command: workspace name autocomplete helper

This helper is a Predictor for the "complete" package that tries to
auto-complete workspace names from the current backend, if it's
initialized and operable.

command/autocomplete.go

@@ -2,6 +2,7 @@ package command
 
 import (
 	"github.com/posener/complete"
+	"github.com/posener/complete/match"
 )
 
 // This file contains some re-usable predictors for auto-complete. The
@@ -31,3 +32,48 @@ func (s completePredictSequence) Predict(a complete.Args) []string {
 
 	return s[idx].Predict(a)
 }
+
+func (m *Meta) completePredictWorkspaceName() complete.Predictor {
+	return complete.PredictFunc(func(a complete.Args) []string {
+		// There are lot of things that can fail in here, so if we encounter
+		// any error then we'll just return nothing and not support autocomplete
+		// until whatever error is fixed. (The user can't actually see the error
+		// here, but other commands should produce a user-visible error before
+		// too long.)
+
+		// We assume here that we want to autocomplete for the current working
+		// directory, since we don't have enough context to know where to
+		// find any config path argument, and it might be _after_ the argument
+		// we're trying to complete here anyway.
+		configPath, err := ModulePath(nil)
+		if err != nil {
+			return nil
+		}
+
+		cfg, err := m.Config(configPath)
+		if err != nil {
+			return nil
+		}
+
+		b, err := m.Backend(&BackendOpts{
+			Config: cfg,
+		})
+		if err != nil {
+			return nil
+		}
+
+		names, _ := b.States()
+
+		if a.Last != "" {
+			// filter for names that match the prefix only
+			filtered := make([]string, 0, len(names))
+			for _, name := range names {
+				if match.Prefix(name, a.Last) {
+					filtered = append(filtered, name)
+				}
+			}
+			names = filtered
+		}
+		return names
+	})
+}

command/autocomplete_test.go

@@ -0,0 +1,60 @@
+package command
+
+import (
+	"io/ioutil"
+	"os"
+	"reflect"
+	"testing"
+
+	"github.com/mitchellh/cli"
+	"github.com/posener/complete"
+)
+
+func TestMetaCompletePredictWorkspaceName(t *testing.T) {
+	// Create a temporary working directory that is empty
+	td := tempDir(t)
+	os.MkdirAll(td, 0755)
+	defer os.RemoveAll(td)
+	defer testChdir(t, td)()
+
+	// make sure a vars file doesn't interfere
+	err := ioutil.WriteFile(DefaultVarsFilename, nil, 0644)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	ui := new(cli.MockUi)
+	meta := &Meta{Ui: ui}
+
+	predictor := meta.completePredictWorkspaceName()
+
+	t.Run("no prefix", func(t *testing.T) {
+		got := predictor.Predict(complete.Args{
+			Last: "",
+		})
+		want := []string{"default"}
+		if !reflect.DeepEqual(got, want) {
+			t.Errorf("wrong result\ngot:  %#v\nwant: %#v", got, want)
+		}
+	})
+
+	t.Run("prefix that matches", func(t *testing.T) {
+		got := predictor.Predict(complete.Args{
+			Last: "def",
+		})
+		want := []string{"default"}
+		if !reflect.DeepEqual(got, want) {
+			t.Errorf("wrong result\ngot:  %#v\nwant: %#v", got, want)
+		}
+	})
+
+	t.Run("prefix that doesn't match", func(t *testing.T) {
+		got := predictor.Predict(complete.Args{
+			Last: "x",
+		})
+		want := []string{}
+		if !reflect.DeepEqual(got, want) {
+			t.Errorf("wrong result\ngot:  %#v\nwant: %#v", got, want)
+		}
+	})
+}