The fetch walks the siblings for Go modules only when one is a Go module at its root; the beside workspace's go directive is the highest among its modules (issue 1921)

Claude-Session: https://claude.ai/code/session_01NcbgfFSnn24M3GcqttoU7i

modules/mesh-build-go/cmd/mesh-build-go/beside_digest_test.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"context"
 	"encoding/json"
 	"os"
 	"path/filepath"
@@ -84,7 +85,7 @@ func TestASiblingsCommitChangesTheDigestOfAPackageWhoseTestsReachAReaderOfTheSib
 // The fetch writes the siblings it placed, which the digest reads.
 func TestTheFetchRecordsTheSiblingsItPlaced(t *testing.T) {
 	// Every checkout writes this go.mod: the placed sibling is a Go module at its root, so the fetch downloads its graph.
-	f := &fakeStep{gomods: "go.mod\n", files: map[string]string{"go.mod": "module git.novox.be/novox/mesh-sdk/go\n\ngo 1.26.0\n"}}
+	f := &fakeStep{gomods: "go.mod\n", files: map[string]string{"go.mod": "module git.novox.be/novox/mesh-sdk/go\n\ngo 1.27.1\n"}}
 	m, states := fetchModule(t, f)
 	args := map[string]any{"repository": "novox/mesh-controller", "commit": commitSHA, "beside": []any{
 		map[string]any{"repository": "novox/mesh-sdk", "commit": commitSHA}}}
@@ -113,6 +114,23 @@ func TestTheFetchRecordsTheSiblingsItPlaced(t *testing.T) {
 			}
 		}
 	}
+	// The go.work's go directive is the highest of the laid modules.
+	if work, _ := os.ReadFile(filepath.Join(m.Work, res.Workspace, "beside-0.work")); !strings.HasPrefix(string(work), "go 1.27.1\n") {
+		t.Errorf("the workspace file begins %q, want go 1.27.1", work)
+	}
+	// No sibling a Go module at its root: nothing is walked, so a directory the walk cannot read does not fail the fetch.
+	plain := t.TempDir()
+	locked := filepath.Join(plain, "beside", "docs", "private")
+	if err := os.MkdirAll(locked, 0o700); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.Chmod(locked, 0); err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() { os.Chmod(locked, 0o700) })
+	if err := m.downloadBeside(context.Background(), plain, filepath.Join(plain, "tree"), nil); err != nil {
+		t.Errorf("a fetch with no Go-module sibling walked the siblings: %v", err)
+	}
 	if n != 1 {
 		t.Errorf("%q ran %d times, want once:\n%s", want, n, strings.Join(f.lines(), "\n"))
 	}

modules/mesh-build-go/cmd/mesh-build-go/workspace.go

@@ -7,6 +7,7 @@ import (
 	"io/fs"
 	"os"
 	"path/filepath"
+	"strconv"
 	"strings"
 )
 
@@ -25,7 +26,7 @@ const zeroVersion = "v0.0.0-00010101000000-000000000000"
 func workspaceOf(first string, candidates []string) (string, error) {
 	paths := map[string]string{}
 	for _, d := range candidates {
-		path, _, err := goModOf(d)
+		path, _, _, err := goModOf(d)
 		if err != nil {
 			return "", err
 		}
@@ -35,11 +36,12 @@ func workspaceOf(first string, candidates []string) (string, error) {
 	}
 	var use, replace strings.Builder
 	laid := map[string]bool{}
+	highest := "1.26.0" // the go directive is the highest among the laid modules, never lower than this
 	queue := []string{first}
 	for len(queue) > 0 {
 		d := queue[0]
 		queue = queue[1:]
-		path, needs, err := goModOf(d)
+		path, needs, goV, err := goModOf(d)
 		if err != nil {
 			return "", err
 		}
@@ -47,6 +49,9 @@ func workspaceOf(first string, candidates []string) (string, error) {
 			continue
 		}
 		laid[path] = true
+		if goNewer(goV, highest) {
+			highest = goV
+		}
 		fmt.Fprintf(&use, "\t%s\n", d)
 		fmt.Fprintf(&replace, "replace %s %s => %s\n", path, zeroVersion, d)
 		for _, need := range needs {
@@ -60,28 +65,48 @@ func workspaceOf(first string, candidates []string) (string, error) {
 			queue = append(queue, dir)
 		}
 	}
-	return "go 1.26.0\n\nuse (\n" + use.String() + ")\n\n" + replace.String(), nil
+	return "go " + highest + "\n\nuse (\n" + use.String() + ")\n\n" + replace.String(), nil
 }
 
-// goModOf is the module path of the go.mod in d and the modules it requires at the zero version.
-func goModOf(d string) (path string, needs []string, err error) {
+// goNewer reports whether the go directive a is higher than b (dotted numbers; a missing part is 0).
+func goNewer(a, b string) bool {
+	x, y := strings.Split(a, "."), strings.Split(b, ".")
+	for i := 0; i < len(x) || i < len(y); i++ {
+		var p, q int
+		if i < len(x) {
+			p, _ = strconv.Atoi(x[i])
+		}
+		if i < len(y) {
+			q, _ = strconv.Atoi(y[i])
+		}
+		if p != q {
+			return p > q
+		}
+	}
+	return false
+}
+
+// goModOf is the module path of the go.mod in d, the modules it requires at the zero version and its go directive.
+func goModOf(d string) (path string, needs []string, goV string, err error) {
 	raw, err := os.ReadFile(filepath.Join(d, "go.mod"))
 	if err != nil {
-		return "", nil, err
+		return "", nil, "", err
 	}
 	for _, line := range strings.Split(string(raw), "\n") {
 		f := strings.Fields(strings.TrimPrefix(strings.TrimSpace(line), "require "))
 		switch {
 		case len(f) == 2 && f[0] == "module" && path == "":
 			path = strings.Trim(f[1], `"`)
+		case len(f) == 2 && f[0] == "go" && goV == "":
+			goV = f[1]
 		case len(f) >= 2 && f[1] == zeroVersion:
 			needs = append(needs, strings.Trim(f[0], `"`))
 		}
 	}
 	if path == "" {
-		return "", nil, fmt.Errorf("%s/go.mod names no module", d)
+		return "", nil, "", fmt.Errorf("%s/go.mod names no module", d)
 	}
-	return path, needs, nil
+	return path, needs, goV, nil
 }
 
 // downloadBeside downloads, where the network is, the Go modules of the graph that the tests of a workspace's siblings
@@ -99,15 +124,21 @@ func (m *Module) downloadBeside(ctx context.Context, root, tree string, dirs []s
 	if err != nil {
 		return err
 	}
-	var candidates, roots []string
-	for _, d := range dirs {
-		candidates = append(candidates, filepath.Join(tree, filepath.FromSlash(d)))
-	}
+	var candidates, roots, all []string
 	for _, e := range entries {
 		dir := filepath.Join(beside, e.Name())
+		all = append(all, dir)
 		if _, err := os.Stat(filepath.Join(dir, "go.mod")); err == nil {
 			roots = append(roots, dir)
 		}
+	}
+	if len(roots) == 0 {
+		return nil // no sibling is a Go module at its root: nothing is walked, nothing downloaded
+	}
+	for _, d := range dirs {
+		candidates = append(candidates, filepath.Join(tree, filepath.FromSlash(d)))
+	}
+	for _, dir := range all {
 		err := filepath.WalkDir(dir, func(p string, d fs.DirEntry, err error) error {
 			if err != nil {
 				return err