cors.go

package engine

import (
	"net/http"
	"strconv"
	"strings"
)

// Cors http handler
type cors struct {

	// Set to true when allowed origins contains a "*"
	allowedOriginsAll bool

	// Normalized list of plain allowed origins
	allowedOrigins []string

	// List of allowed origins containing wildcards
	allowedWOrigins []wildcard

	// Optional origin validator function
	allowOriginFunc func(origin string) bool

	// Set to true when allowed headers contains a "*"
	allowedHeadersAll bool

	// Normalized list of allowed headers
	allowedHeaders []string

	// Normalized list of allowed methods
	allowedMethods []string

	// Normalized list of exposed headers
	exposedHeaders    []string
	allowCredentials  bool
	maxAge            int
	optionPassthrough bool
}

const toLower = 'a' - 'A'

type converter func(string) string

type wildcard struct {
	prefix string
	suffix string
}

func (w wildcard) match(s string) bool {
	return len(s) >= len(w.prefix+w.suffix) && strings.HasPrefix(s, w.prefix) && strings.HasSuffix(s, w.suffix)
}

// convert converts a list of string using the passed converter function
func convert(s []string, c converter) []string {
	out := []string{}
	for _, i := range s {
		out = append(out, c(i))
	}
	return out
}

// parseHeaderList tokenize + normalize a string containing a list of headers
func parseHeaderList(headerList string) []string {
	l := len(headerList)
	h := make([]byte, 0, l)
	upper := true
	// Estimate the number headers in order to allocate the right splice size
	t := 0
	for i := 0; i < l; i++ {
		if headerList[i] == ',' {
			t++
		}
	}
	headers := make([]string, 0, t)
	for i := 0; i < l; i++ {
		b := headerList[i]
		if b >= 'a' && b <= 'z' {
			if upper {
				h = append(h, b-toLower)
			} else {
				h = append(h, b)
			}
		} else if b >= 'A' && b <= 'Z' {
			if !upper {
				h = append(h, b+toLower)
			} else {
				h = append(h, b)
			}
		} else if b == '-' || (b >= '0' && b <= '9') {
			h = append(h, b)
		}

		if b == ' ' || b == ',' || i == l-1 {
			if len(h) > 0 {
				// Flush the found header
				headers = append(headers, string(h))
				h = h[:0]
				upper = true
			}
		} else {
			upper = b == '-'
		}
	}
	return headers
}

// newCors creates a new Cors handler with the provided options.
func newCors(options Options) *cors {
	c := &cors{
		exposedHeaders:    convert(options.ExposedHeaders, http.CanonicalHeaderKey),
		allowOriginFunc:   options.AllowOriginFunc,
		allowCredentials:  options.AllowCredentials,
		maxAge:            options.MaxAge,
		optionPassthrough: options.OptionsPassthrough,
	}

	// Normalize options
	// Note: for origins and methods matching, the spec requires a case-sensitive matching.
	// As it may error prone, we chose to ignore the spec here.

	// Allowed Origins
	if len(options.AllowedOrigins) == 0 {
		// Default is all origins
		c.allowedOriginsAll = true
	} else {
		c.allowedOrigins = []string{}
		c.allowedWOrigins = []wildcard{}
		for _, origin := range options.AllowedOrigins {
			// Normalize
			origin = strings.ToLower(origin)
			if origin == "*" {
				// If "*" is present in the list, turn the whole list into a match all
				c.allowedOriginsAll = true
				c.allowedOrigins = nil
				c.allowedWOrigins = nil
				break
			} else if i := strings.IndexByte(origin, '*'); i >= 0 {
				// Split the origin in two: start and end string without the *
				w := wildcard{origin[0:i], origin[i+1 : len(origin)]}
				c.allowedWOrigins = append(c.allowedWOrigins, w)
			} else {
				c.allowedOrigins = append(c.allowedOrigins, origin)
			}
		}
	}

	// Allowed Headers
	if len(options.AllowedHeaders) == 0 {
		// Use sensible defaults
		c.allowedHeaders = []string{"Origin", "Accept", "Content-Type"}
	} else {
		// Origin is always appended as some browsers will always request for this header at preflight
		c.allowedHeaders = convert(append(options.AllowedHeaders, "Origin"), http.CanonicalHeaderKey)
		for _, h := range options.AllowedHeaders {
			if h == "*" {
				c.allowedHeadersAll = true
				c.allowedHeaders = nil
				break
			}
		}
	}

	// Allowed Methods
	if len(options.AllowedMethods) == 0 {
		// Default is spec's "simple" methods
		c.allowedMethods = []string{"GET", "POST"}
	} else {
		c.allowedMethods = convert(options.AllowedMethods, strings.ToUpper)
	}

	return c
}

// Handler apply the CORS specification on the request, and add relevant CORS headers
// as necessary.
func (c *cors) Handler(h http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		if r.Method == "OPTIONS" {
			c.handlePreflight(w, r)
			// Preflight requests are standalone and should stop the chain as some other
			// middleware may not handle OPTIONS requests correctly. One typical example
			// is authentication middleware ; OPTIONS requests won't carry authentication
			// headers (see #1)
			if c.optionPassthrough {
				w.WriteHeader(204)
			}
		} else {
			c.handleActualRequest(w, r)
			h.ServeHTTP(w, r)
		}
	})
}

// HandlerFunc provides Martini compatible handler
func (c *cors) HandlerFunc(w http.ResponseWriter, r *http.Request) {
	if r.Method == "OPTIONS" {
		c.handlePreflight(w, r)
	} else {
		c.handleActualRequest(w, r)
	}
}

// Negroni compatible interface
func (c *cors) ServeHTTP(w http.ResponseWriter, r *http.Request, next http.HandlerFunc) {
	if r.Method == "OPTIONS" {
		c.handlePreflight(w, r)
		// Preflight requests are standalone and should stop the chain as some other
		// middleware may not handle OPTIONS requests correctly. One typical example
		// is authentication middleware ; OPTIONS requests won't carry authentication
		// headers (see #1)
		if c.optionPassthrough {
			next(w, r)
		}
	} else {
		c.handleActualRequest(w, r)
		next(w, r)
	}
}

// handlePreflight handles pre-flight CORS requests
func (c *cors) handlePreflight(w http.ResponseWriter, r *http.Request) {
	headers := w.Header()
	origin := r.Header.Get("Origin")

	if r.Method != "OPTIONS" {
		return
	}
	// Always set Vary headers
	// see https://github.com/rs/cors/issues/10,
	//     https://github.com/rs/cors/commit/dbdca4d95feaa7511a46e6f1efb3b3aa505bc43f#commitcomment-12352001
	headers.Add("Vary", "Origin")
	headers.Add("Vary", "Access-Control-Request-Method")
	headers.Add("Vary", "Access-Control-Request-Headers")

	if origin == "" {
		return
	}
	if !c.isOriginAllowed(origin) {
		return
	}

	reqMethod := r.Header.Get("Access-Control-Request-Method")
	if !c.isMethodAllowed(reqMethod) {
		return
	}
	reqHeaders := parseHeaderList(r.Header.Get("Access-Control-Request-Headers"))
	if !c.areHeadersAllowed(reqHeaders) {
		return
	}
	headers.Set("Access-Control-Allow-Origin", origin)
	// Spec says: Since the list of methods can be unbounded, simply returning the method indicated
	// by Access-Control-Request-Method (if supported) can be enough
	headers.Set("Access-Control-Allow-Methods", strings.ToUpper(reqMethod))
	if len(reqHeaders) > 0 {
		// Spec says: Since the list of headers can be unbounded, simply returning supported headers
		// from Access-Control-Request-Headers can be enough
		headers.Set("Access-Control-Allow-Headers", strings.Join(reqHeaders, ", "))
	}
	if c.allowCredentials {
		headers.Set("Access-Control-Allow-Credentials", "true")
	}
	if c.maxAge > 0 {
		headers.Set("Access-Control-Max-Age", strconv.Itoa(c.maxAge))
	}
}

// handleActualRequest handles simple cross-origin requests, actual request or redirects
func (c *cors) handleActualRequest(w http.ResponseWriter, r *http.Request) {
	headers := w.Header()
	origin := r.Header.Get("Origin")

	if r.Method == "OPTIONS" {
		return
	}
	// Always set Vary, see https://github.com/rs/cors/issues/10
	headers.Add("Vary", "Origin")
	if origin == "" {
		return
	}
	if !c.isOriginAllowed(origin) {
		return
	}

	// Note that spec does define a way to specifically disallow a simple method like GET or
	// POST. Access-Control-Allow-Methods is only used for pre-flight requests and the
	// spec doesn't instruct to check the allowed methods for simple cross-origin requests.
	// We think it's a nice feature to be able to have control on those methods though.
	if !c.isMethodAllowed(r.Method) {
		return
	}
	headers.Set("Access-Control-Allow-Origin", origin)
	if len(c.exposedHeaders) > 0 {
		headers.Set("Access-Control-Expose-Headers", strings.Join(c.exposedHeaders, ", "))
	}
	if c.allowCredentials {
		headers.Set("Access-Control-Allow-Credentials", "true")
	}
}

// isOriginAllowed checks if a given origin is allowed to perform cross-domain requests
// on the endpoint
func (c *cors) isOriginAllowed(origin string) bool {
	if c.allowOriginFunc != nil {
		return c.allowOriginFunc(origin)
	}
	if c.allowedOriginsAll {
		return true
	}
	origin = strings.ToLower(origin)
	for _, o := range c.allowedOrigins {
		if o == origin {
			return true
		}
	}
	for _, w := range c.allowedWOrigins {
		if w.match(origin) {
			return true
		}
	}
	return false
}

// isMethodAllowed checks if a given method can be used as part of a cross-domain request
// on the endpoing
func (c *cors) isMethodAllowed(method string) bool {
	if len(c.allowedMethods) == 0 {
		// If no method allowed, always return false, even for preflight request
		return false
	}
	method = strings.ToUpper(method)
	if method == "OPTIONS" {
		// Always allow preflight requests
		return true
	}
	for _, m := range c.allowedMethods {
		if m == method {
			return true
		}
	}
	return false
}

// areHeadersAllowed checks if a given list of headers are allowed to used within
// a cross-domain request.
func (c *cors) areHeadersAllowed(requestedHeaders []string) bool {
	if c.allowedHeadersAll || len(requestedHeaders) == 0 {
		return true
	}
	for _, header := range requestedHeaders {
		header = http.CanonicalHeaderKey(header)
		found := false
		for _, h := range c.allowedHeaders {
			if h == header {
				found = true
			}
		}
		if !found {
			return false
		}
	}
	return true
}