cmd/go: change the default value of GO111MODULE to 'on'

This reverts CL 166985, restoring CL 162698.

The bootstrap failure from CL 162698 was fixed in
CL 167077 and CL 167078.

Fixes #30228

Change-Id: I5a4e3081018c51b74b67185e64f20a9c824a564e
Reviewed-on: https://go-review.googlesource.com/c/go/+/167087
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/help_test.go

@@ -12,9 +12,14 @@ import (
 	"testing"
 
 	"cmd/go/internal/help"
+	"cmd/go/internal/modload"
 )
 
 func TestDocsUpToDate(t *testing.T) {
+	if !modload.Enabled() {
+		t.Skipf("help.Help in GOPATH mode is configured by main.main")
+	}
+
 	buf := new(bytes.Buffer)
 	// Match the command in mkalldocs.sh that generates alldocs.go.
 	help.Help(buf, []string{"documentation"})

src/cmd/go/internal/help/help.go

@@ -17,6 +17,7 @@ import (
 	"unicode/utf8"
 
 	"cmd/go/internal/base"
+	"cmd/go/internal/modload"
 )
 
 // Help implements the 'help' command.
@@ -35,8 +36,10 @@ func Help(w io.Writer, args []string) {
 		usage := &base.Command{Long: buf.String()}
 		cmds := []*base.Command{usage}
 		for _, cmd := range base.Go.Commands {
-			if cmd.UsageLine == "gopath-get" {
 			// Avoid duplication of the "get" documentation.
+			if cmd.UsageLine == "module-get" && modload.Enabled() {
+				continue
+			} else if cmd.UsageLine == "gopath-get" && !modload.Enabled() {
 				continue
 			}
 			cmds = append(cmds, cmd)

src/cmd/go/internal/modload/help.go

@@ -19,34 +19,25 @@ including recording and resolving dependencies on other modules.
 Modules replace the old GOPATH-based approach to specifying
 which source files are used in a given build.
 
-Preliminary module support
+Module support
 
-Go 1.11 includes preliminary support for Go modules,
-including a new module-aware 'go get' command.
-We intend to keep revising this support, while preserving compatibility,
-until it can be declared official (no longer preliminary),
-and then at a later point we may remove support for work
-in GOPATH and the old 'go get' command.
+Go 1.13 includes official support for Go modules,
+including a module-aware 'go get' command.
+Module-aware mode is active by default.
 
-The quickest way to take advantage of the new Go 1.11 module support
-is to check out your repository into a directory outside GOPATH/src,
-create a go.mod file (described in the next section) there, and run
-go commands from within that file tree.
-
-For more fine-grained control, the module support in Go 1.11 respects
+For more fine-grained control, Go 1.13 continues to respect
 a temporary environment variable, GO111MODULE, which can be set to one
-of three string values: off, on, or auto (the default).
-If GO111MODULE=off, then the go command never uses the
-new module support. Instead it looks in vendor directories and GOPATH
+of three string values: off, auto, or on (the default).
+If GO111MODULE=on or is unset, then the go command requires the use of
+modules, never consulting GOPATH. We refer to this as the command
+being module-aware or running in "module-aware mode".
+If GO111MODULE=auto, then the go command enables or disables module
+support based on the current directory. Module support is enabled only
+when the current directory is outside GOPATH/src and itself contains a
+go.mod file or is below a directory containing a go.mod file.
+If GO111MODULE=off, then the go command never uses
+module support. Instead it looks in vendor directories and GOPATH
 to find dependencies; we now refer to this as "GOPATH mode."
-If GO111MODULE=on, then the go command requires the use of modules,
-never consulting GOPATH. We refer to this as the command being
-module-aware or running in "module-aware mode".
-If GO111MODULE=auto or is unset, then the go command enables or
-disables module support based on the current directory.
-Module support is enabled only when the current directory is outside
-GOPATH/src and itself contains a go.mod file or is below a directory
-containing a go.mod file.
 
 In module-aware mode, GOPATH no longer defines the meaning of imports
 during a build, but it still stores downloaded dependencies (in GOPATH/pkg/mod)

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

@@ -33,7 +33,7 @@ import (
 
 var (
 	cwd            string // TODO(bcmills): Is this redundant with base.Cwd?
-	MustUseModules = mustUseModules()
+	mustUseModules = true
 	initialized    bool
 
 	modRoot     string
@@ -78,16 +78,6 @@ func BinDir() string {
 	return filepath.Join(gopath, "bin")
 }
 
-// mustUseModules reports whether we are invoked as vgo
-// (as opposed to go).
-// If so, we only support builds with go.mod files.
-func mustUseModules() bool {
-	name := os.Args[0]
-	name = name[strings.LastIndex(name, "/")+1:]
-	name = name[strings.LastIndex(name, `\`)+1:]
-	return strings.HasPrefix(name, "vgo")
-}
-
 var inGOPATH bool // running in GOPATH/src
 
 // Init determines whether module mode is enabled, locates the root of the
@@ -104,15 +94,14 @@ func Init() {
 	switch env {
 	default:
 		base.Fatalf("go: unknown environment setting GO111MODULE=%s", env)
-	case "", "auto":
-		// leave MustUseModules alone
-	case "on":
-		MustUseModules = true
+	case "auto":
+		mustUseModules = false
+	case "on", "":
+		mustUseModules = true
 	case "off":
-		if !MustUseModules {
+		mustUseModules = false
 		return
 	}
-	}
 
 	// Disable any prompting for passwords by Git.
 	// Only has an effect for 2.3.0 or later, but avoiding
@@ -158,7 +147,7 @@ func Init() {
 		}
 	}
 
-	if inGOPATH && !MustUseModules {
+	if inGOPATH && !mustUseModules {
 		if CmdModInit {
 			die() // Don't init a module that we're just going to ignore.
 		}
@@ -175,8 +164,8 @@ func Init() {
 	} else {
 		modRoot = findModuleRoot(cwd)
 		if modRoot == "" {
-			if !MustUseModules {
-				// GO111MODULE is 'auto' (or unset), and we can't find a module root.
+			if !mustUseModules {
+				// GO111MODULE is 'auto', and we can't find a module root.
 				// Stay in GOPATH mode.
 				return
 			}
@@ -275,7 +264,7 @@ func init() {
 // (usually through MustModRoot).
 func Enabled() bool {
 	Init()
-	return modRoot != "" || MustUseModules
+	return modRoot != "" || mustUseModules
 }
 
 // ModRoot returns the root of the main module.
@@ -308,7 +297,7 @@ func die() {
 	if os.Getenv("GO111MODULE") == "off" {
 		base.Fatalf("go: modules disabled by GO111MODULE=off; see 'go help modules'")
 	}
-	if inGOPATH && !MustUseModules {
+	if inGOPATH && !mustUseModules {
 		base.Fatalf("go: modules disabled inside GOPATH/src by GO111MODULE=auto; see 'go help modules'")
 	}
 	if cwd != "" {

src/cmd/go/main.go

@@ -49,7 +49,7 @@ func init() {
 		fix.CmdFix,
 		fmtcmd.CmdFmt,
 		generate.CmdGenerate,
-		get.CmdGet,
+		modget.CmdGet,
 		work.CmdInstall,
 		list.CmdList,
 		modcmd.CmdMod,
@@ -89,17 +89,10 @@ func main() {
 		base.Usage()
 	}
 
-	if modload.MustUseModules {
-		// If running with modules force-enabled, change get now to change help message.
-		*get.CmdGet = *modget.CmdGet
-	}
-
 	if args[0] == "get" || args[0] == "help" {
-		// Replace get with module-aware get if appropriate.
-		// Note that if MustUseModules is true, this happened already above,
-		// but no harm in doing it again.
-		if modload.Init(); modload.Enabled() {
-			*get.CmdGet = *modget.CmdGet
+		if modload.Init(); !modload.Enabled() {
+			// Replace module-aware get with GOPATH get if appropriate.
+			*modget.CmdGet = *get.CmdGet
 		}
 	}
 

src/cmd/go/mkalldocs.sh

@@ -8,6 +8,6 @@ set -e
 go build -o go.latest
 # If the command used to generate alldocs.go changes, update TestDocsUpToDate in
 # help_test.go.
-./go.latest help documentation >alldocs.go
+GO111MODULE='' ./go.latest help documentation >alldocs.go
 gofmt -w alldocs.go
 rm go.latest

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

@@ -17,7 +17,7 @@ cd $GOPATH/src/example.com/x/y
 ! go mod init
 stderr 'go: modules disabled inside GOPATH/src by GO111MODULE=auto; see ''go help modules'''
 
-env GO111MODULE=on
+env GO111MODULE=
 
 # Derive module path from location inside GOPATH.
 cd $GOPATH/src/example.com/x/y

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

@@ -24,12 +24,18 @@ exec $WORK/testimport.exe other/x/y/z/w .
 stdout w2.go
 
 # GO111MODULE=on outside GOPATH/src
+env GO111MODULE=
+exec $WORK/testimport.exe other/x/y/z/w .
+stdout w2.go
 env GO111MODULE=on
 exec $WORK/testimport.exe other/x/y/z/w .
 stdout w2.go
 
 # GO111MODULE=on in GOPATH/src
 cd $GOPATH/src
+env GO111MODULE=
+exec $WORK/testimport.exe x/y/z/w .
+stdout w1.go
 env GO111MODULE=on
 exec $WORK/testimport.exe x/y/z/w .
 stdout w1.go