Add XSLT podcast post

v2/content/posts/2023/xslt-podcast/index.md

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+title = "Creating a Podcast From a Mastodon Account With XSLT"
+date = 2023-03-01T18:33:33+10:00
+
+#[extra]
+#updated = 2023-01-11T21:11:28+10:00
++++
+
+{% aside(title="Just want the feed?", float="right") %}
+Here you go:<br>
+[ATPrewind podcast feed](https://files.wezm.net/aptrewind.rss)
+{% end %}
+
+I recently discovered the [ATPrewind account on Mastodon][ATPrewind]. It's an account
+sharing "gems discovered while re-listening to [@atpfm] from the very first
+episode. By [@joshua]". ATP is a tech Podcast that's been running for about 10
+years. Each post (so far) from ATPrewind includes a short clip from the show in the
+form of a little video.
+
+This post describes how I was nerd sniped into creating a podcast from the ATPrewind posts.
+
+<!-- more -->
+
+It all started when [I posted the following][my-post] on Mastodon:
+
+> Ahh this ATP Rewind account is gold https://social.tupo.space/@ATPrewind
+>
+> Keep up the great work @joshua
+
+[Kashyap replied][Kashyap]:
+
+> Indeed! This should also be a podcast 🙃
+
+This was a great idea and it got me thinking about how to do it with the least
+amount of effort.
+
+In this day and age I imagine many programmers would reach for
+their favourite programming language and code up something to generate a
+podcast feed (real podcasts are just RSS). Perhaps using the Mastodon API or
+similar and then work out a way to host their program.
+
+With [my recent experience with Deno Deploy][deno-deploy] fresh in my mind I
+considered using it. However I opted for a decidedly late 90s solution: [XSLT].
+According to Wikipedia "XSLT is a language originally designed for transforming
+XML documents into other XML documents". The 'originally' refers to fact that
+you can now generate any text with it, not just XML.
+Since it was created in the era of "XML ALL THE THINGS", XSL templates are
+themselves XML documents. It also makes extensive use of [XPath] expressions to
+select nodes and extract their content.
+
+### Creating a Podcast Feed
+
+Every Mastodon account has an RSS feed so I created an XSL template to process
+the ATPrewind RSS feed and add the missing elements required to turn it into a
+valid podcast feed. With some help from [this Dr. Drang][drdrang] post this is
+what I came up with:
+
+```xml
+<?xml version="1.0" encoding="utf-8" ?>
+<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:media="http://search.yahoo.com/mrss/">
+
+<xsl:output method="xml" encoding="utf-8" indent="yes"/>
+
+<!-- First, get everything. -->
+<xsl:template match="node() | @*">
+   <xsl:copy>
+       <xsl:apply-templates select="node() | @*"/>
+   </xsl:copy>
+</xsl:template>
+
+<!-- Update items to include title and enclosure elements. -->
+<xsl:template match="/rss/channel/item">
+   <item>
+     <xsl:apply-templates select="node()" />
+     <title>Post on <xsl:value-of select="pubDate"/></title>
+     <enclosure url="{media:content/@url}" length="{media:content/@fileSize}" type="audio/mp4; codecs=&quot;mp4a.40.2&quot;" />
+  </item>
+</xsl:template>
+
+</xsl:stylesheet>
+```
+
+XML noise aside this document should be fairly self explanatory. It copies all
+nodes from the source RSS feed, then processes each `item` element. To each one
+it adds a `title` element derived from the text "Post on" and the publication
+date of the post, and an `enclosure` element derived from the `media:content`
+element.
+
+I lie a bit by saying that the enclosure MIME type is `audio/mp4;
+codecs="mp4a.40.2"`. I.e. AAC-LC in MP4 container for the video in each post.
+Running `curl -L https://social.tupo.space/@ATPrewind.rss | xsltproc
+podcast.xsl` produces the podcast feed.
+
+It took a few tries to get it to work but eventually I was able to convince
+[Overcast] that it was a real podcast. One trick that I'm exploiting here is
+that the MP4 container for video and audio is the same, the audio only version
+is just lacking the video stream. I figured that podcast players might still be
+able to play the video and at least for Overcast it works:
+
+{{ figure(image="posts/2023/xslt-podcast/overcast-screenshot.png", link="posts/2023/xslt-podcast/overcast-screenshot.png", alt="Screenshot of Overcast showing the podcast 'episodes'.", caption="Screenshot of Overcast showing the 'episodes'.", width=393) }}
+
+
+### Deployment
+
+To deploy this contraption I created a Docker image with `curl` and `libxslt`
+installed:
+
+```dockerfile
+FROM wezm-alpine:3.17.2
+
+# UID needs to match owner of /home/rss/feeds volume
+ARG PUID=1000
+ARG PGID=1000
+ARG USER=rss
+
+RUN addgroup -g ${PGID} ${USER} && \
+    adduser -D -u ${PUID} -G ${USER} -h /home/${USER} -D ${USER}
+
+RUN apk --update add curl libxslt
+
+COPY ./entrypoint.sh /home/${USER}/entrypoint.sh
+COPY ./podcast.xsl /home/${USER}/podcast.xsl
+
+WORKDIR /home/${USER}
+
+USER ${USER}
+
+VOLUME ["/home/rss/feeds"]
+ENTRYPOINT ["./entrypoint.sh"]
+```
+
+I made a small script as the entrypoint to the container:
+
+```sh
+#!/bin/sh
+
+set -e
+
+trap 'exit' TERM INT
+
+while true; do
+  curl -L https://social.tupo.space/@ATPrewind.rss | xsltproc podcast.xsl - > /home/rss/feeds/atprewind.rss
+  sleep 3600 # 1 hour
+done
+```
+
+It regenerates the podcast feed each hour. I bind mount the directory for
+files.wezm.net into the container in my Docker Compose config, which takes
+advantage of the fact that nginx is already serving that directory.
+
+Some may argue that the Docker part of this is not in the spirit of, "least
+amount of effort", but I already have all this set up on my server so adding
+one more container is very little effort. I've written previously about [my
+Alpine Linux server][server] if you'd like to read more.
+
+Once I pushed the Docker image the podcast was live. The URL is:<br>
+<https://files.wezm.net/aptrewind.rss> if you'd like to subscribe.
+
+### Future Work
+
+I whipped all this up before work today and made an assumption that might not
+always hold: there is a single video media attachment on each post. So far
+that's true but I should update the XSL template to only try to generate an
+`enclosure` element if the attachment is present. It would also be a good idea
+to filter for audio and video only in case a post appears with images.
+
+For now I shall eagerly look forward to the next post appearing in Overcast.
+
+[@atpfm]: https://mastodon.social/@atpfm
+[@joshua]: https://social.tupo.space/@joshua
+[ATPrewind]: https://social.tupo.space/@ATPrewind
+[Overcast]: https://overcast.fm/
+[XPath]: https://www.w3.org/TR/xpath-31/
+[XSLT]: https://www.w3.org/TR/xslt-30/
+[deno-deploy]: https://www.youtube.com/watch?v=d-tsfUVg4II
+[drdrang]: https://leancrew.com/all-this/2022/08/filtering-my-rss-reading/
+[server]: https://www.wezm.net/technical/2019/02/alpine-linux-docker-infrastructure/
+[my-post]: https://mastodon.decentralised.social/@wezm/109940341596949214
+[Kashyap]: https://mastodon.social/@kgrz/109942702497796855