Fix infinite loop case for envvar backed opts (#50)

This is a regression introduced by the feature "Allow required options to be satisfied by an nev var".

The bug lies in `optsMatcher`, which iterates over its options, and keeps looping until no more options can match or there is no more input to feed them.

If one of the options can be satsified from an en var, it doesn't consume any input, and returns true, causing an inifinite loop.

The proposed fix is to exclude an option (in `optsMatcher`) whenever  it has been satisfied by an env var.

fsm.go

@@ -142,11 +142,17 @@ func dot(s *state, visited map[*state]bool) []string {
 type parseContext struct {
 	args          map[*arg][]string
 	opts          map[*opt][]string
+	excludedOpts  map[*opt]struct{}
 	rejectOptions bool
 }
 
 func newParseContext() parseContext {
-	return parseContext{map[*arg][]string{}, map[*opt][]string{}, false}
+	return parseContext{
+		args:          map[*arg][]string{},
+		opts:          map[*opt][]string{},
+		excludedOpts:  map[*opt]struct{}{},
+		rejectOptions: false,
+	}
 }
 
 func (pc parseContext) merge(o parseContext) {

matchers.go

@@ -86,6 +86,10 @@ func (o *optMatcher) match(args []string, c *parseContext) (bool, []string) {
 			idx += consumed
 
 		default:
+			if o.theOne.valueSetFromEnv {
+				//  exclude opt
+				c.excludedOpts[o.theOne] = struct{}{}
+			}
 			return o.theOne.valueSetFromEnv, args
 		}
 	}
@@ -227,6 +231,9 @@ func (om optsMatcher) try(args []string, c *parseContext) (bool, []string) {
 		return false, args
 	}
 	for _, o := range om.options {
+		if _, exclude := c.excludedOpts[o]; exclude {
+			continue
+		}
 		if ok, nargs := (&optMatcher{theOne: o, optionsIdx: om.optionsIndex}).match(args, c); ok {
 			return ok, nargs
 		}

spec_n_parse_test.go

@@ -1254,6 +1254,33 @@ func TestEnvOverrideOk(t *testing.T) {
 	}
 }
 
+// Anti-regression test for infinite loop case with envvar backed opts
+// https://github.com/jawher/mow.cli/pull/49
+func TestEnvOptSeq(t *testing.T) {
+	defer os.Unsetenv("envopt")
+
+	var envopt *string
+	var arg *string
+
+	init := func(c *Cmd) {
+		defer os.Unsetenv("envopt")
+
+		envopt = c.String(StringOpt{
+			Name:   "e envopt",
+			Value:  "envdefault",
+			EnvVar: "envopt",
+		})
+
+		arg = c.StringArg("ARG", "", "")
+	}
+
+	os.Setenv("envopt", "envval")
+	okCmd(t, "", init, []string{"argval"})
+
+	assert.Equal(t, "envval", *envopt)
+	assert.Equal(t, "argval", *arg)
+}
+
 // Test that not setting an environment variable correctly causes
 // required options to fail if no value is supplied in args.
 func TestEnvOverrideFail(t *testing.T) {