There | char protection is fine because this is trying to serialized as a wikilink.

The interwiki matcher has,

{
  "prefix": "stats",
  "local": "",
  "url": "https://stats.wikimedia.org/$1"
},

which the href matches.

Maybe the right thing is to add (v2\/)? to https://github.com/wikimedia/parsoid/blob/master/lib/html2wt/LinkHandler.js#L197
but that doesn't seem very general.

Maybe there needs to be a hardcoded exception for statswiki which doesn't seem to be a wiki.

I don't think this is specific to this interwiki. You get the same problem with e.g. this:

<a href="https://www.google.com/search?q=a|b">x</a>

…which serializes to x instead of [https://www.google.com/search?q=a|b x] (I guess it tries to serialize to [[google:a|b|x]], but that's invalid).

{
  "prefix": "google",
  "url": "https://www.google.com/search?q=$1",
  "protorel": ""
},

is in the interwiki map, so that is also trying to serialize to the wikilink form and then bailing.

We can add another clause to https://github.com/wikimedia/parsoid/blob/master/lib/html2wt/LinkHandler.js#L190-L200
so that it tests for | and falls back to extlink if it's present. Or, percent encode the | since interwiki targeta are just pattern matched into the url where the $1 occurs.

In T199926#4448354, @Arlolra wrote:

Or, percent encode the | since interwiki targeta are just pattern matched into the url where the $1 occurs.

I just tried and this does not actually work! :o Wikitext syntax like [[google:a%7cb]] or [[stats:a%7cb]] does not produce a link. I am guessing that we still validate this "title" against the usual rules? In that case, links with < >, [ ], or other characters invalid in page titles probably cause the same issue as |.

Change 447728 had a related patch set uploaded (by Sbailey; owner: Sbailey):
[mediawiki/services/parsoid@master] html -> wt: Parsoid sometimes trips up on | chars in hrefs

https://gerrit.wikimedia.org/r/447728

I am guessing that we still validate this "title" against the usual rules?

Yeah, it gets passed to Title::newFromText() which applies the valid title regexp, so we should bail more generally on env.isValidLinkTarget

<a href="https://stats.wikimedia.org/v2/#/fr.wikipedia.org/reading/page-views-by-country/normal|map|2-Year~2016060100~2018071100|~total">9</a>
<a href="https://stats.wikimedia.org/v2/#/fr.wikipedia.org/reading/page-views-by-country/normal%7Cmap%7C2-Year~2016060100~2018071100%7C~total">10</a>

What's fun here is that the bar occurs in fragment identifier, so these are two distinct cases. The former should fall back to extlink syntax because the bar would divide the link content in the wrong place. The latter can serialize to the wikilink form, since the it wouldn't be part of the link target.

[[stats:v2/#/fr.wikipedia.org/reading/page-views-by-country/normal%7Cmap%7C2-Year~2016060100~2018071100%7C~total|10]]

Change 447728 merged by jenkins-bot:
[mediawiki/services/parsoid@master] html -> wt: Parsoid sometimes trips up on | chars in hrefs

https://gerrit.wikimedia.org/r/447728

Mentioned in SAL (#wikimedia-operations) [2018-08-20T20:36:52Z] <arlolra> Updated Parsoid to 129d71f (T130224, T199926)

Change 451198 had a related patch set uploaded (by Arlolra; owner: Arlolra):
[mediawiki/services/parsoid@master] Remove unnecessary pattern from interwiki checks

https://gerrit.wikimedia.org/r/451198

Change 451198 merged by jenkins-bot:
[mediawiki/services/parsoid@master] Remove unnecessary pattern from interwiki checks

https://gerrit.wikimedia.org/r/451198

Mentioned in SAL (#wikimedia-operations) [2019-01-08T19:11:39Z] <arlolra> Updated Parsoid to 2c5dc7b (T197616, T205491, T209772, T199926, T209194, T204622)