cmd/go: refactor modload.InitMod

InitMod is split into two functions. LoadModFile parses an existing
go.mod file and loads the build list (or checks vendor/modules.txt for
consistency in vendor mode). CreateModFile creates a new go.mod file,
possibly inferring the module path and importing a vendor
configuration file.

Some logic is moved from runInit to CreateModFile. init-specific logic
is removed from other functions.

This CL shouldn't cause substantial differences in behavior, though
some error messages are slightly different.

For #41712

Change-Id: Ia684945cfcf5beca30bbb81e7144fc246c4f27ed
Reviewed-on: https://go-review.googlesource.com/c/go/+/264621
Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

7 files changed, 73 insertions, 72 deletions

diff --git a/src/cmd/go/internal/list/list.go b/src/cmd/go/internal/list/list.go
index 9fd9d7446d..1c77e4d478 100644
--- a/src/cmd/go/internal/list/list.go
+++ b/src/cmd/go/internal/list/list.go
@@ -415,7 +415,7 @@ func runList(ctx context.Context, cmd *base.Command, args []string) {
 			base.Fatalf("go list -m: not using modules")
 		}
 
-		modload.InitMod(ctx) // Parses go.mod and sets cfg.BuildMod.
+		modload.LoadModFile(ctx) // Parses go.mod and sets cfg.BuildMod.
 		if cfg.BuildMod == "vendor" {
 			const actionDisabledFormat = "go list -m: can't %s using the vendor directory\n\t(Use -mod=mod or -mod=readonly to bypass.)"
 
diff --git a/src/cmd/go/internal/modcmd/download.go b/src/cmd/go/internal/modcmd/download.go
index 050a2e0e12..e2e8ba6825 100644
--- a/src/cmd/go/internal/modcmd/download.go
+++ b/src/cmd/go/internal/modcmd/download.go
@@ -87,7 +87,7 @@ func runDownload(ctx context.Context, cmd *base.Command, args []string) {
 	if len(args) == 0 {
 		args = []string{"all"}
 	} else if modload.HasModRoot() {
-		modload.InitMod(ctx) // to fill Target
+		modload.LoadModFile(ctx) // to fill Target
 		targetAtLatest := modload.Target.Path + "@latest"
 		targetAtUpgrade := modload.Target.Path + "@upgrade"
 		targetAtPatch := modload.Target.Path + "@patch"
diff --git a/src/cmd/go/internal/modcmd/init.go b/src/cmd/go/internal/modcmd/init.go
index 7cfc0e6f5b..7384f3f293 100644
--- a/src/cmd/go/internal/modcmd/init.go
+++ b/src/cmd/go/internal/modcmd/init.go
@@ -10,8 +10,6 @@ import (
 	"cmd/go/internal/base"
 	"cmd/go/internal/modload"
 	"context"
-	"os"
-	"strings"
 )
 
 var cmdInit = &base.Command{
@@ -33,21 +31,14 @@ func init() {
 }
 
 func runInit(ctx context.Context, cmd *base.Command, args []string) {
-	modload.CmdModInit = true
 	if len(args) > 1 {
 		base.Fatalf("go mod init: too many arguments")
 	}
+	var modPath string
 	if len(args) == 1 {
-		modload.CmdModModule = args[0]
+		modPath = args[0]
 	}
+
 	modload.ForceUseModules = true
-	modFilePath := modload.ModFilePath()
-	if _, err := os.Stat(modFilePath); err == nil {
-		base.Fatalf("go mod init: go.mod already exists")
-	}
-	if strings.Contains(modload.CmdModModule, "@") {
-		base.Fatalf("go mod init: module path must not contain '@'")
-	}
-	modload.InitMod(ctx) // does all the hard work
-	modload.WriteGoMod()
+	modload.CreateModFile(ctx, modPath) // does all the hard work
 }
diff --git a/src/cmd/go/internal/modload/buildlist.go b/src/cmd/go/internal/modload/buildlist.go
index 95a68637c6..76e5fe0173 100644
--- a/src/cmd/go/internal/modload/buildlist.go
+++ b/src/cmd/go/internal/modload/buildlist.go
@@ -37,7 +37,7 @@ var buildList []module.Version
 //
 // The caller must not modify the returned list.
 func LoadAllModules(ctx context.Context) []module.Version {
-	InitMod(ctx)
+	LoadModFile(ctx)
 	ReloadBuildList()
 	WriteGoMod()
 	return buildList
diff --git a/src/cmd/go/internal/modload/init.go b/src/cmd/go/internal/modload/init.go
index 9baaf41124..7a8d826994 100644
--- a/src/cmd/go/internal/modload/init.go
+++ b/src/cmd/go/internal/modload/init.go
@@ -50,9 +50,6 @@ var (
 
 	gopath string
 
-	CmdModInit   bool   // running 'go mod init'
-	CmdModModule string // module argument for 'go mod init'
-
 	// RootMode determines whether a module root is needed.
 	RootMode Root
 
@@ -163,9 +160,9 @@ func Init() {
 		os.Setenv("GIT_SSH_COMMAND", "ssh -o ControlMaster=no")
 	}
 
-	if CmdModInit {
-		// Running 'go mod init': go.mod will be created in current directory.
-		modRoot = base.Cwd
+	if modRoot != "" {
+		// modRoot set before Init was called ("go mod init" does this).
+		// No need to search for go.mod.
 	} else if RootMode == NoRoot {
 		if cfg.ModFile != "" && !base.InGOFLAGS("-modfile") {
 			base.Fatalf("go: -modfile cannot be used with commands that ignore the current module")
@@ -202,8 +199,7 @@ func Init() {
 		base.Fatalf("go: -modfile=%s: file does not have .mod extension", cfg.ModFile)
 	}
 
-	// We're in module mode. Install the hooks to make it work.
-
+	// We're in module mode. Set any global variables that need to be set.
 	list := filepath.SplitList(cfg.BuildContext.GOPATH)
 	if len(list) == 0 || list[0] == "" {
 		base.Fatalf("missing $GOPATH")
@@ -270,10 +266,6 @@ func WillBeEnabled() bool {
 		return false
 	}
 
-	if CmdModInit {
-		// Running 'go mod init': go.mod will be created in current directory.
-		return true
-	}
 	if modRoot := findModuleRoot(base.Cwd); modRoot == "" {
 		// GO111MODULE is 'auto', and we can't find a module root.
 		// Stay in GOPATH mode.
@@ -347,16 +339,16 @@ func die() {
 	base.Fatalf("go: cannot find main module; see 'go help modules'")
 }
 
-// InitMod sets Target and, if there is a main module, parses the initial build
-// list from its go.mod file. If InitMod is called by 'go mod init', InitMod
-// will populate go.mod in memory, possibly importing dependencies from a
-// legacy configuration file. For other commands, InitMod may make other
-// adjustments in memory, like adding a go directive. WriteGoMod should be
-// called later to write changes out to disk.
+// LoadModFile sets Target and, if there is a main module, parses the initial
+// build list from its go.mod file.
+//
+// LoadModFile may make changes in memory, like adding a go directive and
+// ensuring requirements are consistent. WriteGoMod should be called later to
+// write changes out to disk or report errors in readonly mode.
 //
-// As a side-effect, InitMod sets a default for cfg.BuildMod if it does not
+// As a side-effect, LoadModFile sets a default for cfg.BuildMod if it does not
 // already have an explicit value.
-func InitMod(ctx context.Context) {
+func LoadModFile(ctx context.Context) {
 	if len(buildList) > 0 {
 		return
 	}
@@ -369,13 +361,6 @@ func InitMod(ctx context.Context) {
 		return
 	}
 
-	if CmdModInit {
-		// Running go mod init: do legacy module conversion
-		legacyModInit()
-		modFileToBuildList()
-		return
-	}
-
 	gomod := ModFilePath()
 	data, err := lockedfile.Read(gomod)
 	if err != nil {
@@ -408,6 +393,50 @@ func InitMod(ctx context.Context) {
 	}
 }
 
+// CreateModFile initializes a new module by creating a go.mod file.
+//
+// If modPath is empty, CreateModFile will attempt to infer the path from the
+// directory location within GOPATH.
+//
+// If a vendoring configuration file is present, CreateModFile will attempt to
+// translate it to go.mod directives. The resulting build list may not be
+// exactly the same as in the legacy configuration (for example, we can't get
+// packages at multiple versions from the same module).
+func CreateModFile(ctx context.Context, modPath string) {
+	modRoot = base.Cwd
+	Init()
+	modFilePath := ModFilePath()
+	if _, err := os.Stat(modFilePath); err == nil {
+		base.Fatalf("go: %s already exists", modFilePath)
+	}
+
+	if modPath == "" {
+		var err error
+		modPath, err = findModulePath(modRoot)
+		if err != nil {
+			base.Fatalf("go: %v", err)
+		}
+	} else if err := checkModulePathLax(modPath); err != nil {
+		base.Fatalf("go: %v", err)
+	}
+
+	fmt.Fprintf(os.Stderr, "go: creating new go.mod: module %s\n", modPath)
+	modFile = new(modfile.File)
+	modFile.AddModuleStmt(modPath)
+	addGoStmt() // Add the go directive before converted module requirements.
+
+	convertedFrom, err := convertLegacyConfig(modPath)
+	if convertedFrom != "" {
+		fmt.Fprintf(os.Stderr, "go: copying requirements from %s\n", base.ShortPath(convertedFrom))
+	}
+	if err != nil {
+		base.Fatalf("go: %v", err)
+	}
+
+	modFileToBuildList()
+	WriteGoMod()
+}
+
 // checkModulePathLax checks that the path meets some minimum requirements
 // to avoid confusing users or the module cache. The requirements are weaker
 // than those of module.CheckPath to allow room for weakening module path
@@ -574,34 +603,23 @@ func setDefaultBuildMod() {
 	cfg.BuildMod = "readonly"
 }
 
-func legacyModInit() {
-	if modFile == nil {
-		path, err := findModulePath(modRoot)
-		if err != nil {
-			base.Fatalf("go: %v", err)
-		}
-		fmt.Fprintf(os.Stderr, "go: creating new go.mod: module %s\n", path)
-		modFile = new(modfile.File)
-		modFile.AddModuleStmt(path)
-		addGoStmt() // Add the go directive before converted module requirements.
-	}
-
+// convertLegacyConfig imports module requirements from a legacy vendoring
+// configuration file, if one is present.
+func convertLegacyConfig(modPath string) (from string, err error) {
 	for _, name := range altConfigs {
 		cfg := filepath.Join(modRoot, name)
 		data, err := ioutil.ReadFile(cfg)
 		if err == nil {
 			convert := modconv.Converters[name]
 			if convert == nil {
-				return
+				return "", nil
 			}
-			fmt.Fprintf(os.Stderr, "go: copying requirements from %s\n", base.ShortPath(cfg))
 			cfg = filepath.ToSlash(cfg)
-			if err := modconv.ConvertLegacyConfig(modFile, cfg, data); err != nil {
-				base.Fatalf("go: %v", err)
-			}
-			return
+			err := modconv.ConvertLegacyConfig(modFile, cfg, data)
+			return name, err
 		}
 	}
+	return "", nil
 }
 
 // addGoStmt adds a go directive to the go.mod file if it does not already include one.
@@ -681,14 +699,6 @@ func findAltConfig(dir string) (root, name string) {
 }
 
 func findModulePath(dir string) (string, error) {
-	if CmdModModule != "" {
-		// Running go mod init x/y/z; return x/y/z.
-		if err := module.CheckImportPath(CmdModModule); err != nil {
-			return "", err
-		}
-		return CmdModModule, nil
-	}
-
 	// TODO(bcmills): once we have located a plausible module path, we should
 	// query version control (if available) to verify that it matches the major
 	// version of the most recent tag.
diff --git a/src/cmd/go/internal/modload/load.go b/src/cmd/go/internal/modload/load.go
index 4611fc7f6e..dc816540b9 100644
--- a/src/cmd/go/internal/modload/load.go
+++ b/src/cmd/go/internal/modload/load.go
@@ -169,7 +169,7 @@ type PackageOpts struct {
 // LoadPackages identifies the set of packages matching the given patterns and
 // loads the packages in the import graph rooted at that set.
 func LoadPackages(ctx context.Context, opts PackageOpts, patterns ...string) (matches []*search.Match, loadedPackages []string) {
-	InitMod(ctx)
+	LoadModFile(ctx)
 	if opts.Tags == nil {
 		opts.Tags = imports.Tags()
 	}
@@ -494,7 +494,7 @@ func pathInModuleCache(dir string) string {
 // ImportFromFiles adds modules to the build list as needed
 // to satisfy the imports in the named Go source files.
 func ImportFromFiles(ctx context.Context, gofiles []string) {
-	InitMod(ctx)
+	LoadModFile(ctx)
 
 	tags := imports.Tags()
 	imports, testImports, err := imports.ScanFiles(gofiles, tags)
diff --git a/src/cmd/go/testdata/script/mod_init_path.txt b/src/cmd/go/testdata/script/mod_init_path.txt
index 637c29f4bc..ccdfc92317 100644
--- a/src/cmd/go/testdata/script/mod_init_path.txt
+++ b/src/cmd/go/testdata/script/mod_init_path.txt
@@ -1,7 +1,7 @@
 env GO111MODULE=on
 
 ! go mod init .
-stderr 'malformed import path'
+stderr '^go: invalid module path "\.": is a local import path$'
 
 cd x
 go mod init example.com/x