add ability to ignore unknown flags (#160)

* add ability to ignore unknown flags

* add testcases

* add 2 more patterns to the testcase

* handle --unknownflag=val arg and -u=val arg scenario

* add ParseErrorsWhiteList to extend error handling during parsing

2 files changed, 134 insertions, 7 deletions

diff --git a/flag.go b/flag.go
index 7f460b5..f0acbb8 100644
--- a/flag.go
+++ b/flag.go
@@ -123,6 +123,12 @@ const (
 	PanicOnError
 )
 
+// ParseErrorsWhitelist defines the parsing errors that can be ignored
+type ParseErrorsWhitelist struct {
+	// UnknownFlags will ignore unknown flags errors and continue parsing rest of the flags
+	UnknownFlags bool
+}
+
 // NormalizedName is a flag name that has been normalized according to rules
 // for the FlagSet (e.g. making '-' and '_' equivalent).
 type NormalizedName string
@@ -138,6 +144,9 @@ type FlagSet struct {
 	// help/usage messages.
 	SortFlags bool
 
+	// ParseErrorsWhitelist is used to configure a whitelist of errors
+	ParseErrorsWhitelist ParseErrorsWhitelist
+
 	name              string
 	parsed            bool
 	actual            map[NormalizedName]*Flag
@@ -899,6 +908,25 @@ func (f *FlagSet) usage() {
 	}
 }
 
+//--unknown (args will be empty)
+//--unknown --next-flag ... (args will be --next-flag ...)
+//--unknown arg ... (args will be arg ...)
+func stripUnknownFlagValue(args []string) []string {
+	if len(args) == 0 {
+		//--unknown
+		return args
+	}
+
+	first := args[0]
+	if first[0] == '-' {
+		//--unknown --next-flag ...
+		return args
+	}
+
+	//--unknown arg ... (args will be arg ...)
+	return args[1:]
+}
+
 func (f *FlagSet) parseLongArg(s string, args []string, fn parseFunc) (a []string, err error) {
 	a = args
 	name := s[2:]
@@ -910,13 +938,24 @@ func (f *FlagSet) parseLongArg(s string, args []string, fn parseFunc) (a []strin
 	split := strings.SplitN(name, "=", 2)
 	name = split[0]
 	flag, exists := f.formal[f.normalizeFlagName(name)]
+
 	if !exists {
-		if name == "help" { // special case for nice help message.
+		switch {
+		case name == "help":
 			f.usage()
 			return a, ErrHelp
+		case f.ParseErrorsWhitelist.UnknownFlags:
+			// --unknown=unknownval arg ...
+			// we do not want to lose arg in this case
+			if len(split) >= 2 {
+				return a, nil
+			}
+
+			return stripUnknownFlagValue(a), nil
+		default:
+			err = f.failf("unknown flag: --%s", name)
+			return
 		}
-		err = f.failf("unknown flag: --%s", name)
-		return
 	}
 
 	var value string
@@ -954,13 +993,25 @@ func (f *FlagSet) parseSingleShortArg(shorthands string, args []string, fn parse
 
 	flag, exists := f.shorthands[c]
 	if !exists {
-		if c == 'h' { // special case for nice help message.
+		switch {
+		case c == 'h':
 			f.usage()
 			err = ErrHelp
 			return
+		case f.ParseErrorsWhitelist.UnknownFlags:
+			// '-f=arg arg ...'
+			// we do not want to lose arg in this case
+			if len(shorthands) > 2 && shorthands[1] == '=' {
+				outShorts = ""
+				return
+			}
+
+			outArgs = stripUnknownFlagValue(outArgs)
+			return
+		default:
+			err = f.failf("unknown shorthand flag: %q in -%s", c, shorthands)
+			return
 		}
-		err = f.failf("unknown shorthand flag: %q in -%s", c, shorthands)
-		return
 	}
 
 	var value string
diff --git a/flag_test.go b/flag_test.go
index d587752..3c13de3 100644
--- a/flag_test.go
+++ b/flag_test.go
@@ -389,7 +389,78 @@ func testParseAll(f *FlagSet, t *testing.T) {
 	}
 	if !reflect.DeepEqual(got, want) {
 		t.Errorf("f.ParseAll() fail to restore the args")
-		t.Errorf("Got: %v", got)
+		t.Errorf("Got:  %v", got)
+		t.Errorf("Want: %v", want)
+	}
+}
+
+func testParseWithUnknownFlags(f *FlagSet, t *testing.T) {
+	if f.Parsed() {
+		t.Error("f.Parse() = true before Parse")
+	}
+	f.ParseErrorsWhitelist.UnknownFlags = true
+
+	f.BoolP("boola", "a", false, "bool value")
+	f.BoolP("boolb", "b", false, "bool2 value")
+	f.BoolP("boolc", "c", false, "bool3 value")
+	f.BoolP("boold", "d", false, "bool4 value")
+	f.BoolP("boole", "e", false, "bool4 value")
+	f.StringP("stringa", "s", "0", "string value")
+	f.StringP("stringz", "z", "0", "string value")
+	f.StringP("stringx", "x", "0", "string value")
+	f.StringP("stringy", "y", "0", "string value")
+	f.StringP("stringo", "o", "0", "string value")
+	f.Lookup("stringx").NoOptDefVal = "1"
+	args := []string{
+		"-ab",
+		"-cs=xx",
+		"--stringz=something",
+		"--unknown1",
+		"unknown1Value",
+		"-d=true",
+		"-x",
+		"--unknown2=unknown2Value",
+		"-u=unknown3Value",
+		"-p",
+		"unknown4Value",
+		"-q", //another unknown with bool value
+		"-y",
+		"ee",
+		"--unknown7=unknown7value",
+		"--stringo=ovalue",
+		"--unknown8=unknown8value",
+		"--boole",
+		"--unknown6",
+	}
+	want := []string{
+		"boola", "true",
+		"boolb", "true",
+		"boolc", "true",
+		"stringa", "xx",
+		"stringz", "something",
+		"boold", "true",
+		"stringx", "1",
+		"stringy", "ee",
+		"stringo", "ovalue",
+		"boole", "true",
+	}
+	got := []string{}
+	store := func(flag *Flag, value string) error {
+		got = append(got, flag.Name)
+		if len(value) > 0 {
+			got = append(got, value)
+		}
+		return nil
+	}
+	if err := f.ParseAll(args, store); err != nil {
+		t.Errorf("expected no error, got %s", err)
+	}
+	if !f.Parsed() {
+		t.Errorf("f.Parse() = false after Parse")
+	}
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("f.ParseAll() fail to restore the args")
+		t.Errorf("Got:  %v", got)
 		t.Errorf("Want: %v", want)
 	}
 }
@@ -500,6 +571,11 @@ func TestParseAll(t *testing.T) {
 	testParseAll(GetCommandLine(), t)
 }
 
+func TestIgnoreUnknownFlags(t *testing.T) {
+	ResetForTesting(func() { t.Error("bad parse") })
+	testParseWithUnknownFlags(GetCommandLine(), t)
+}
+
 func TestFlagSetParse(t *testing.T) {
 	testParse(NewFlagSet("test", ContinueOnError), t)
 }