share.go

// Copyright 2011 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package main

import (
	"bytes"
	"crypto/sha256"
	"encoding/base64"
	"fmt"
	"io"
	"net/http"
	"strings"
)

const (
	// This salt is not meant to be kept secret (it’s checked in after all). It’s
	// a tiny bit of paranoia to avoid whatever problems a collision may cause.
	salt = "Go playground salt\n"

	maxSnippetSize = 64 * 1024
)

type snippet struct {
	Body []byte `datastore:",noindex"` // golang.org/issues/23253
}

func (s *snippet) ID() string {
	h := sha256.New()
	io.WriteString(h, salt)
	h.Write(s.Body)
	sum := h.Sum(nil)
	b := make([]byte, base64.URLEncoding.EncodedLen(len(sum)))
	base64.URLEncoding.Encode(b, sum)
	// Web sites don’t always linkify a trailing underscore, making it seem like
	// the link is broken. If there is an underscore at the end of the substring,
	// extend it until there is not.
	hashLen := 11
	for hashLen <= len(b) && b[hashLen-1] == '_' {
		hashLen++
	}
	return string(b)[:hashLen]
}

func (s *server) handleShare(w http.ResponseWriter, r *http.Request) {
	w.Header().Set("Access-Control-Allow-Origin", "*")
	if r.Method == "OPTIONS" {
		// This is likely a pre-flight CORS request.
		return
	}
	if r.Method != "POST" {
		http.Error(w, "Requires POST", http.StatusMethodNotAllowed)
		return
	}
	if !allowShare(r) {
		http.Error(w, "Either this isn't available in your country due to legal reasons, or our IP geolocation is wrong.",
			http.StatusUnavailableForLegalReasons)
		return
	}

	var body bytes.Buffer
	_, err := io.Copy(&body, io.LimitReader(r.Body, maxSnippetSize+1))
	r.Body.Close()
	if err != nil {
		s.log.Errorf("reading Body: %v", err)
		http.Error(w, "Server Error", http.StatusInternalServerError)
		return
	}
	if body.Len() > maxSnippetSize {
		http.Error(w, "Snippet is too large", http.StatusRequestEntityTooLarge)
		return
	}

	snip := &snippet{Body: body.Bytes()}
	id := snip.ID()
	if err := s.db.PutSnippet(r.Context(), id, snip); err != nil {
		s.log.Errorf("putting Snippet: %v", err)
		http.Error(w, http.StatusText(http.StatusInternalServerError), http.StatusInternalServerError)
		return
	}

	fmt.Fprint(w, id)
}

// golang/go#65081: the IP prefixes below have been observed in proxied traffic
// from go.dev to play.golang.org, and as of 2024-01-16 are incorrectly
// identified as CN by X-AppEngine-Country. Using geoIP tooling, these were all
// verified as having temporarily been categorized as CN in early January, but
// are currently US.
//
// While this is being investigated, hard-code a temporary allow list for these
// IPs to get the playground working again.
//
// Per https://www.gstatic.com/ipranges/goog.json, these are Google IPs.
var temporaryAllowListIPPrefixes = []string{
	"2600:1900:2001:2",
	"2600:1900:2001:3",
	"2600:1900:2000:1b",
	"2600:1900:2000:1d",
	"2600:1900:2000:38",
	"2600:1900:2000:37",
	"2600:1900:2000:9",
}

func allowShare(r *http.Request) bool {
	if r.Header.Get("X-AppEngine-Country") != "CN" {
		return true
	}
	for _, forward := range strings.Split(r.Header.Get("X-Forwarded-For"), ",") {
		for _, prefix := range temporaryAllowListIPPrefixes {
			if strings.HasPrefix(strings.TrimSpace(forward), prefix) {
				return true
			}
		}
	}
	return false
}