[dev.cmdgo] cmd/go: add go mod initwork command

This command is used to create a go.work file with a set of modules
given in the arguments to the command.

For #45713
Change-Id: I09f8cefc5849dd43c234dc4a37091791fcc02ebe
Reviewed-on: https://go-review.googlesource.com/c/go/+/334936
Trust: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/alldocs.go

@@ -1034,6 +1034,7 @@
 // 	edit        edit go.mod from tools or scripts
 // 	graph       print module requirement graph
 // 	init        initialize new module in current directory
+// 	initwork    initialize workspace file
 // 	tidy        add missing and remove unused modules
 // 	vendor      make vendored copy of dependencies
 // 	verify      verify dependencies have expected content
@@ -1229,6 +1230,23 @@
 // See https://golang.org/ref/mod#go-mod-init for more about 'go mod init'.
 //
 //
+// Initialize workspace file
+//
+// Usage:
+//
+// 	go mod initwork [moddirs]
+//
+// go mod initwork initializes and writes a new go.work file in the current
+// directory, in effect creating a new workspace at the current directory.
+//
+// go mod initwork optionally accepts paths to the workspace modules as arguments.
+// If the argument is omitted, an empty workspace with no modules will be created.
+//
+// See the workspaces design proposal at
+// https://go.googlesource.com/proposal/+/master/design/45713-workspace.md for
+// more information.
+//
+//
 // Add missing and remove unused modules
 //
 // Usage:

src/cmd/go/internal/modcmd/initwork.go

@@ -0,0 +1,54 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// go mod initwork
+
+package modcmd
+
+import (
+	"cmd/go/internal/base"
+	"cmd/go/internal/modload"
+	"context"
+	"path/filepath"
+)
+
+var _ = modload.TODOWorkspaces("Add more documentation below.T hough this is" +
+	"enough for those trying workspaces out, there should be more through" +
+	"documentation if the proposal is accepted.")
+
+var cmdInitwork = &base.Command{
+	UsageLine: "go mod initwork [moddirs]",
+	Short:     "initialize workspace file",
+	Long: `go mod initwork initializes and writes a new go.work file in the current
+directory, in effect creating a new workspace at the current directory.
+
+go mod initwork optionally accepts paths to the workspace modules as arguments.
+If the argument is omitted, an empty workspace with no modules will be created.
+
+See the workspaces design proposal at
+https://go.googlesource.com/proposal/+/master/design/45713-workspace.md for
+more information.
+`,
+	Run: runInitwork,
+}
+
+func init() {
+	base.AddModCommonFlags(&cmdInitwork.Flag)
+	base.AddWorkfileFlag(&cmdInitwork.Flag)
+}
+
+func runInitwork(ctx context.Context, cmd *base.Command, args []string) {
+	modload.InitWorkfile()
+
+	modload.ForceUseModules = true
+
+	// TODO(matloob): support using the -workfile path
+	// To do that properly, we'll have to make the module directories
+	// make dirs relative to workFile path before adding the paths to
+	// the directory entries
+
+	workFile := filepath.Join(base.Cwd(), "go.work")
+
+	modload.CreateWorkFile(ctx, workFile, args)
+}

src/cmd/go/internal/modcmd/mod.go

@@ -25,6 +25,7 @@ See 'go help modules' for an overview of module functionality.
 		cmdEdit,
 		cmdGraph,
 		cmdInit,
+		cmdInitwork,
 		cmdTidy,
 		cmdVendor,
 		cmdVerify,

src/cmd/go/internal/modload/init.go

@@ -767,6 +767,24 @@ func CreateModFile(ctx context.Context, modPath string) {
 	}
 }
 
+// CreateWorkFile initializes a new workspace by creating a go.work file.
+func CreateWorkFile(ctx context.Context, workFile string, modDirs []string) {
+	_ = TODOWorkspaces("Report an error if the file already exists.")
+
+	goV := LatestGoVersion() // Use current Go version by default
+	workF := new(modfile.WorkFile)
+	workF.Syntax = new(modfile.FileSyntax)
+	workF.AddGoStmt(goV)
+
+	for _, dir := range modDirs {
+		_ = TODOWorkspaces("Add the module path of the module.")
+		workF.AddDirectory(dir, "")
+	}
+
+	data := modfile.Format(workF.Syntax)
+	lockedfile.Write(workFile, bytes.NewReader(data), 0644)
+}
+
 // fixVersion returns a modfile.VersionFixer implemented using the Query function.
 //
 // It resolves commit hashes and branch names to versions,

src/cmd/go/testdata/script/work.txt

@@ -1,3 +1,6 @@
+go mod initwork ./a ./b
+cmp go.work go.work.want
+
 go run example.com/b
 stdout 'Hello from module A'
 
@@ -31,14 +34,13 @@ directory (
   b
   ../src/a
 )
--- go.work --
+-- go.work.want --
 go 1.17
 
 directory (
-  ./a
+	./a
-  ./b
+	./b
 )
-
 -- a/go.mod --
 
 module example.com/a