form.go

/*
Copyright 2015 Gravitational, Inc.

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.
*/
// Form is a minimalist HTTP web form parser library based on functional arguments.
package form

import (
	"fmt"
	"mime"
	"mime/multipart"
	"net/http"
	"time"

	"strconv"
)

// Param is a functional argument parameter passed to the Parse function
type Param func(r *http.Request) error

// Parse takes http.Request and form arguments that it needs to extract
//
//   import (
//        "github.com/gravitational/form"
//   )
//
//   var duration time.Duration
//   var count int
//   name := "default" // a simple way to set default argument
//
//   err := form.Parse(r,
//      form.Duration("duration", &duration),
//      form.Int("count", &count, Required()), // notice the "Required" argument
//      form.String("name", &name),
//      )
//
//   if err != nil {
//        // handle error here
//   }
func Parse(r *http.Request, params ...Param) error {
	mtype, _, err := mime.ParseMediaType(r.Header.Get("Content-Type"))
	if err != nil {
		return err
	}
	if mtype == "multipart/form-data" {
		if err := r.ParseMultipartForm(maxMemoryBytes); err != nil {
			return err
		}
	} else {
		if err := r.ParseForm(); err != nil {
			return err
		}
	}
	for _, p := range params {
		if err := p(r); err != nil {
			return err
		}
	}
	return nil
}

// Duration extracts duration expressed as a Go duration string e.g. "1s"
func Duration(name string, out *time.Duration, predicates ...Predicate) Param {
	return func(r *http.Request) error {
		for _, p := range predicates {
			if err := p.Pass(name, r); err != nil {
				return err
			}
		}
		v := r.Form.Get(name)
		if v == "" {
			return nil
		}
		d, err := time.ParseDuration(v)
		if err != nil {
			return &BadParameterError{Param: name, Message: err.Error()}
		}
		*out = d
		return nil
	}
}

// Time extracts duration expressed as in RFC 3339 format
func Time(name string, out *time.Time, predicates ...Predicate) Param {
	return func(r *http.Request) error {
		for _, p := range predicates {
			if err := p.Pass(name, r); err != nil {
				return err
			}
		}
		v := r.Form.Get(name)
		if v == "" {
			return nil
		}
		var t time.Time
		if err := t.UnmarshalText([]byte(v)); err != nil {
			return &BadParameterError{Param: name, Message: err.Error()}
		}
		*out = t
		return nil
	}
}

// String extracts the argument by name as is without any changes
func String(name string, out *string, predicates ...Predicate) Param {
	return func(r *http.Request) error {
		for _, p := range predicates {
			if err := p.Pass(name, r); err != nil {
				return err
			}
		}
		*out = r.Form.Get(name)
		return nil
	}
}

// Int extracts the integer argument in decimal format e.g. "10"
func Int(name string, out *int, predicates ...Predicate) Param {
	return func(r *http.Request) error {
		v := r.Form.Get(name)
		for _, p := range predicates {
			if err := p.Pass(name, r); err != nil {
				return err
			}
		}
		if v == "" {
			return nil
		}
		p, err := strconv.Atoi(v)
		if err != nil {
			return &BadParameterError{Param: name, Message: err.Error()}
		}
		*out = p
		return nil
	}
}

// StringSlice extracts the string slice of arguments by name
func StringSlice(name string, out *[]string, predicates ...Predicate) Param {
	return func(r *http.Request) error {
		for _, p := range predicates {
			if err := p.Pass(name, r); err != nil {
				return err
			}
		}
		*out = make([]string, len(r.Form[name]))
		copy(*out, r.Form[name])
		return nil
	}
}

// FileSlice reads the files uploaded with name parameter and initialized
// the slice of files. The files should be closed by the callee after
// usage, by executing f.Close() on each of them
// files slice will be nil if there's an error
func FileSlice(name string, files *Files, predicates ...Predicate) Param {
	return func(r *http.Request) error {
		err := r.ParseMultipartForm(maxMemoryBytes)
		if err != nil {
			return err
		}
		if r.MultipartForm == nil && r.MultipartForm.File == nil {
			return fmt.Errorf("missing form")
		}
		for _, p := range predicates {
			if err := p.Pass(name, r); err != nil {
				return err
			}
		}

		fhs := r.MultipartForm.File[name]
		if len(fhs) == 0 {
			*files = []*FileWrapper{}
			return nil
		}

		*files = make([]*FileWrapper, len(fhs))
		for i, fh := range fhs {
			f, err := fh.Open()
			if err != nil {
				files.Close()
				return err
			}
			(*files)[i] = &FileWrapper{f, fh.Filename}
		}
		return nil
	}
}

// Predicate provides an extensible way to check various conditions on a variable
// e.g. setting minimums and maximums, or parsing some regular expressions
type Predicate interface {
	Pass(param string, r *http.Request) error
}

// PredicateFunc takes a func and converts it into a Predicate-compatible interface
type PredicateFunc func(param string, r *http.Request) error

func (p PredicateFunc) Pass(param string, r *http.Request) error {
	return p(param, r)
}

// Required checker parameter ensures that the parameter is indeed supplied by user
// it returns MissingParameterError when parameter is not present
func Required() Predicate {
	return PredicateFunc(func(param string, r *http.Request) error {
		if r.Form.Get(param) == "" {
			return &MissingParameterError{Param: param}
		}
		return nil
	})
}

// MissingParameterError is an error that indicates that required parameter was not
// supplied by user.
type MissingParameterError struct {
	Param string
}

func (p *MissingParameterError) Error() string {
	return fmt.Sprintf("missing required parameter: '%v'", p.Param)
}

// BadParameterError is returned whenever the parameter format does not match
// required restrictions.
type BadParameterError struct {
	Param   string // Param is a paramter name
	Message string // Message is an error message presented to user
}

func (p *BadParameterError) Error() string {
	return fmt.Sprintf("bad parameter '%v', error: %v", p.Param, p.Message)
}

const maxMemoryBytes = 64 * 1024

type FileWrapper struct {
	multipart.File
	name string
}

// Name returns file name as set during upload
func (f *FileWrapper) Name() string {
	return f.name
}

// Files is a slice of multipart.File that provides additional
// convenient method to close all files as a single operation
type Files []*FileWrapper

func (fs *Files) Close() error {
	e := &FilesCloseError{}
	for _, f := range *fs {
		if f != nil {
			if err := f.Close(); err != nil {
				e.Errors = append(e.Errors, err)
			}
		}
	}
	if len(e.Errors) != 0 {
		return e
	}
	return nil
}

type FilesCloseError struct {
	Errors []error
}

func (p *FilesCloseError) Error() string {
	return fmt.Sprintf("failed to close files, error: %v", p.Errors)
}