Add RSS feed support

This adds a project-local RSS plugin that generates RSS 2.0 feeds from
configured Lektor sources, including feed titles, item dates, authors,
body content, and stable permalink IDs.

It also adds latest and all-post RSS feed configurations and advertises
both feeds from the base template alongside the existing Atom feeds.

4 files changed, 301 insertions, 0 deletions

diff --git a/configs/rss.ini b/configs/rss.ini
new file mode 100644
index 0000000..ed86ddc
--- /dev/null
+++ b/configs/rss.ini
@@ -0,0 +1,18 @@
+[latest]
+name = Still Thinking About It — Latest posts
+source_path = /blog
+url_path = /blog/latest.rss.xml
+item_model = blog-post
+item_date_field = publication_date
+blog_author_field = author
+item_author_field = author
+
+[all]
+name = Still Thinking About It — All posts
+source_path = /blog
+url_path = /blog/all.rss.xml
+item_model = blog-post
+item_date_field = publication_date
+blog_author_field = author
+item_author_field = author
+limit = 0
diff --git a/packages/rss/lektor_rss.py b/packages/rss/lektor_rss.py
new file mode 100644
index 0000000..224d6f8
--- /dev/null
+++ b/packages/rss/lektor_rss.py
@@ -0,0 +1,260 @@
+from __future__ import annotations
+
+from datetime import date, datetime, time, timezone
+from typing import Any
+
+from feedgenerator.django.utils.feedgenerator import Rss201rev2Feed
+from lektor.build_programs import BuildProgram
+from lektor.context import get_ctx, url_to
+from lektor.db import F
+from lektor.environment import Expression
+from lektor.pluginsystem import Plugin
+from lektor.sourceobj import VirtualSourceObject
+from lektor.utils import build_url
+
+
+def get_field(record: Any, field: str, default: Any = None) -> Any:
+    if field in record:
+        return record[field]
+    return default
+
+
+def get_item_title(item: Any, field: str) -> str:
+    if field in item:
+        return str(item[field])
+    return item.record_label
+
+
+def get_item_body(item: Any, field: str) -> str:
+    if field not in item:
+        raise RuntimeError(f"Body field {field!r} not found in {item!r}")
+
+    with get_ctx().changed_base_url(item.url_path):
+        body = item[field]
+
+        if hasattr(body, "__html__"):
+            return str(body.__html__())
+
+        return str(body)
+
+
+def get_item_date(item: Any, field: str) -> datetime:
+    value = get_field(item, field)
+
+    if value is None:
+        raise RuntimeError(f"Publication date field {field!r} not found in {item!r}")
+
+    if isinstance(value, datetime):
+        if value.tzinfo is None:
+            return value.replace(tzinfo=timezone.utc)
+        return value
+
+    if isinstance(value, date):
+        return datetime.combine(value, time.min, tzinfo=timezone.utc)
+
+    raise TypeError(f"Publication date field {field!r} must be a date or datetime")
+
+
+class RssFeedSource(VirtualSourceObject):
+    def __init__(
+        self,
+        parent: Any,
+        feed_id: str,
+        plugin: RssPlugin,
+    ) -> None:
+        super().__init__(parent)
+        self.feed_id = feed_id
+        self.plugin = plugin
+
+    @property
+    def path(self) -> str:
+        return f"{self.parent.path}@rss/{self.feed_id}"
+
+    @property
+    def url_path(self) -> str:
+        configured_path = self.plugin.get_rss_config(
+            self.feed_id,
+            "url_path",
+        )
+
+        if configured_path:
+            return configured_path
+
+        return build_url([self.parent.url_path, self.filename])
+
+    @property
+    def feed_name(self) -> str:
+        return self.plugin.get_rss_config(self.feed_id, "name") or self.feed_id
+
+    def iter_source_filenames(self):
+        return self.parent.iter_source_filenames()
+
+    def __getattr__(self, name: str) -> Any:
+        try:
+            return self.plugin.get_rss_config(self.feed_id, name)
+        except KeyError as exc:
+            raise AttributeError(name) from exc
+
+
+class RssFeedBuilderProgram(BuildProgram):
+    def produce_artifacts(self) -> None:
+        self.declare_artifact(
+            self.source.url_path,
+            sources=list(self.source.iter_source_filenames()),
+        )
+
+    def build_artifact(self, artifact: Any) -> None:
+        ctx = get_ctx()
+        source = self.source
+        blog = source.parent
+
+        summary = get_field(
+            blog,
+            source.blog_summary_field,
+            "",
+        )
+
+        if hasattr(summary, "__html__"):
+            summary = str(summary.__html__())
+        else:
+            summary = str(summary)
+
+        blog_author = str(
+            get_field(
+                blog,
+                source.blog_author_field,
+                "",
+            )
+            or ""
+        )
+
+        feed = Rss201rev2Feed(
+            title=source.feed_name,
+            link=url_to(blog, external=True),
+            description=summary,
+            feed_url=url_to(source, external=True),
+            language="en",
+        )
+
+        if source.items:
+            expression = Expression(ctx.env, source.items)
+            items = expression.evaluate(ctx.pad)
+        else:
+            items = blog.children
+
+        if source.item_model:
+            items = items.filter(F._model == source.item_model)
+
+        items = items.order_by(f"-{source.item_date_field}").limit(int(source.limit))
+
+        for item in items:
+            item_url = url_to(item, external=True)
+
+            item_author = (
+                get_field(
+                    item,
+                    source.item_author_field,
+                )
+                or blog_author
+            )
+
+            feed.add_item(
+                title=get_item_title(
+                    item,
+                    source.item_title_field,
+                ),
+                link=item_url,
+                description=get_item_body(
+                    item,
+                    source.item_body_field,
+                ),
+                author_name=str(item_author),
+                pubdate=get_item_date(
+                    item,
+                    source.item_date_field,
+                ),
+                unique_id=item_url,
+                unique_id_is_permalink=True,
+            )
+
+        with artifact.open("wb") as file:
+            feed.write(file, "utf-8")
+
+
+class RssPlugin(Plugin):
+    name = "RSS"
+    description = "Generate RSS 2.0 feeds."
+
+    defaults = {
+        "source_path": "/",
+        "name": None,
+        "url_path": None,
+        "filename": "feed.rss",
+        "blog_author_field": "author",
+        "blog_summary_field": "summary",
+        "items": None,
+        "limit": 50,
+        "item_title_field": "title",
+        "item_body_field": "body",
+        "item_author_field": "author",
+        "item_date_field": "pub_date",
+        "item_model": None,
+    }
+
+    def get_rss_config(
+        self,
+        feed_id: str,
+        key: str,
+    ) -> Any:
+        return self.get_config().get(
+            f"{feed_id}.{key}",
+            self.defaults[key],
+        )
+
+    def on_setup_env(self, **extra: Any) -> None:
+        self.env.add_build_program(
+            RssFeedSource,
+            RssFeedBuilderProgram,
+        )
+
+        @self.env.virtualpathresolver("rss")
+        def rss_path_resolver(
+            node: Any,
+            pieces: list[str],
+        ) -> RssFeedSource | None:
+            if len(pieces) != 1:
+                return None
+
+            feed_id = pieces[0]
+
+            if feed_id not in self.get_config().sections():
+                return None
+
+            source_path = self.get_rss_config(
+                feed_id,
+                "source_path",
+            )
+
+            if node.path != source_path:
+                return None
+
+            return RssFeedSource(
+                node,
+                feed_id,
+                self,
+            )
+
+        @self.env.generator
+        def generate_rss_feeds(source: Any):
+            for feed_id in self.get_config().sections():
+                source_path = self.get_rss_config(
+                    feed_id,
+                    "source_path",
+                )
+
+                if source.path == source_path:
+                    yield RssFeedSource(
+                        source,
+                        feed_id,
+                        self,
+                    )
diff --git a/packages/rss/setup.py b/packages/rss/setup.py
new file mode 100644
index 0000000..b036f8b
--- /dev/null
+++ b/packages/rss/setup.py
@@ -0,0 +1,15 @@
+from setuptools import setup
+
+setup(
+    name="lektor-rss",
+    version="0.1.0",
+    py_modules=["lektor_rss"],
+    entry_points={
+        "lektor.plugins": [
+            "rss = lektor_rss:RssPlugin",
+        ]
+    },
+    install_requires=[
+        "feedgenerator",
+    ],
+)
diff --git a/templates/base.html b/templates/base.html
index 3ac053c..051abf7 100644
--- a/templates/base.html
+++ b/templates/base.html
@@ -102,6 +102,14 @@
               type="application/atom+xml"
               title="{{ bag('blog.name') }} — All posts"
               href="{{ '/blog@atom/all'|url(absolute=True) }}">
+        <link rel="alternate"
+              type="application/rss+xml"
+              title="{{ bag('blog.name') }} — Latest posts"
+              href="{{ '/blog@rss/latest'|url(absolute=True) }}">
+        <link rel="alternate"
+              type="application/rss+xml"
+              title="{{ bag('blog.name') }} — All posts"
+              href="{{ '/blog@rss/all'|url(absolute=True) }}">
         <link rel="shortcut icon"
               type="image/png"
               href="{{ '/static/images/favicon.png' | asseturl }}">