Improve error message in the case of an unknown linter (alecthomas#370)

* Fix error message in the case of an unknown linter

Discovered in alecthomas#369

When an unknown linter is enabled, raise an error identifying which linter(s) are unknown, rather than failing with:

    WARNING: invalid command ""

Note: linters defined with the --linter flag are not considered unknown and will not fail with this error.

* address feedback

- move validateLinters into linters.go
- test the happy path too
- remove unnecessary setup in test

* clean up error handling

linters.go

@@ -182,6 +182,21 @@ func defaultEnabled() []string {
 	return enabled
 }
 
+func validateLinters(linters map[string]*Linter, config *Config) error {
+	var unknownLinters []string
+	for name := range linters {
+		if _, isDefault := defaultLinters[name]; !isDefault {
+			if _, isCustom := config.Linters[name]; !isCustom {
+				unknownLinters = append(unknownLinters, name)
+			}
+		}
+	}
+	if len(unknownLinters) > 0 {
+		return fmt.Errorf("unknown linters: %s", strings.Join(unknownLinters, ", "))
+	}
+	return nil
+}
+
 const vetPattern = `^(?:vet:.*?\.go:\s+(?P<path>.*?\.go):(?P<line>\d+):(?P<col>\d+):\s*(?P<message>.*))|(?:(?P<path>.*?\.go):(?P<line>\d+):\s*(?P<message>.*))$`
 
 var defaultLinters = map[string]LinterConfig{

linters_test.go

@@ -35,6 +35,24 @@ func TestGetLinterByName(t *testing.T) {
 	assert.Equal(t, config.IsFast, overrideConfig.IsFast)
 }
 
+func TestValidateLinters(t *testing.T) {
+	originalConfig := *config
+	defer func() { config = &originalConfig }()
+
+	config = &Config{
+		Enable: []string{"_dummylinter_"},
+	}
+
+	err := validateLinters(lintersFromConfig(config), config)
+	require.Error(t, err, "expected unknown linter error for _dummylinter_")
+
+	config = &Config{
+		Enable: defaultEnabled(),
+	}
+	err = validateLinters(lintersFromConfig(config), config)
+	require.NoError(t, err)
+}
+
 func functionName(i interface{}) string {
 	return runtime.FuncForPC(reflect.ValueOf(i).Pointer()).Name()
 }

main.go

@@ -201,6 +201,9 @@ Severity override map (default is "warning"):
 	paths := resolvePaths(*pathsArg, config.Skip)
 
 	linters := lintersFromConfig(config)
+	err := validateLinters(linters, config)
+	kingpin.FatalIfError(err, "")
+
 	issues, errch := runLinters(linters, paths, config.Concurrency, exclude, include)
 	status := 0
 	if config.JSON {