feat: Implement RSS feed API

Makefile

@@ -263,14 +263,13 @@ shell-lint:
 # Format all tracked and untracked Go files (skips ignored/generated files).
 go-format: go-install-tools
 	git ls-files --cached --others --exclude-standard '*.go' | xargs -r -I {} go tool golines -w --max-len=120 {}
-
+	go list -f '{{.Dir}}/...' -m | xargs -t golangci-lint run --fix
 
 lint-fix: node-install go-format
 	npm run lint-fix -w frontend
 	terraform fmt -recursive .
 	npx prettier . --write
 	npx stylelint "frontend/src/**/*.css" --fix
-	go list -f '{{.Dir}}/...' -m | xargs -t golangci-lint run --fix
 
 style-lint:
 	npx stylelint "frontend/src/**/*.css"

backend/pkg/httpserver/get_saved_search_rss.go

@@ -15,19 +15,181 @@
 package httpserver
 
 import (
+	"bytes"
 	"context"
+	"encoding/xml"
+	"errors"
+	"fmt"
+	"log/slog"
+	"net/http"
+	"net/url"
+	"strconv"
+	"time"
 
+	"github.com/GoogleChrome/webstatus.dev/lib/backendtypes"
 	"github.com/GoogleChrome/webstatus.dev/lib/gen/openapi/backend"
 )
 
-// GetSubscriptionRSS returns a "not supported" error for now.
+// RSS struct for marshaling.
+type RSS struct {
+	XMLName xml.Name `xml:"rss"`
+	Version string   `xml:"version,attr"`
+	AtomNS  string   `xml:"xmlns:atom,attr"`
+	Channel Channel  `xml:"channel"`
+}
+
+type AtomLink struct {
+	Rel  string `xml:"rel,attr"`
+	Href string `xml:"href,attr"`
+}
+
+type Channel struct {
+	Title       string     `xml:"title"`
+	Link        string     `xml:"link"`
+	Description string     `xml:"description"`
+	AtomLinks   []AtomLink `xml:"atom:link"`
+	Items       []Item     `xml:"item"`
+}
+
+type GUID struct {
+	Value       string `xml:",chardata"`
+	IsPermaLink string `xml:"isPermaLink,attr"`
+}
+
+type Item struct {
+	Description string `xml:"description"`
+	GUID        GUID   `xml:"guid"`
+	PubDate     string `xml:"pubDate"`
+}
+
+// GetSubscriptionRSS handles the request to get an RSS feed for a subscription.
 // nolint: ireturn // Signature generated from OpenAPI.
 func (s *Server) GetSubscriptionRSS(
-	_ context.Context,
-	_ backend.GetSubscriptionRSSRequestObject,
+	ctx context.Context,
+	request backend.GetSubscriptionRSSRequestObject,
 ) (backend.GetSubscriptionRSSResponseObject, error) {
-	return backend.GetSubscriptionRSS500JSONResponse{
-		Code:    500,
-		Message: "Not supported",
+	sub, err := s.wptMetricsStorer.GetSavedSearchSubscriptionPublic(ctx, request.SubscriptionId)
+	if err != nil {
+		if errors.Is(err, backendtypes.ErrEntityDoesNotExist) {
+			return backend.GetSubscriptionRSS404JSONResponse{
+				Code:    http.StatusNotFound,
+				Message: "Subscription not found",
+			}, nil
+		}
+
+		return backend.GetSubscriptionRSS500JSONResponse{
+			Code:    http.StatusInternalServerError,
+			Message: "Internal server error",
+		}, nil
+	}
+
+	search, err := s.wptMetricsStorer.GetSavedSearchPublic(ctx, sub.Subscribable.Id)
+	if err != nil {
+		if errors.Is(err, backendtypes.ErrEntityDoesNotExist) {
+			return backend.GetSubscriptionRSS404JSONResponse{
+				Code:    http.StatusNotFound,
+				Message: "Saved search not found",
+			}, nil
+		}
+		slog.ErrorContext(ctx, "failed to get saved search", "error", err)
+
+		return backend.GetSubscriptionRSS500JSONResponse{
+			Code:    http.StatusInternalServerError,
+			Message: "Internal server error",
+		}, nil
+	}
+
+	snapshotType := string(sub.Frequency)
+	pageSize := getPageSizeOrDefault(request.Params.PageSize)
+	events, nextPageToken, err := s.wptMetricsStorer.ListSavedSearchNotificationEvents(
+		ctx,
+		search.Id,
+		snapshotType,
+		pageSize,
+		request.Params.PageToken,
+	)
+	if err != nil {
+		slog.ErrorContext(ctx, "failed to list notification events", "error", err)
+
+		return backend.GetSubscriptionRSS500JSONResponse{
+			Code:    http.StatusInternalServerError,
+			Message: "Internal server error",
+		}, nil
+	}
+
+	channelLink := s.baseURL.String() + "/features?q=" + url.QueryEscape(search.Query)
+
+	rss := RSS{
+		XMLName: xml.Name{Local: "rss", Space: ""},
+		Version: "2.0",
+		AtomNS:  "http://www.w3.org/2005/Atom",
+		Channel: Channel{
+			Title:       fmt.Sprintf("WebStatus.dev - %s", search.Name),
+			Link:        channelLink,
+			Description: fmt.Sprintf("RSS feed for saved search: %s", search.Name),
+			Items:       make([]Item, 0, len(events)),
+			AtomLinks:   nil,
+		},
+	}
+
+	selfURL := s.baseURL.JoinPath("v1", "subscriptions", request.SubscriptionId, "rss")
+	selfQuery := selfURL.Query()
+	if request.Params.PageToken != nil {
+		selfQuery.Set("page_token", *request.Params.PageToken)
+	}
+	if request.Params.PageSize != nil {
+		selfQuery.Set("page_size", strconv.Itoa(*request.Params.PageSize))
+	}
+	if len(selfQuery) > 0 {
+		selfURL.RawQuery = selfQuery.Encode()
+	}
+
+	rss.Channel.AtomLinks = append(rss.Channel.AtomLinks, AtomLink{
+		Rel:  "self",
+		Href: selfURL.String(),
+	})
+
+	if nextPageToken != nil {
+		u := s.baseURL.JoinPath("v1", "subscriptions", request.SubscriptionId, "rss")
+		q := u.Query()
+		q.Set("page_token", *nextPageToken)
+		q.Set("page_size", strconv.Itoa(pageSize))
+		u.RawQuery = q.Encode()
+
+		rss.Channel.AtomLinks = append(rss.Channel.AtomLinks, AtomLink{
+			Rel:  "next",
+			Href: u.String(),
+		})
+	}
+
+	for _, e := range events {
+		rss.Channel.Items = append(rss.Channel.Items, Item{
+			Description: string(e.Summary),
+			GUID: GUID{
+				Value:       e.ID,
+				IsPermaLink: "false",
+			},
+			PubDate: e.Timestamp.Format(time.RFC1123Z),
+		})
+	}
+
+	xmlBytes, err := xml.MarshalIndent(rss, "", "  ")
+	if err != nil {
+		slog.ErrorContext(ctx, "failed to marshal RSS XML", "error", err)
+
+		return backend.GetSubscriptionRSS500JSONResponse{
+			Code:    http.StatusInternalServerError,
+			Message: "Internal server error",
+		}, nil
+	}
+
+	var buf bytes.Buffer
+	buf.Grow(len(xml.Header) + len(xmlBytes))
+	buf.WriteString(xml.Header)
+	buf.Write(xmlBytes)
+
+	return backend.GetSubscriptionRSS200ApplicationrssXmlResponse{
+		Body:          bytes.NewReader(buf.Bytes()),
+		ContentLength: int64(buf.Len()),
 	}, nil
 }