Prevent panic on flagSet access from custom BashComplete

Fixes #944

command.go

@@ -100,7 +100,7 @@ func (c *Command) Run(ctx *Context) (err error) {
 		c.UseShortOptionHandling = true
 	}
 
-	set, err := c.parseFlags(ctx.Args())
+	set, err := c.parseFlags(ctx.Args(), ctx.shellComplete)
 
 	context := NewContext(ctx.App, set, ctx)
 	context.Command = c
@@ -174,7 +174,7 @@ func (c *Command) useShortOptionHandling() bool {
 	return c.UseShortOptionHandling
 }
 
-func (c *Command) parseFlags(args Args) (*flag.FlagSet, error) {
+func (c *Command) parseFlags(args Args, shellComplete bool) (*flag.FlagSet, error) {
 	set, err := c.newFlagSet()
 	if err != nil {
 		return nil, err
@@ -185,7 +185,8 @@ func (c *Command) parseFlags(args Args) (*flag.FlagSet, error) {
 	}
 
 	err = parseIter(set, c, args.Tail())
-	if err != nil {
+	// Continue parsing flags on failure during shell completion
+	if err != nil && !shellComplete {
 		return nil, err
 	}
 

command_test.go

@@ -1,6 +1,7 @@
 package cli
 
 import (
+	"bytes"
 	"errors"
 	"flag"
 	"fmt"
@@ -329,3 +330,49 @@ func TestCommandSkipFlagParsing(t *testing.T) {
 		expect(t, args, c.expectedArgs)
 	}
 }
+
+func TestCommand_Run_CustomShellCompleteAcceptsMalformedFlags(t *testing.T) {
+	cases := []struct {
+		testArgs    args
+		expectedOut string
+	}{
+		{testArgs: args{"--undefined"}, expectedOut: "found 0 args"},
+		{testArgs: args{"--number"}, expectedOut: "found 0 args"},
+		{testArgs: args{"--number", "fourty-two"}, expectedOut: "found 0 args"},
+		{testArgs: args{"--number", "42"}, expectedOut: "found 0 args"},
+		{testArgs: args{"--number", "42", "newArg"}, expectedOut: "found 1 args"},
+	}
+
+	for _, c := range cases {
+		var outputBuffer bytes.Buffer
+		app := &App{
+			Writer:               &outputBuffer,
+			EnableBashCompletion: true,
+			Commands: []*Command{
+				{
+					Name:  "bar",
+					Usage: "this is for testing",
+					Flags: []Flag{
+						&IntFlag{
+							Name:  "number",
+							Usage: "A number to parse",
+						},
+					},
+					BashComplete: func(c *Context) {
+						fmt.Fprintf(c.App.Writer, "found %d args", c.NArg())
+					},
+				},
+			},
+		}
+
+		osArgs := args{"foo", "bar"}
+		osArgs = append(osArgs, c.testArgs...)
+		osArgs = append(osArgs, "--generate-bash-completion")
+
+		err := app.Run(osArgs)
+		stdout := outputBuffer.String()
+		expect(t, err, nil)
+		expect(t, stdout, c.expectedOut)
+	}
+
+}