gocomplete: suppress error output by default

Prevent the Go command completion from jumbling up the shell prompt on
errors by setting the Writer of the standard lib default log.Logger to
ioutil.Discard unless the system environment variable `GOCOMP_VERBOSE`
has been explicitly set to "1" (e.g. for debugging purposes).

Fixes #139

gocomplete/complete.go

@@ -2,10 +2,17 @@
 package main
 
 import (
+	"io/ioutil"
+	"log"
+	"os"
+
 	"github.com/posener/complete/v2"
 	"github.com/posener/complete/v2/predict"
 )
 
+// envVerbose is the sys env var that controls error output verbosity.
+const envVerbose = "GOCOMP_VERBOSE"
+
 var (
 	ellipsis   = predict.Set{"./..."}
 	anyPackage = complete.PredictFunc(predictPackages)
@@ -15,6 +22,10 @@ var (
 )
 
 func main() {
+	if os.Getenv(envVerbose) != "1" {
+		log.SetOutput(ioutil.Discard)
+	}
+
 	build := &complete.Command{
 		Flags: map[string]complete.Predictor{
 			"o": anyFile,

gocomplete/tests_test.go

@@ -1,8 +1,14 @@
 package main
 
 import (
+	"fmt"
+	"io"
+	"io/ioutil"
+	"log"
 	"os"
 	"sort"
+	"strconv"
+	"strings"
 	"testing"
 
 	"bou.ke/monkey"
@@ -21,7 +27,11 @@ func TestPredictions(t *testing.T) {
 		{
 			name:      "predict tests ok",
 			predictor: predictTest,
-			want:      []string{"TestPredictions", "Example"},
+			want: []string{
+				"TestPredictions",
+				"Example",
+				"TestErrorSupression",
+			},
 		},
 		{
 			name:      "predict benchmark ok",
@@ -64,3 +74,71 @@ func equal(s1, s2 []string) bool {
 	}
 	return true
 }
+
+func TestErrorSupression(t *testing.T) {
+	defer monkey.Patch(os.Exit, func(int) {}).Unpatch()
+
+	// Completion API environment variable names.
+	const envLine, envPoint = "COMP_LINE", "COMP_POINT"
+
+	// line should work out to
+	//
+	// * on most POSIX:
+	//     go test /tmp/
+	// * on MacOS X:
+	//     go test /var/folders/<randomized_pathname>/T//
+	// * on Windows:
+	//     go test C:\Users\<username>\AppData\Local\Temp\
+	//
+	// which should trigger "failed importing directory: ... no
+	// buildable Go source files..." error messages.
+	var line = "go test " + os.TempDir() + string(os.PathSeparator)
+
+	defer os.Unsetenv(envLine)
+	defer os.Unsetenv(envPoint)
+	os.Setenv(envLine, line)
+	os.Setenv(envPoint, strconv.Itoa(len(line)))
+
+	tests := []struct {
+		verbose string
+		wantErr bool
+	}{{
+		verbose: "",
+		wantErr: false,
+	}, {
+		verbose: "1",
+		wantErr: true,
+	}}
+	for _, tt := range tests {
+		t.Run(fmt.Sprintf(
+			"%s=%q", envVerbose, tt.verbose,
+		), func(t *testing.T) {
+			// Discard completion (stdout).
+			r, w, err := os.Pipe()
+			if err != nil {
+				t.Fatal(err)
+			}
+			defer w.Close()
+			defer func(o *os.File) { os.Stdout = o }(os.Stdout)
+			os.Stdout = w
+			go io.Copy(ioutil.Discard, r)
+
+			// "Redirect" stderr into a buffer.
+			b := &strings.Builder{}
+			log.SetOutput(b)
+
+			defer os.Unsetenv(envVerbose)
+			os.Setenv(envVerbose, tt.verbose)
+
+			main()
+
+			gotErr := b.Len() != 0
+			if tt.wantErr && !gotErr {
+				t.Fatal("want something in stderr, got nothing")
+			} else if !tt.wantErr && gotErr {
+				t.Fatalf("want nothing in stderr, got %d bytes",
+					b.Len())
+			}
+		})
+	}
+}