feat(feed): auto-discover feed URLs from HTML pages

When an HTML page is provided instead of a direct feed URL, parse
<link rel="alternate"> tags to find RSS/Atom feeds. Atom is preferred
over RSS when both are present.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

1 files changed, 59 insertions, 4 deletions

diff --git a/backend/feed/feed.go b/backend/feed/feed.go
index 4349d1e..2d84798 100644
--- a/backend/feed/feed.go
+++ b/backend/feed/feed.go
@@ -2,7 +2,10 @@ package feed
 
 import (
 	"context"
+	"errors"
 	"fmt"
+	"net/http"
+	"net/url"
 	"time"
 
 	"github.com/mmcdole/gofeed"
@@ -10,15 +13,67 @@ import (
 	"undef.ninja/x/feedaka/db"
 )
 
-func Fetch(ctx context.Context, url string) (*gofeed.Feed, error) {
+// FetchResult holds the result of fetching a feed, including the resolved URL.
+type FetchResult struct {
+	Feed *gofeed.Feed
+	URL  string
+}
+
+func Fetch(ctx context.Context, rawURL string) (*FetchResult, error) {
 	fp := gofeed.NewParser()
 	ctx, cancel := context.WithTimeout(ctx, 10*time.Second)
 	defer cancel()
-	feed, err := fp.ParseURLWithContext(url, ctx)
+
+	f, err := fp.ParseURLWithContext(rawURL, ctx)
+	if err == nil {
+		return &FetchResult{Feed: f, URL: rawURL}, nil
+	}
+
+	if !errors.Is(err, gofeed.ErrFeedTypeNotDetected) {
+		return nil, fmt.Errorf("failed to fetch %s: %w", rawURL, err)
+	}
+
+	discoveredURL, discoverErr := discoverFeedURL(ctx, rawURL)
+	if discoverErr != nil {
+		return nil, fmt.Errorf("failed to fetch %s: not a feed and auto-discovery failed: %w", rawURL, discoverErr)
+	}
+
+	f, err = fp.ParseURLWithContext(discoveredURL, ctx)
 	if err != nil {
-		return nil, fmt.Errorf("failed to fetch %s: %w", url, err)
+		return nil, fmt.Errorf("failed to fetch discovered feed %s: %w", discoveredURL, err)
 	}
-	return feed, nil
+
+	return &FetchResult{Feed: f, URL: discoveredURL}, nil
+}
+
+func discoverFeedURL(ctx context.Context, rawURL string) (string, error) {
+	req, err := http.NewRequestWithContext(ctx, "GET", rawURL, nil)
+	if err != nil {
+		return "", err
+	}
+
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		return "", err
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
+		return "", fmt.Errorf("HTTP %d", resp.StatusCode)
+	}
+
+	base, err := url.Parse(rawURL)
+	if err != nil {
+		return "", err
+	}
+
+	links := discoverFeeds(resp.Body, base)
+	feedURL := selectFeed(links)
+	if feedURL == "" {
+		return "", fmt.Errorf("no feed links found in HTML")
+	}
+
+	return feedURL, nil
 }
 
 func Sync(ctx context.Context, queries *db.Queries, feedID int64, f *gofeed.Feed) error {