Enable passing stdin to gh gist create -

Goals:
- print command usage information on `gh gist create` when no args nor stdin
- disallow passing `-` as argument multiple times
- utilize Cobra's Args to do argument validation

command/gist.go

@@ -10,6 +10,7 @@ import (
 
 	"github.com/MakeNowJust/heredoc"
 	"github.com/cli/cli/api"
+	"github.com/cli/cli/pkg/cmdutil"
 	"github.com/cli/cli/utils"
 	"github.com/spf13/cobra"
 )
@@ -52,6 +53,22 @@ By default, gists are private; use '--public' to make publicly listed ones.`,
 	# create a gist from output piped from another command
 	$ cat cool.txt | gh gist create
 	`),
+	Args: func(cmd *cobra.Command, args []string) error {
+		if len(args) > 0 {
+			return nil
+		}
+
+		info, err := os.Stdin.Stat()
+		if err != nil {
+			return fmt.Errorf("failed to check STDIN: %w", err)
+		}
+
+		stdinIsTTY := (info.Mode() & os.ModeCharDevice) == os.ModeCharDevice
+		if stdinIsTTY {
+			return cmdutil.FlagError{Err: errors.New("no filenames passed and nothing on STDIN")}
+		}
+		return nil
+	},
 	RunE: gistCreate,
 }
 
@@ -83,15 +100,12 @@ func gistCreate(cmd *cobra.Command, args []string) error {
 		return fmt.Errorf("did not understand arguments: %w", err)
 	}
 
-	info, err := os.Stdin.Stat()
-	if err != nil {
-		return fmt.Errorf("failed to check STDIN: %w", err)
-	}
-	if (info.Mode() & os.ModeCharDevice) != os.ModeCharDevice {
-		args = append(args, "-")
+	fileArgs := args
+	if len(args) == 0 {
+		fileArgs = []string{"-"}
 	}
 
-	files, err := processFiles(os.Stdin, args)
+	files, err := processFiles(os.Stdin, fileArgs)
 	if err != nil {
 		return fmt.Errorf("failed to collect files for posting: %w", err)
 	}
@@ -128,11 +142,11 @@ func processOpts(cmd *cobra.Command) (*Opts, error) {
 	}, err
 }
 
-func processFiles(stdin io.Reader, filenames []string) (map[string]string, error) {
+func processFiles(stdin io.ReadCloser, filenames []string) (map[string]string, error) {
 	fs := map[string]string{}
 
 	if len(filenames) == 0 {
-		return fs, errors.New("no filenames passed and nothing on STDIN")
+		return nil, errors.New("no files passed")
 	}
 
 	for i, f := range filenames {
@@ -145,6 +159,7 @@ func processFiles(stdin io.Reader, filenames []string) (map[string]string, error
 			if err != nil {
 				return fs, fmt.Errorf("failed to read from stdin: %w", err)
 			}
+			stdin.Close()
 		} else {
 			content, err = ioutil.ReadFile(f)
 			if err != nil {

command/gist_test.go

@@ -6,6 +6,8 @@ import (
 	"io/ioutil"
 	"strings"
 	"testing"
+
+	"github.com/stretchr/testify/assert"
 )
 
 func TestGistCreate(t *testing.T) {
@@ -43,10 +45,11 @@ func TestGistCreate(t *testing.T) {
 
 func TestGistCreate_stdin(t *testing.T) {
 	fakeStdin := strings.NewReader("hey cool how is it going")
-	files, err := processFiles(fakeStdin, []string{"-"})
+	files, err := processFiles(ioutil.NopCloser(fakeStdin), []string{"-"})
 	if err != nil {
 		t.Fatalf("unexpected error processing files: %s", err)
 	}
 
-	eq(t, files["gistfile0.txt"], "hey cool how is it going")
+	assert.Equal(t, 1, len(files))
+	assert.Equal(t, "hey cool how is it going", files["gistfile0.txt"])
 }