cmd/go: ignore /tmp/go.mod

Two different people have created /tmp/go.mod for experimentation
and then had other tests that create fresh work directories
below /tmp fail unexpectedly because the go command finds
/tmp/go.mod. Refuse to use /tmp/go.mod. /tmp/anything/go.mod is fine.

Fixes #26708.

Change-Id: I2a4f61ea63099cff59fbf9e8798e5dcefefd5557
Reviewed-on: https://go-review.googlesource.com/129063
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -150,8 +150,20 @@ func Init() {
 		ModRoot = cwd
 	} else {
 		ModRoot, _ = FindModuleRoot(cwd, "", MustUseModules)
-		if ModRoot == "" && !MustUseModules {
-			return
+		if !MustUseModules {
+			if ModRoot == "" {
+				return
+			}
+			if search.InDir(ModRoot, os.TempDir()) == "." {
+				// If you create /tmp/go.mod for experimenting,
+				// then any tests that create work directories under /tmp
+				// will find it and get modules when they're not expecting them.
+				// It's a bit of a peculiar thing to disallow but quite mysterious
+				// when it happens. See golang.org/issue/26708.
+				ModRoot = ""
+				fmt.Fprintf(os.Stderr, "go: warning: ignoring go.mod in system temp root %v\n", os.TempDir())
+				return
+			}
 		}
 	}
 

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

@@ -65,6 +65,15 @@ cd $GOPATH/foo/bar/baz
 go env GOMOD
 ! stdout .+
 
+# GO111MODULE=auto should ignore and warn about /tmp/go.mod
+env GO111MODULE=auto
+cp $GOPATH/src/x/y/z/go.mod $WORK/tmp/go.mod
+mkdir $WORK/tmp/mydir
+cd $WORK/tmp/mydir
+go env GOMOD
+! stdout .+
+stderr '^go: warning: ignoring go.mod in system temp root '
+
 -- $GOPATH/src/x/y/z/go.mod --
 module x/y/z
 -- $GOPATH/src/x/y/z/w/w.txt --