cmd/go: enable module mode without a main module when GO111MODULE=on

This is as minimal a change as I could comfortably make to enable 'go
get' outside of a module for 1.12.

In general, commands invoked in module mode while outside of a module
operate as though they are in a module with an initially-empty go.mod
file. ('go env GOMOD' reports os.DevNull.)

Commands that operate on the current directory (such as 'go list' and
'go get -u' without arguments) fail: without a module definition, we
don't know the package path. Likewise, commands whose sole purpose is
to write files within the main module (such as 'go mod edit' and 'go
mod vendor') fail, since we don't know where to write their output.

Since the go.sum file for the main module is authoritative, we do not
check go.sum files when operating outside of a module. I plan to
revisit that when the tree opens for 1.13.

We may also want to revisit the behavior of 'go list': it would be
useful to be able to query individual packages (and dependencies of
those packages) within versioned modules, but today we only allow
versioned paths in conjunction with the '-m' flag.

Fixes #24250

RELNOTE=yes

Change-Id: I028c323ddea27693a92ad0aa4a6a55d5e3f43f2c
Reviewed-on: https://go-review.googlesource.com/c/148517
Run-TryBot: Bryan C. Mills <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Russ Cox <[email protected]>

Author: Bryan C. Mills

src/cmd/go/internal/clean/clean.go

@@ -105,10 +105,7 @@ func init() {
 }
 
 func runClean(cmd *base.Command, args []string) {
-	if len(args) == 0 && modload.Failed() {
-		// Don't try to clean current directory,
-		// which will cause modload to base.Fatalf.
-	} else {
+	if len(args) > 0 || !modload.Enabled() || modload.HasModRoot() {
 		for _, pkg := range load.PackagesAndErrors(args) {
 			clean(pkg)
 		}

src/cmd/go/internal/envcmd/env.go

@@ -115,8 +115,10 @@ func findEnv(env []cfg.EnvVar, name string) string {
 // ExtraEnvVars returns environment variables that should not leak into child processes.
 func ExtraEnvVars() []cfg.EnvVar {
 	gomod := ""
-	if modload.Init(); modload.ModRoot != "" {
-		gomod = filepath.Join(modload.ModRoot, "go.mod")
+	if modload.HasModRoot() {
+		gomod = filepath.Join(modload.ModRoot(), "go.mod")
+	} else if modload.Enabled() {
+		gomod = os.DevNull
 	}
 	return []cfg.EnvVar{
 		{Name: "GOMOD", Value: gomod},

src/cmd/go/internal/load/pkg.go

@@ -1515,7 +1515,9 @@ func (p *Package) load(stk *ImportStack, bp *build.Package, err error) {
 	}
 
 	if cfg.ModulesEnabled {
-		p.Module = ModPackageModuleInfo(p.ImportPath)
+		if !p.Internal.CmdlineFiles {
+			p.Module = ModPackageModuleInfo(p.ImportPath)
+		}
 		if p.Name == "main" {
 			p.Internal.BuildInfo = ModPackageBuildInfo(p.ImportPath, p.Deps)
 		}

src/cmd/go/internal/modcmd/edit.go

@@ -157,8 +157,7 @@ func runEdit(cmd *base.Command, args []string) {
 	if len(args) == 1 {
 		gomod = args[0]
 	} else {
-		modload.MustInit()
-		gomod = filepath.Join(modload.ModRoot, "go.mod")
+		gomod = filepath.Join(modload.ModRoot(), "go.mod")
 	}
 
 	if *editModule != "" {

src/cmd/go/internal/modcmd/vendor.go

@@ -43,7 +43,7 @@ func runVendor(cmd *base.Command, args []string) {
 	}
 	pkgs := modload.LoadVendor()
 
-	vdir := filepath.Join(modload.ModRoot, "vendor")
+	vdir := filepath.Join(modload.ModRoot(), "vendor")
 	if err := os.RemoveAll(vdir); err != nil {
 		base.Fatalf("go mod vendor: %v", err)
 	}

src/cmd/go/internal/modget/get.go

@@ -281,8 +281,8 @@ func runGet(cmd *base.Command, args []string) {
 				base.Errorf("go get %s: %v", arg, err)
 				continue
 			}
-			if !str.HasFilePathPrefix(abs, modload.ModRoot) {
-				base.Errorf("go get %s: directory %s is outside module root %s", arg, abs, modload.ModRoot)
+			if !str.HasFilePathPrefix(abs, modload.ModRoot()) {
+				base.Errorf("go get %s: directory %s is outside module root %s", arg, abs, modload.ModRoot())
 				continue
 			}
 			// TODO: Check if abs is inside a nested module.

src/cmd/go/internal/modload/build.go

@@ -98,8 +98,8 @@ func moduleInfo(m module.Version, fromBuildList bool) *modinfo.ModulePublic {
 			Path:    m.Path,
 			Version: m.Version,
 			Main:    true,
-			Dir:     ModRoot,
-			GoMod:   filepath.Join(ModRoot, "go.mod"),
+			Dir:     ModRoot(),
+			GoMod:   filepath.Join(ModRoot(), "go.mod"),
 		}
 		if modFile.Go != nil {
 			info.GoVersion = modFile.Go.Version
@@ -117,7 +117,7 @@ func moduleInfo(m module.Version, fromBuildList bool) *modinfo.ModulePublic {
 	}
 
 	if cfg.BuildMod == "vendor" {
-		info.Dir = filepath.Join(ModRoot, "vendor", m.Path)
+		info.Dir = filepath.Join(ModRoot(), "vendor", m.Path)
 		return info
 	}
 
@@ -171,7 +171,7 @@ func moduleInfo(m module.Version, fromBuildList bool) *modinfo.ModulePublic {
 		if filepath.IsAbs(r.Path) {
 			info.Replace.Dir = r.Path
 		} else {
-			info.Replace.Dir = filepath.Join(ModRoot, r.Path)
+			info.Replace.Dir = filepath.Join(ModRoot(), r.Path)
 		}
 	}
 	complete(info.Replace)

src/cmd/go/internal/modload/import.go

@@ -67,8 +67,8 @@ func Import(path string) (m module.Version, dir string, err error) {
 	// -mod=vendor is special.
 	// Everything must be in the main module or the main module's vendor directory.
 	if cfg.BuildMod == "vendor" {
-		mainDir, mainOK := dirInModule(path, Target.Path, ModRoot, true)
-		vendorDir, vendorOK := dirInModule(path, "", filepath.Join(ModRoot, "vendor"), false)
+		mainDir, mainOK := dirInModule(path, Target.Path, ModRoot(), true)
+		vendorDir, vendorOK := dirInModule(path, "", filepath.Join(ModRoot(), "vendor"), false)
 		if mainOK && vendorOK {
 			return module.Version{}, "", fmt.Errorf("ambiguous import: found %s in multiple directories:\n\t%s\n\t%s", path, mainDir, vendorDir)
 		}