internal/imports: handle un-downloaded modules

Now that gopls is passing GOPROXY=off, running go list -m gives an error
if any modules aren't downloaded. We need to pass -e to get results for
the modules that we do have. Also add the missing error handling that
resulted in silent failure. That, in turn, reveals that we need to
explicitly ignore an expected error.

Fixes golang/go#43333.

Change-Id: I77604650b67a3c480d8231c65f0486f22e4a6722
Reviewed-on: https://go-review.googlesource.com/c/tools/+/283172
Trust: Heschi Kreinick <heschi@google.com>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>

Author: heschi

gopls/internal/regtest/completion_test.go

@@ -286,3 +286,40 @@ func _() {
 		)
 	})
 }
+
+// Test that completions still work with an undownloaded module, golang/go#43333.
+func TestUndownloadedModule(t *testing.T) {
+	// mod.com depends on example.com, but only in a file that's hidden by a
+	// build tag, so the IWL won't download example.com. That will cause errors
+	// in the go list -m call performed by the imports package.
+	const files = `
+-- go.mod --
+module mod.com
+
+go 1.14
+
+require example.com v1.2.3
+-- go.sum --
+example.com v1.2.3 h1:ihBTGWGjTU3V4ZJ9OmHITkU9WQ4lGdQkMjgyLFk0FaY=
+example.com v1.2.3/go.mod h1:Y2Rc5rVWjWur0h3pd9aEvK5Pof8YKDANh9gHA2Maujo=
+-- useblah.go --
+// +build hidden
+
+package pkg
+import "example.com/blah"
+var _ = blah.Name
+-- mainmod/mainmod.go --
+package mainmod
+
+const Name = "mainmod"
+`
+	withOptions(ProxyFiles(proxy)).run(t, files, func(t *testing.T, env *Env) {
+		env.CreateBuffer("import.go", "package pkg\nvar _ = mainmod.Name\n")
+		env.SaveBuffer("import.go")
+		content := env.ReadWorkspaceFile("import.go")
+		if !strings.Contains(content, `import "mod.com/mainmod`) {
+			t.Errorf("expected import of mod.com/mainmod in %q", content)
+		}
+	})
+
+}

internal/imports/mod.go

@@ -86,7 +86,12 @@ func (r *ModuleResolver) init() error {
 		r.modsByDir = []*gocommand.ModuleJSON{mainMod, r.dummyVendorMod}
 	} else {
 		// Vendor mode is off, so run go list -m ... to find everything.
-		r.initAllMods()
+		err := r.initAllMods()
+		// We expect an error when running outside of a module with
+		// GO111MODULE=on. Other errors are fatal.
+		if err != nil && !strings.Contains(err.Error(), "working directory is not part of a module") {
+			return err
+		}
 	}
 
 	if gmc := r.env.Env["GOMODCACHE"]; gmc != "" {
@@ -161,7 +166,7 @@ func (r *ModuleResolver) init() error {
 }
 
 func (r *ModuleResolver) initAllMods() error {
-	stdout, err := r.env.invokeGo(context.TODO(), "list", "-m", "-json", "...")
+	stdout, err := r.env.invokeGo(context.TODO(), "list", "-m", "-e", "-json", "...")
 	if err != nil {
 		return err
 	}