[feature] Add opt-in RSS feed for account's latest Public posts (#897)

* start adding rss functionality

* add gorilla/feeds dependency

* first bash at building rss feed
still needs work, this is an interim commit

* tidy up a bit

* add publicOnly option to GetAccountLastPosted

* implement rss endpoint

* fix test

* add initial user docs for rss

* update rss logo

* docs update

* add rssFeed to frontend

* feed -> feed.rss

* enableRSS

* increase rss logo size a lil bit

* add rss toggle

* move emojify to text package

* fiddle with rss feed formatting

* add Text field to test statuses

* move status to rss item to typeconverter

* update bun schema for enablerss

* simplify 304 checking

* assume account not rss

* update tests

* update swagger docs

* allow more characters in title, trim nicer

* update last posted to be more consistent

1 files changed, 154 insertions, 0 deletions

diff --git a/internal/web/rss.go b/internal/web/rss.go
new file mode 100644
index 000000000..64be7685c
--- /dev/null
+++ b/internal/web/rss.go
@@ -0,0 +1,154 @@
+/*
+   GoToSocial
+   Copyright (C) 2021-2022 GoToSocial Authors admin@gotosocial.org
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU Affero General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU Affero General Public License for more details.
+
+   You should have received a copy of the GNU Affero General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+package web
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"net/http"
+	"strings"
+	"time"
+
+	"github.com/gin-gonic/gin"
+	"github.com/superseriousbusiness/gotosocial/internal/api"
+	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
+	"github.com/superseriousbusiness/gotosocial/internal/log"
+)
+
+const appRSSUTF8 = string(api.AppRSSXML + "; charset=utf-8")
+
+func (m *Module) GetRSSETag(urlPath string, lastModified time.Time, getRSSFeed func() (string, gtserror.WithCode)) (string, error) {
+	if cachedETag, ok := m.eTagCache.Get(urlPath); ok && !lastModified.After(cachedETag.lastModified) {
+		// only return our cached etag if the file wasn't
+		// modified since last time, otherwise generate a
+		// new one; eat fresh!
+		return cachedETag.eTag, nil
+	}
+
+	rssFeed, errWithCode := getRSSFeed()
+	if errWithCode != nil {
+		return "", fmt.Errorf("error getting rss feed: %s", errWithCode)
+	}
+
+	eTag, err := generateEtag(bytes.NewReader([]byte(rssFeed)))
+	if err != nil {
+		return "", fmt.Errorf("error generating etag: %s", err)
+	}
+
+	// put new entry in cache before we return
+	m.eTagCache.Set(urlPath, eTagCacheEntry{
+		eTag:         eTag,
+		lastModified: lastModified,
+	})
+
+	return eTag, nil
+}
+
+func extractIfModifiedSince(header string) time.Time {
+	if header == "" {
+		return time.Time{}
+	}
+
+	t, err := http.ParseTime(header)
+	if err != nil {
+		log.Errorf("couldn't parse if-modified-since %s: %s", header, err)
+		return time.Time{}
+	}
+
+	return t
+}
+
+func (m *Module) rssFeedGETHandler(c *gin.Context) {
+	// set this Cache-Control header to instruct clients to validate the response with us
+	// before each reuse (https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Cache-Control)
+	c.Header(cacheControlHeader, cacheControlNoCache)
+	ctx := c.Request.Context()
+
+	if _, err := api.NegotiateAccept(c, api.AppRSSXML); err != nil {
+		api.ErrorHandler(c, gtserror.NewErrorNotAcceptable(err, err.Error()), m.processor.InstanceGet)
+		return
+	}
+
+	// usernames on our instance will always be lowercase
+	username := strings.ToLower(c.Param(usernameKey))
+	if username == "" {
+		err := errors.New("no account username specified")
+		api.ErrorHandler(c, gtserror.NewErrorBadRequest(err, err.Error()), m.processor.InstanceGet)
+		return
+	}
+
+	ifNoneMatch := c.Request.Header.Get(ifNoneMatchHeader)
+	ifModifiedSince := extractIfModifiedSince(c.Request.Header.Get(ifModifiedSinceHeader))
+
+	getRssFeed, accountLastPostedPublic, errWithCode := m.processor.AccountGetRSSFeedForUsername(ctx, username)
+	if errWithCode != nil {
+		api.ErrorHandler(c, errWithCode, m.processor.InstanceGet)
+		return
+	}
+
+	var rssFeed string
+	cacheKey := c.Request.URL.Path
+	cacheEntry, ok := m.eTagCache.Get(cacheKey)
+
+	if !ok || cacheEntry.lastModified.Before(accountLastPostedPublic) {
+		// we either have no cache entry for this, or we have an expired cache entry; generate a new one
+		rssFeed, errWithCode = getRssFeed()
+		if errWithCode != nil {
+			api.ErrorHandler(c, errWithCode, m.processor.InstanceGet)
+			return
+		}
+
+		eTag, err := generateEtag(bytes.NewBufferString(rssFeed))
+		if err != nil {
+			api.ErrorHandler(c, gtserror.NewErrorInternalError(err), m.processor.InstanceGet)
+			return
+		}
+
+		cacheEntry.lastModified = accountLastPostedPublic
+		cacheEntry.eTag = eTag
+		m.eTagCache.Put(cacheKey, cacheEntry)
+	}
+
+	c.Header(eTagHeader, cacheEntry.eTag)
+	c.Header(lastModifiedHeader, accountLastPostedPublic.Format(http.TimeFormat))
+
+	if ifNoneMatch == cacheEntry.eTag {
+		c.AbortWithStatus(http.StatusNotModified)
+		return
+	}
+
+	lmUnix := cacheEntry.lastModified.Unix()
+	imsUnix := ifModifiedSince.Unix()
+	if lmUnix <= imsUnix {
+		c.AbortWithStatus(http.StatusNotModified)
+		return
+	}
+
+	if rssFeed == "" {
+		// we had a cache entry already so we didn't call to get the rss feed yet
+		rssFeed, errWithCode = getRssFeed()
+		if errWithCode != nil {
+			api.ErrorHandler(c, errWithCode, m.processor.InstanceGet)
+			return
+		}
+	}
+
+	c.Data(http.StatusOK, appRSSUTF8, []byte(rssFeed))
+}