cmd/go: fix error message for go mod in GOPATH mode

Checks if modules are enabled in GOPATH mode for go mod [graph, verify].
Added tests for GO111MODULE=[auto, off].

Fixes: #31237

Change-Id: I91efccfa10d0b2385ec2af1ea133deaa8234ba37
Reviewed-on: https://go-review.googlesource.com/c/go/+/174697
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/internal/modcmd/graph.go

@@ -8,6 +8,7 @@ package modcmd
 
 import (
 	"bufio"
+	"cmd/go/internal/cfg"
 	"os"
 	"sort"
 
@@ -33,6 +34,14 @@ func runGraph(cmd *base.Command, args []string) {
 	if len(args) > 0 {
 		base.Fatalf("go mod graph: graph takes no arguments")
 	}
+	// Checks go mod expected behavior
+	if !modload.Enabled() {
+		if cfg.Getenv("GO111MODULE") == "off" {
+			base.Fatalf("go: modules disabled by GO111MODULE=off; see 'go help modules'")
+		} else {
+			base.Fatalf("go: cannot find main module; see 'go help modules'")
+		}
+	}
 	modload.LoadBuildList()
 
 	reqs := modload.MinReqs()

src/cmd/go/internal/modcmd/verify.go

@@ -6,6 +6,7 @@ package modcmd
 
 import (
 	"bytes"
+	"cmd/go/internal/cfg"
 	"fmt"
 	"io/ioutil"
 	"os"
@@ -36,6 +37,14 @@ func runVerify(cmd *base.Command, args []string) {
 		// NOTE(rsc): Could take a module pattern.
 		base.Fatalf("go mod verify: verify takes no arguments")
 	}
+	// Checks go mod expected behavior
+	if !modload.Enabled() {
+		if cfg.Getenv("GO111MODULE") == "off" {
+			base.Fatalf("go: modules disabled by GO111MODULE=off; see 'go help modules'")
+		} else {
+			base.Fatalf("go: cannot find main module; see 'go help modules'")
+		}
+	}
 	ok := true
 	for _, mod := range modload.LoadBuildList()[1:] {
 		ok = verifyMod(mod) && ok

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

@@ -0,0 +1,31 @@
+env GO111MODULE=off
+
+# This script tests that running go mod with
+# GO111MODULE=off when outside of GOPATH will fatal
+# with an error message, even with some source code in the directory and a go.mod.
+! go mod init
+stderr 'go mod init: modules disabled by GO111MODULE=off; see ''go help modules'''
+! go mod graph
+stderr 'go: modules disabled by GO111MODULE=off; see ''go help modules'''
+! go mod verify
+stderr 'go: modules disabled by GO111MODULE=off; see ''go help modules'''
+
+# Same result in an empty directory
+mkdir z
+cd z
+! go mod init
+stderr 'go mod init: modules disabled by GO111MODULE=off; see ''go help modules'''
+! go mod graph
+stderr 'go: modules disabled by GO111MODULE=off; see ''go help modules'''
+! go mod verify
+stderr 'go: modules disabled by GO111MODULE=off; see ''go help modules'''
+
+-- sample.go --
+package sample
+
+func main() {}
+
+-- go.mod --
+module sample
+
+go 1.12