cmd/cue: allow cue mod tidy in non-module root

The module-root-finding code somewhat duplicates similar
code in both `internal/mod/modpkgload` and `cue/load` but
the former isn't suitable because it's defined in terms of `io/fs.FS`
and reusing the latter would require adding new public API.

Also avoid writing the module file when there's no change.

Signed-off-by: Roger Peppe <[email protected]>
Change-Id: I2fb76abedc43f6be41e8eb16762d5e400acfa77d
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1173826
Reviewed-by: Daniel Martí <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

cmd/cue/cmd/modtidy.go

@@ -15,6 +15,7 @@
 package cmd
 
 import (
+	"bytes"
 	"context"
 	"fmt"
 	"os"
@@ -53,12 +54,11 @@ func runModTidy(cmd *Command, args []string) error {
 		return fmt.Errorf("no registry configured to upload to")
 	}
 	ctx := context.Background()
-	// TODO don't assume we're running in the module's root directory.
-	wd, err := os.Getwd()
+	modRoot, err := findModuleRoot()
 	if err != nil {
 		return err
 	}
-	mf, err := modload.Load(ctx, os.DirFS(wd), ".", reg)
+	mf, err := modload.Load(ctx, os.DirFS(modRoot), ".", reg)
 	if err != nil {
 		return err
 	}
@@ -67,12 +67,43 @@ func runModTidy(cmd *Command, args []string) error {
 	if err != nil {
 		return fmt.Errorf("internal error: invalid module.cue file generated: %v", err)
 	}
-	if err := os.WriteFile(filepath.Join("cue.mod", "module.cue"), data, 0o666); err != nil {
+	modPath := filepath.Join(modRoot, "cue.mod", "module.cue")
+	oldData, err := os.ReadFile(modPath)
+	if err != nil {
+		// Shouldn't happen because modload.Load returns an error
+		// if it can't load the module file.
+		return err
+	}
+	if bytes.Equal(data, oldData) {
+		return nil
+	}
+	if err := os.WriteFile(modPath, data, 0o666); err != nil {
 		return err
 	}
 	return nil
 }
 
+func findModuleRoot() (string, error) {
+	// TODO this logic is duplicated in multiple places. We should
+	// consider deduplicating it.
+	dir, err := os.Getwd()
+	if err != nil {
+		return "", err
+	}
+	for {
+		if _, err := os.Stat(filepath.Join(dir, "cue.mod")); err == nil {
+			return dir, nil
+		} else if !os.IsNotExist(err) {
+			return "", err
+		}
+		dir1 := filepath.Dir(dir)
+		if dir1 == dir {
+			return "", fmt.Errorf("module root not found")
+		}
+		dir = dir1
+	}
+}
+
 func modCacheDir() (string, error) {
 	if dir := os.Getenv("CUE_MODCACHE"); dir != "" {
 		return dir, nil

cmd/cue/cmd/testdata/script/modtidy_non_root.txtar

@@ -0,0 +1,17 @@
+# Check that cue mod tidy works OK even when not
+# run in the module's root directory.
+
+cd x/y
+exec cue mod tidy
+cd $WORK
+cmp cue.mod/module.cue want-module
+
+-- want-module --
+module: "main.org@v0"
+-- cue.mod/module.cue --
+module: "main.org@v0"
+-- x/y/z.cue --
+package y
+-- _registry/example.com_v0.0.1/cue.mod/module.cue --
+// This file is just here to ensure that CUE_REGISTRY etc is set.
+module: "example.com@v0"