S1: keep no package.json or go.mod outside the named tree in a narrowed build

A file outside the key changed what is built (novox/hq ADR 0499 clause 4); a build that needs one names it among its paths.

go/buildstep/narrow.go

@@ -9,8 +9,10 @@ import (
 )
 
 // narrowKept are the files of a module's directory a narrowed build keeps beside its named paths: its manifest and
-// its lock files.
-var narrowKept = []string{"module.json", "package.json", "package-lock.json", "go.mod", "go.sum"}
+// its lock files, each an input of the module's key (ModuleTree's Manifest and Locks). package.json and go.mod are
+// not kept: neither is in the named tree, so a build that needs one names it among its artifact's paths, which puts
+// it in the key (novox/hq ADR 0499 clause 4).
+var narrowKept = append([]string{"module.json"}, lockFiles...)
 
 // Narrow leaves in the fetched tree only what a build of an artifact that names its paths may see (novox/hq ADR 0499
 // clause 4): r.Dir's named paths, its manifest and lock files, and .git; everything else is removed, so a file outside

go/buildstep/narrow_test.go

@@ -23,7 +23,8 @@ func filesUnder(t *testing.T, root string) []string {
 }
 
 // A build of an artifact that names its paths sees only those paths, its manifest and its lock files (novox/hq ADR 0499
-// clause 4): Narrow removes everything else of the fetched tree but .git; with no paths it changes nothing.
+// clause 4): Narrow removes everything else of the fetched tree but .git, package.json and go.mod among it, since
+// neither is in the named tree nor among the lock files; with no paths it changes nothing.
 func TestNarrowLeavesOnlyTheNamedPathsTheManifestAndTheLocks(t *testing.T) {
 	tree := t.TempDir()
 	for _, f := range []string{".git/HEAD", "module.json", "package.json", "package-lock.json", "go.sum", "src/index.ts",
@@ -39,7 +40,7 @@ func TestNarrowLeavesOnlyTheNamedPathsTheManifestAndTheLocks(t *testing.T) {
 	if err := Narrow(tree, FetchRequest{Dir: ".", Paths: []string{"src", "test", "tsconfig.json"}}); err != nil {
 		t.Fatal(err)
 	}
-	want := ".git/HEAD,go.sum,module.json,package-lock.json,package.json,src/deep/a.ts,src/index.ts,test/a.test.ts,tsconfig.json"
+	want := ".git/HEAD,go.sum,module.json,package-lock.json,src/deep/a.ts,src/index.ts,test/a.test.ts,tsconfig.json"
 	if got := strings.Join(filesUnder(t, tree), ","); got != want {
 		t.Fatalf("the narrowed tree holds %s\nwant %s", got, want)
 	}
@@ -61,7 +62,21 @@ func TestNarrowKeepsAModuleDirectorysPathsAndFailsOnAnAbsentOne(t *testing.T) {
 	if err := Narrow(tree, FetchRequest{Dir: "modules/a", Paths: []string{"src"}}); err != nil {
 		t.Fatal(err)
 	}
-	if got := strings.Join(filesUnder(t, tree), ","); got != ".git/config,modules/a/go.mod,modules/a/module.json,modules/a/src/x.ts" {
+	if got := strings.Join(filesUnder(t, tree), ","); got != ".git/config,modules/a/module.json,modules/a/src/x.ts" {
+		t.Fatalf("the narrowed tree holds %s", got)
+	}
+}
+
+// A build that needs package.json or go.mod names it among its paths, and so keys on it (ADR 0499 clause 4).
+func TestNarrowKeepsAManifestOfTheLanguageOnlyWhenNamed(t *testing.T) {
+	tree := t.TempDir()
+	for _, f := range []string{"module.json", "package.json", "go.mod", "src/a.ts"} {
+		writeTreeFile(t, tree, f, "x")
+	}
+	if err := Narrow(tree, FetchRequest{Dir: ".", Paths: []string{"src", "package.json"}}); err != nil {
+		t.Fatal(err)
+	}
+	if got := strings.Join(filesUnder(t, tree), ","); got != "module.json,package.json,src/a.ts" {
 		t.Fatalf("the narrowed tree holds %s", got)
 	}
 }