internal/lsp/cache: set GO111MODULE=auto in 1.16

1.16 wants to set GO111MODULE=on. There are, AFAIK, two differences
between "auto" and "on". First, if you're in a directory outside of GOPATH
and with no go.mod, "on" will run in module mode with GOMOD=os.DevNull.
I don't think we care very much about that. Second, if you're in GOPATH
with no go.mod, "on" will run in module mode, breaking GOPATH mode.

Breaking GOPATH mode may be desirable for the go command generally, but
for gopls I think it will lead to an unnecessarily bad user experience.
Users will find out when they do their first build or test; there's IMO
no need to also break their editor.

Flip the default back to "auto".

Change-Id: I280e001a9f7e80d65e68c0cb94353d70a7f5425e
Reviewed-on: https://go-review.googlesource.com/c/tools/+/255781
Trust: Heschi Kreinick <heschi@google.com>
Run-TryBot: Heschi Kreinick <heschi@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>

internal/lsp/cache/snapshot.go

@@ -138,7 +138,7 @@ func (s *snapshot) configWithDir(ctx context.Context, dir string) *packages.Conf
 	cfg := &packages.Config{
 		Context:    ctx,
 		Dir:        dir,
-		Env:        append([]string{}, env...),
+		Env:        append(append([]string{}, env...), "GO111MODULE="+s.view.go111module),
 		BuildFlags: append([]string{}, buildFlags...),
 		Mode: packages.NeedName |
 			packages.NeedFiles |

internal/lsp/cache/view.go

@@ -150,6 +150,9 @@ type View struct {
 	// `go env` variables that need to be tracked by gopls.
 	gocache, gomodcache, gopath, goprivate string
 
+	// The value of GO111MODULE we want to run with.
+	go111module string
+
 	// goEnv is the `go env` output collected when a view is created.
 	// It includes the values of the environment variables above.
 	goEnv map[string]string
@@ -492,6 +495,8 @@ func (v *View) populateProcessEnv(ctx context.Context, modFH, sumFH source.FileH
 	for k, v := range v.goEnv {
 		pe.Env[k] = v
 	}
+	pe.Env["GO111MODULE"] = v.go111module
+
 	modmod, err := v.needsModEqualsMod(ctx, modFH)
 	if err != nil {
 		return cleanup, err
@@ -795,8 +800,25 @@ func (v *View) setBuildInformation(ctx context.Context, options *source.Options)
 	if err != nil {
 		return err
 	}
+
+	v.go111module = os.Getenv("GO111MODULE")
+	for _, kv := range options.Env {
+		split := strings.SplitN(kv, "=", 2)
+		if len(split) != 2 {
+			continue
+		}
+		if split[0] == "GO111MODULE" {
+			v.go111module = split[1]
+		}
+	}
+	// If using 1.16, change the default back to auto. The primary effect of
+	// GO111MODULE=on is to break GOPATH, which we aren't too interested in.
+	if v.goversion >= 16 && v.go111module == "" {
+		v.go111module = "auto"
+	}
+
 	// Make sure to get the `go env` before continuing with initialization.
-	modFile, err := v.setGoEnv(ctx, options.Env)
+	modFile, err := v.setGoEnv(ctx, append(options.Env, "GO111MODULE="+v.go111module))
 	if err != nil {
 		return err
 	}