internal/lsp: do not invoke the Go command when checking common errors

There's no need to do this more than once per view.

Change-Id: I3160adc602764204155dd0e506fd554aeb55d639
Reviewed-on: https://go-review.googlesource.com/c/tools/+/215321
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>

internal/lsp/cache/view.go

@@ -502,6 +502,10 @@ func (v *view) addIgnoredFile(uri span.URI) {
 	v.ignoredURIs[uri] = struct{}{}
 }
 
+func (v *view) ModFile() string {
+	return v.gomod
+}
+
 func (v *view) BackgroundContext() context.Context {
 	v.mu.Lock()
 	defer v.mu.Unlock()

internal/lsp/cmd/cmd_test.go

@@ -20,10 +20,6 @@ import (
 )
 
 func TestMain(m *testing.M) {
-	if runtime.GOOS == "android" {
-		fmt.Fprintln(os.Stderr, "skipping test: cmd tests open too many files on android")
-		os.Exit(0)
-	}
 	testenv.ExitIfSmallMachine()
 	os.Exit(m.Run())
 }

internal/lsp/source/errors.go

@@ -31,25 +31,13 @@ func checkCommonErrors(ctx context.Context, v View) (string, error) {
 	//  1. The user is in GOPATH mode and is working outside their GOPATH
 	//  2. The user is in module mode and has opened a subdirectory of their module
 	//
-
-	// TODO(rstambler): Get the values for GOPATH and GOMOD from
-	// the view, once it's possible to do so: golang.org/cl/214417.
 	gopath := os.Getenv("GOPATH")
 	folder := v.Folder().Filename()
 
-	// Invoke `go env GOMOD` inside of the directory of the view.
-	b, err := InvokeGo(ctx, folder, v.Config(ctx).Env, "env", "GOMOD")
-	if err != nil {
-		return "", err
-	}
-	modFile := strings.TrimSpace(b.String())
-	if modFile == filepath.FromSlash("/dev/null") {
-		modFile = ""
-	}
-	modRoot := filepath.Dir(modFile)
+	modRoot := filepath.Dir(v.ModFile())
 
 	// Not inside of a module.
-	inAModule := modFile != ""
+	inAModule := v.ModFile() != "" && v.ModFile() != os.DevNull
 
 	// The user may have a multiple directories in their GOPATH.
 	var inGopath bool

internal/lsp/source/view.go

@@ -92,6 +92,9 @@ type View interface {
 	// Folder returns the root folder for this view.
 	Folder() span.URI
 
+	// ModFile is the path to the go.mod file for the view, if any.
+	ModFile() string
+
 	// LookupBuiltin returns the go/ast.Object for the given name in the builtin package.
 	LookupBuiltin(ctx context.Context, name string) (*ast.Object, error)