cmd/p", "no go.mod"},
+		{"a workspace", map[string]string{"go.mod": "module x\n", "go.work": "go 1.22\n", "p/main.go": "package main\n"},
+			"p", "go.work"},
+		{"a local replacement outside the tree", map[string]string{
+			"go.mod": "module x\n\nreplace y => ../y\n", "p/main.go": "package main\n\nimport _ \"y\"\n"}, "p", "outside"},
+		{"a cgo header outside the directory", map[string]string{
+			"go.mod": "module x\n", "p/main.go": "package main\n\n// #include \"../h/h.h\"\nimport \"C\"\n"}, "p", "cgo"},
+		{"a file that does not parse", map[string]string{"go.mod": "module x\n", "p/main.go": "package main\n\nimport (\n"},
+			"p", "main.go"},
+		{"a package that leaves the tree", map[string]string{"go.mod": "module x\n"}, "../elsewhere", "leaves"},
+	} {
+		_, err := GoBuildSource(aGoTree(t, c.files), c.pkg)
+		if err == nil || !strings.Contains(err.Error(), c.says) {
+			t.Errorf("%s: %v, wanted a refusal saying %q", c.what, err, c.says)
+		}
+	}
+	// A cgo header in the package's own directory is held already, and is no refusal.
+	if _, err := GoBuildSource(aGoTree(t, map[string]string{"go.mod": "module x\n",
+		"p/main.go": "package main\n\n// #include \"h.h\"\nimport \"C\"\n"}), "p"); err != nil {
+		t.Errorf("a header in the package's directory was refused: %v", err)
+	}
+}
+
+func TestABuildSourceHoldsWhatItsEntriesSay(t *testing.T) {
+	entries := []string{"./", "cmd/p/", "web/**", "go.mod"}
+	for file, want := range map[string]bool{
+		"main.go": true, "main_test.go": false, "cmd/p/x.go": true, "cmd/p/x_test.go": false, "cmd/p/sub/y.go": false,
+		"cmd/px/x.go": false, "web/a/b/c.css": true, "web": true, "webx/a": false, "go.mod": true, "docs/x.md": false,
+		"/cmd/p/x.go": true, "cmd/p/../q/x.go": false,
+	} {
+		if SourceHolds(entries, file) != want {
+			t.Errorf("%s: held %v, wanted %v", file, !want, want)
+		}
+	}
+	if !SourceHolds([]string{"**"}, "anything/at/all") {
+		t.Error("the whole tree does not hold a file")
+	}
+}
+
+// **A build handed its build source reads nothing else** (rule 3): the narrowed tree holds the source's
+// files and no others — never .git — and its fingerprint changes with them and only with them.
+func TestANarrowedTreeHoldsTheBuildSourceAndNothingElse(t *testing.T) {
+	files := map[string]string{}
+	for k, v := range aProgram {
+		files[k] = v
+	}
+	files[".git/HEAD"] = "ref: refs/heads/main\n"
+	src := aGoTree(t, files)
+	entries, err := GoBuildSource(src, "cmd/prog")
+	if err != nil {
+		t.Fatal(err)
+	}
+	dst := filepath.Join(t.TempDir(), "narrow")
+	one, err := narrowTree(src, dst, entries)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for file, want := range map[string]bool{"cmd/prog/main.go": true, "lib/lib.go": true, "emb/static/deep/style.css": true,
+		"lib/lib_test.go": false, "unrelated/u.go": false, "README.md": false, ".git/HEAD": false} {
+		_, err := os.Stat(filepath.Join(dst, filepath.FromSlash(file)))
+		if (err == nil) != want {
+			t.Errorf("%s: copied %v, wanted %v", file, err == nil, want)
+		}
+	}
+	// Outside the build source: one fingerprint.
+	files["README.md"] = "changed"
+	files["unrelated/u.go"] = "package unrelated\n\nconst Changed = 1\n"
+	again, err := narrowTree(aGoTree(t, files), filepath.Join(t.TempDir(), "n"), entries)
+	if err != nil || again != one {
+		t.Fatalf("a change outside the build source changed its fingerprint: %s %s %v", one, again, err)
+	}
+	// Inside it: another.
+	files["lib/lib.go"] = "package lib\n\nconst X = 2\n"
+	moved, err := narrowTree(aGoTree(t, files), filepath.Join(t.TempDir(), "n"), entries)
+	if err != nil || moved == one {
+		t.Fatalf("a change inside the build source kept its fingerprint: %s %v", moved, err)
+	}
+}
+
+// This repository's own programs: the route proxy's build source is not the controller's, and neither
+// holds the other's command.
+func TestThisRepositorysProgramsHaveBuildSourcesOfTheirOwn(t *testing.T) {
+	root := filepath.Join("..", "..")
+	proxy, err := GoBuildSource(root, "examples/route-proxy")
+	if err != nil {
+		t.Fatal(err)
+	}
+	controller, err := GoBuildSource(root, "cmd/mesh-controller")
+	if err != nil {
+		t.Fatal(err)
+	}
+	builder, err := GoBuildSource(root, "cmd/mesh-builder")
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, c := range []struct {
+		entries []string
+		name    string
+		file    string
+		held    bool
+	}{
+		{proxy, "the route proxy", "examples/route-proxy/main.go", true},
+		{proxy, "the route proxy", "internal/broker/broker.go", true},
+		{proxy, "the route proxy", "cmd/mesh-controller/main.go", false},
+		{proxy, "the route proxy", "internal/conditions/condition.go", false},
+		{proxy, "the route proxy", "README.md", false},
+		{controller, "the controller", "cmd/mesh-controller/main.go", true},
+		{controller, "the controller", "internal/conditions/condition.go", true},
+		{controller, "the controller", "internal/inventory/migrations/0001-nodes.sql", true},
+		{controller, "the controller", "examples/route-proxy/main.go", false},
+		{controller, "the controller", "cmd/mesh-builder/main.go", false},
+		{controller, "the controller", "README.md", false},
+		{builder, "the build seat's program", "cmd/mesh-builder/main.go", true},
+		{builder, "the build seat's program", "internal/conditions/condition.go", false},
+		{builder, "the build seat's program", "cmd/mesh-controller/main.go", false},
+	} {
+		if SourceHolds(c.entries, c.file) != c.held {
+			t.Errorf("%s: %s held %v, wanted %v", c.name, c.file, !c.held, c.held)
+		}
+	}
+}

internal/builder/source.go

@@ -9,6 +9,8 @@ import (
 	"path/filepath"
 	"sort"
 	"strings"
+
+	"github.com/novox/mesh-controller/internal/catalogue"
 )
 
 // A build's source fingerprint: what it was made from, hashed (novox/hq issue 280).
@@ -49,10 +51,132 @@ type sourceInputs struct {
 	toolchains map[string]string
 	// unpinned is why this build has no fingerprint: empty when it has one.
 	unpinned string
+
+	// prefix is the module's directory within its repository, empty at the root.
+	prefix string
+	// own is the module's build source in its own repository, relative to the module's directory, and
+	// whole when an artifact's is not known (novox/hq ADR 0267).
+	own      map[string]bool
+	ownIsAll bool
+	// read is, per context (repository and ref), the build source read there; nil for one read whole.
+	read   map[string]map[string]bool
+	readAs map[string]catalogue.ArtifactContext
 }
 
 func newSourceInputs(module string) *sourceInputs {
-	return &sourceInputs{module: module, contexts: map[string]string{}, toolchains: map[string]string{}}
+	return &sourceInputs{module: module, contexts: map[string]string{}, toolchains: map[string]string{},
+		own: map[string]bool{}, read: map[string]map[string]bool{}, readAs: map[string]catalogue.ArtifactContext{}}
+}
+
+// ownHas adds entries, relative to the module's directory, to its build source in its own repository.
+func (s *sourceInputs) ownHas(entries ...string) {
+	if s == nil {
+		return
+	}
+	for _, e := range entries {
+		s.own[e] = true
+	}
+}
+
+// ownWhole says an artifact's build source in the module's own repository is not known: the module's
+// whole directory is its source, as it was before.
+func (s *sourceInputs) ownWhole() {
+	if s != nil {
+		s.ownIsAll = true
+	}
+}
+
+func contextKey(c catalogue.ArtifactContext) string { return c.Repository + "#" + c.Ref }
+
+// readIn adds entries to the build source read in a context; one read whole stays whole.
+func (s *sourceInputs) readIn(c catalogue.ArtifactContext, entries []string) {
+	if s == nil {
+		return
+	}
+	key := contextKey(c)
+	s.readAs[key] = c
+	set, known := s.read[key]
+	if known && set == nil {
+		return
+	}
+	if set == nil {
+		set = map[string]bool{}
+		s.read[key] = set
+	}
+	for _, e := range entries {
+		set[e] = true
+	}
+}
+
+// readWhole says a context is read whole by an artifact.
+func (s *sourceInputs) readWhole(c catalogue.ArtifactContext) {
+	if s == nil {
+		return
+	}
+	key := contextKey(c)
+	s.readAs[key] = c
+	s.read[key] = nil
+}
+
+// buildSources is what the build says it was made from, per repository: its own (module.json always,
+// and every artifact's) unless an artifact's is not known, and each context not read whole.
+func (s *sourceInputs) buildSources() []BuildSource {
+	if s == nil {
+		return nil
+	}
+	var out []BuildSource
+	if !s.ownIsAll {
+		own := []string{withPrefix(s.prefix, ManifestName)}
+		for e := range s.own {
+			own = append(own, withPrefix(s.prefix, e))
+		}
+		sort.Strings(own)
+		out = append(out, BuildSource{Paths: compactSorted(own)})
+	}
+	var keys []string
+	for k := range s.read {
+		keys = append(keys, k)
+	}
+	sort.Strings(keys)
+	for _, k := range keys {
+		set := s.read[k]
+		if set == nil {
+			continue
+		}
+		var paths []string
+		for e := range set {
+			paths = append(paths, e)
+		}
+		sort.Strings(paths)
+		c := s.readAs[k]
+		out = append(out, BuildSource{Repository: c.Repository, Ref: c.Ref, Paths: paths})
+	}
+	return out
+}
+
+// withPrefix is an entry relative to the module's directory made relative to its repository's root.
+func withPrefix(prefix, entry string) string {
+	entry = strings.TrimPrefix(entry, "./")
+	if prefix == "" {
+		if entry == "" {
+			return "./"
+		}
+		return entry
+	}
+	if entry == "" {
+		return prefix + "/"
+	}
+	return prefix + "/" + entry
+}
+
+func compactSorted(in []string) []string {
+	var out []string
+	for i, e := range in {
+		if i == 0 || e != in[i-1] {
+			out = append(out, e)
+		}
+	}
+	return out
 }
 
 // notPinned marks the build as one its source does not pin; the first reason stands.