feat: implement gno mod init command (gnolang#955)

Implement `gno mod init` command.

Usage: 
```sh
$ gno mod init <module-path>
```
If no `<module-path>` is given, tries to determine package name from
existing `*.gno` files.

<details><summary>Checklists...</summary>

## Contributors Checklist

- [x] Added new tests, or not needed, or not feasible
- [x] Provided an example (e.g. screenshot) to aid review or the PR is
self-explanatory
- [x] Updated the official documentation or not needed
- [x] No breaking changes were made, or a `BREAKING CHANGE: xxx` message
was included in the description
- [ ] Added references to related issues and PRs
- [x] Provided any useful hints for running manual tests
- [ ] Added new benchmarks to [generated
graphs](https://gnoland.github.io/benchmarks), if any. More info
[here](../.benchmarks/README.md).

## Maintainers Checklist

- [x] Checked that the author followed the guidelines in
`CONTRIBUTING.md`
- [x] Checked the conventional-commit (especially PR title and verb,
presence of `BREAKING CHANGE:` in the body)
- [x] Ensured that this PR is not a significant change or confirmed that
the review/consideration process was appropriate for the change
</details>

gnovm/cmd/gno/mod.go

@@ -30,6 +30,7 @@ func newModCmd(io *commands.IO) *commands.Command {
 
 	cmd.AddSubCommands(
 		newModDownloadCmd(io),
+		newModInitCmd(),
 	)
 
 	return cmd
@@ -51,6 +52,20 @@ func newModDownloadCmd(io *commands.IO) *commands.Command {
 	)
 }
 
+func newModInitCmd() *commands.Command {
+	return commands.NewCommand(
+		commands.Metadata{
+			Name:       "init",
+			ShortUsage: "init [module-path]",
+			ShortHelp:  "Initialize gno.mod file in current directory",
+		},
+		commands.NewEmptyConfig(),
+		func(_ context.Context, args []string) error {
+			return execModInit(args)
+		},
+	)
+}
+
 func (c *modDownloadCfg) RegisterFlags(fs *flag.FlagSet) {
 	fs.StringVar(
 		&c.remote,
@@ -111,10 +126,29 @@ func execModDownload(cfg *modDownloadCfg, args []string, io *commands.IO) error
 	}
 
 	// write go.mod file
-	err = gomod.WriteToPath(path)
+	err = gomod.WriteToPath(filepath.Join(path, "go.mod"))
 	if err != nil {
 		return fmt.Errorf("write go.mod file: %w", err)
 	}
 
 	return nil
 }
+
+func execModInit(args []string) error {
+	if len(args) > 1 {
+		return flag.ErrHelp
+	}
+	var modPath string
+	if len(args) == 1 {
+		modPath = args[0]
+	}
+	dir, err := os.Getwd()
+	if err != nil {
+		return err
+	}
+	if err := gnomod.CreateGnoModFile(dir, modPath); err != nil {
+		return fmt.Errorf("create gno.mod file: %w", err)
+	}
+
+	return nil
+}

gnovm/cmd/gno/mod_test.go

@@ -9,7 +9,7 @@ func TestModApp(t *testing.T) {
 			errShouldBe: "flag: help requested",
 		},
 
-		// test gno.mod
+		// test gno.mod download
 		{
 			args:                 []string{"mod", "download"},
 			testDir:              "../../tests/integ/empty-dir",
@@ -72,6 +72,71 @@ func TestModApp(t *testing.T) {
 			simulateExternalRepo: true,
 			errShouldContain:     "fetch: writepackage: querychain",
 		},
+
+		// test gno.mod init with no module name
+		{
+			args:                 []string{"mod", "init"},
+			testDir:              "../../tests/integ/valid1",
+			simulateExternalRepo: true,
+		},
+		{
+			args:                 []string{"mod", "init"},
+			testDir:              "../../tests/integ/empty-dir",
+			simulateExternalRepo: true,
+			errShouldBe:          "create gno.mod file: cannot determine package name",
+		},
+		{
+			args:                 []string{"mod", "init"},
+			testDir:              "../../tests/integ/empty-gno1",
+			simulateExternalRepo: true,
+			recoverShouldContain: "expected 'package', found 'EOF'",
+		},
+		{
+			args:                 []string{"mod", "init"},
+			testDir:              "../../tests/integ/empty-gno2",
+			simulateExternalRepo: true,
+			recoverShouldContain: "expected 'package', found 'EOF'",
+		},
+		{
+			args:                 []string{"mod", "init"},
+			testDir:              "../../tests/integ/empty-gno3",
+			simulateExternalRepo: true,
+			recoverShouldContain: "expected 'package', found 'EOF'",
+		},
+		{
+			args:                 []string{"mod", "init"},
+			testDir:              "../../tests/integ/empty-gnomod",
+			simulateExternalRepo: true,
+			errShouldBe:          "create gno.mod file: gno.mod file already exists",
+		},
+
+		// test gno.mod init with module name
+		{
+			args:                 []string{"mod", "init", "gno.land/p/demo/foo"},
+			testDir:              "../../tests/integ/empty-dir",
+			simulateExternalRepo: true,
+		},
+		{
+			args:                 []string{"mod", "init", "gno.land/p/demo/foo"},
+			testDir:              "../../tests/integ/empty-gno1",
+			simulateExternalRepo: true,
+		},
+		{
+			args:                 []string{"mod", "init", "gno.land/p/demo/foo"},
+			testDir:              "../../tests/integ/empty-gno2",
+			simulateExternalRepo: true,
+		},
+		{
+			args:                 []string{"mod", "init", "gno.land/p/demo/foo"},
+			testDir:              "../../tests/integ/empty-gno3",
+			simulateExternalRepo: true,
+		},
+		{
+			args:                 []string{"mod", "init", "gno.land/p/demo/foo"},
+			testDir:              "../../tests/integ/empty-gnomod",
+			simulateExternalRepo: true,
+			errShouldBe:          "create gno.mod file: gno.mod file already exists",
+		},
 	}
 	testMainCaseRun(t, tc)
 }

gnovm/docs/go-gno-compatibility.md

@@ -361,6 +361,7 @@ Additional native types:
 | go list           |                  |                                                                       |
 | go mod            |                  |                                                                       |
 | + go mod download | gno mod download | same behavior                                                         |
+| + go mod init     | gno mod init     | same behavior                                                         |
 |                   | gno precompile   |                                                                       |
 | go work           |                  |                                                                       |
 |                   | gno repl         |                                                                       |

gnovm/pkg/gnomod/file.go

@@ -110,7 +110,9 @@ func (f *File) FetchDeps(path string, remote string, verbose bool) error {
 		if err != nil {
 			return err
 		}
-		err = goMod.WriteToPath(PackageDir(path, mod))
+		pkgPath := PackageDir(path, mod)
+		goModFilePath := filepath.Join(pkgPath, "go.mod")
+		err = goMod.WriteToPath(goModFilePath)
 		if err != nil {
 			return err
 		}
@@ -119,10 +121,10 @@ func (f *File) FetchDeps(path string, remote string, verbose bool) error {
 	return nil
 }
 
-// WriteToPath writes go.mod file in the given absolute path
+// WriteToPath writes file to the given absolute file path
 // TODO: Find better way to do this. Try to use `modfile`
 // package to manage this.
-func (f *File) WriteToPath(absPath string) error {
+func (f *File) WriteToPath(absFilePath string) error {
 	if f.Module == nil {
 		return errors.New("writing go.mod: module not found")
 	}
@@ -150,10 +152,9 @@ func (f *File) WriteToPath(absPath string) error {
 		data += ")\n"
 	}
 
-	modPath := filepath.Join(absPath, "go.mod")
-	err := os.WriteFile(modPath, []byte(data), 0o644)
+	err := os.WriteFile(absFilePath, []byte(data), 0o644)
 	if err != nil {
-		return fmt.Errorf("writefile %q: %w", modPath, err)
+		return fmt.Errorf("writefile %q: %w", absFilePath, err)
 	}
 
 	return nil

gnovm/pkg/gnomod/gnomod.go

@@ -1,7 +1,9 @@
 package gnomod
 
 import (
+	"errors"
 	"fmt"
+	"io/ioutil"
 	"os"
 	"path/filepath"
 	"strings"
@@ -163,6 +165,68 @@ func GnoToGoMod(f File) (*File, error) {
 	return &f, nil
 }
 
+func CreateGnoModFile(rootDir, modPath string) error {
+	if !filepath.IsAbs(rootDir) {
+		return fmt.Errorf("dir %q is not absolute", rootDir)
+	}
+
+	modFilePath := filepath.Join(rootDir, "gno.mod")
+	if _, err := os.Stat(modFilePath); err == nil {
+		return errors.New("gno.mod file already exists")
+	}
+
+	if modPath == "" {
+		// Check .gno files for package name
+		// and use it as modPath
+		files, err := ioutil.ReadDir(rootDir)
+		if err != nil {
+			fmt.Errorf("read dir %q: %w", rootDir, err)
+		}
+
+		var pkgName gnolang.Name
+		for _, file := range files {
+			if file.IsDir() || !strings.HasSuffix(file.Name(), ".gno") || strings.HasSuffix(file.Name(), "_filetest.gno") {
+				continue
+			}
+
+			fpath := filepath.Join(rootDir, file.Name())
+			bz, err := os.ReadFile(fpath)
+			if err != nil {
+				return fmt.Errorf("read file %q: %w", fpath, err)
+			}
+
+			pn := gnolang.PackageNameFromFileBody(file.Name(), string(bz))
+			if strings.HasSuffix(string(pkgName), "_test") {
+				pkgName = pkgName[:len(pkgName)-len("_test")]
+			}
+			if pkgName == "" {
+				pkgName = pn
+			}
+			if pkgName != pn {
+				return fmt.Errorf("package name mismatch: [%q] and [%q]", pkgName, pn)
+			}
+		}
+		if pkgName == "" {
+			return errors.New("cannot determine package name")
+		}
+		modPath = string(pkgName)
+	}
+	if err := module.CheckImportPath(modPath); err != nil {
+		return err
+	}
+
+	modFile := &File{
+		Module: &modfile.Module{
+			Mod: module.Version{
+				Path: modPath,
+			},
+		},
+	}
+	modFile.WriteToPath(filepath.Join(rootDir, "gno.mod"))
+
+	return nil
+}
+
 func isReplaced(mod module.Version, repl []*modfile.Replace) (module.Version, bool) {
 	for _, r := range repl {
 		hasNoVersion := r.Old.Path == mod.Path && r.Old.Version == ""