config.go

/*
Copyright 2018 The Kubernetes Authors.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

// Package config simplifies the declaration of configuration options.
// Right now the implementation maps them directly command line
// flags. When combined with test/e2e/framework/viper in a test suite,
// those flags then can also be read from a config file.
//
// Instead of defining flags one-by-one, developers annotate a
// structure with tags and then call a single function. This is the
// same approach as in https://godoc.org/github.com/jessevdk/go-flags,
// but implemented so that a test suite can continue to use the normal
// "flag" package.
//
// For example, a file storage/csi.go might define:
//
//     var scaling struct {
//             NumNodes int  `default:"1" description:"number of nodes to run on"`
//             Master string
//     }
//     _ = config.AddOptions(&scaling, "storage.csi.scaling")
//
// This defines the following command line flags:
//
//     -storage.csi.scaling.numNodes=<int>  - number of nodes to run on (default: 1)
//     -storage.csi.scaling.master=<string>
//
// All fields in the structure must be exported and have one of the following
// types (same as in the `flag` package):
// - bool
// - time.Duration
// - float64
// - string
// - int
// - int64
// - uint
// - uint64
// - and/or nested or embedded structures containing those basic types.
//
// Each basic entry may have a tag with these optional keys:
//
//     usage:   additional explanation of the option
//     default: the default value, in the same format as it would
//              be given on the command line and true/false for
//              a boolean
//
// The names of the final configuration options are a combination of an
// optional common prefix for all options in the structure and the
// name of the fields, concatenated with a dot. To get names that are
// consistent with the command line flags defined by `ginkgo`, the
// initial character of each field name is converted to lower case.
//
// There is currently no support for aliases, so renaming the fields
// or the common prefix will be visible to users of the test suite and
// may breaks scripts which use the old names.
//
// The variable will be filled with the actual values by the test
// suite before running tests. Beware that the code which registers
// Ginkgo tests cannot use those config options, because registering
// tests and options both run before the E2E test suite handles
// parameters.
package config

import (
	"flag"
	"fmt"
	"reflect"
	"strconv"
	"time"
	"unicode"
	"unicode/utf8"
)

// CommandLine is the flag set that AddOptions adds to. Usually this
// is the same as the default in the flag package, but can also be
// something else (for example during testing).
var CommandLine = flag.CommandLine

// AddOptions analyzes the options value and creates the necessary
// flags to populate it.
//
// The prefix can be used to root the options deeper in the overall
// set of options, with a dot separating different levels.
//
// The function always returns true, to enable this simplified
// registration of options:
// _ = AddOptions(...)
//
// It panics when it encounters an error, like unsupported types
// or option name conflicts.
func AddOptions(options interface{}, prefix string) bool {
	optionsType := reflect.TypeOf(options)
	if optionsType == nil {
		panic("options parameter without a type - nil?!")
	}
	if optionsType.Kind() != reflect.Ptr || optionsType.Elem().Kind() != reflect.Struct {
		panic(fmt.Sprintf("need a pointer to a struct, got instead: %T", options))
	}
	addStructFields(optionsType.Elem(), reflect.Indirect(reflect.ValueOf(options)), prefix)
	return true
}

func addStructFields(structType reflect.Type, structValue reflect.Value, prefix string) {
	for i := 0; i < structValue.NumField(); i++ {
		entry := structValue.Field(i)
		addr := entry.Addr()
		structField := structType.Field(i)
		name := structField.Name
		r, n := utf8.DecodeRuneInString(name)
		name = string(unicode.ToLower(r)) + name[n:]
		usage := structField.Tag.Get("usage")
		def := structField.Tag.Get("default")
		if prefix != "" {
			name = prefix + "." + name
		}
		if structField.PkgPath != "" {
			panic(fmt.Sprintf("struct entry %q not exported", name))
		}
		ptr := addr.Interface()
		if structField.Anonymous {
			// Entries in embedded fields are treated like
			// entries, in the struct itself, i.e. we add
			// them with the same prefix.
			addStructFields(structField.Type, entry, prefix)
			continue
		}
		if structField.Type.Kind() == reflect.Struct {
			// Add nested options.
			addStructFields(structField.Type, entry, name)
			continue
		}
		// We could switch based on structField.Type. Doing a
		// switch after getting an interface holding the
		// pointer to the entry has the advantage that we
		// immediately have something that we can add as flag
		// variable.
		//
		// Perhaps generics will make this entire switch redundant someday...
		switch ptr := ptr.(type) {
		case *bool:
			var defValue bool
			parseDefault(&defValue, name, def)
			CommandLine.BoolVar(ptr, name, defValue, usage)
		case *time.Duration:
			var defValue time.Duration
			parseDefault(&defValue, name, def)
			CommandLine.DurationVar(ptr, name, defValue, usage)
		case *float64:
			var defValue float64
			parseDefault(&defValue, name, def)
			CommandLine.Float64Var(ptr, name, defValue, usage)
		case *string:
			CommandLine.StringVar(ptr, name, def, usage)
		case *int:
			var defValue int
			parseDefault(&defValue, name, def)
			CommandLine.IntVar(ptr, name, defValue, usage)
		case *int64:
			var defValue int64
			parseDefault(&defValue, name, def)
			CommandLine.Int64Var(ptr, name, defValue, usage)
		case *uint:
			var defValue uint
			parseDefault(&defValue, name, def)
			CommandLine.UintVar(ptr, name, defValue, usage)
		case *uint64:
			var defValue uint64
			parseDefault(&defValue, name, def)
			CommandLine.Uint64Var(ptr, name, defValue, usage)
		default:
			panic(fmt.Sprintf("unsupported struct entry type %q: %T", name, entry.Interface()))
		}
	}
}

// parseDefault is necessary because "flag" wants the default in the
// actual type and cannot take a string. It would be nice to reuse the
// existing code for parsing from the "flag" package, but it isn't
// exported.
func parseDefault(value interface{}, name, def string) {
	if def == "" {
		return
	}
	checkErr := func(err error, value interface{}) {
		if err != nil {
			panic(fmt.Sprintf("invalid default %q for %T entry %s: %s", def, value, name, err))
		}
	}
	switch value := value.(type) {
	case *bool:
		v, err := strconv.ParseBool(def)
		checkErr(err, *value)
		*value = v
	case *time.Duration:
		v, err := time.ParseDuration(def)
		checkErr(err, *value)
		*value = v
	case *float64:
		v, err := strconv.ParseFloat(def, 64)
		checkErr(err, *value)
		*value = v
	case *int:
		v, err := strconv.Atoi(def)
		checkErr(err, *value)
		*value = v
	case *int64:
		v, err := strconv.ParseInt(def, 0, 64)
		checkErr(err, *value)
		*value = v
	case *uint:
		v, err := strconv.ParseUint(def, 0, strconv.IntSize)
		checkErr(err, *value)
		*value = uint(v)
	case *uint64:
		v, err := strconv.ParseUint(def, 0, 64)
		checkErr(err, *value)
		*value = v
	default:
		panic(fmt.Sprintf("%q: setting defaults not supported for type %T", name, value))
	}
}