[fuzz result] [commonmark-pandoc] footnotes in footnotes

[notriddle]

This is loosely related to jgm/pandoc#2053, but commonmark_x is different from pandoc flavored markdown. I'm not really sure which repository to report this against, but since commonmark-pandoc lives here, this is where I'm reporting it.

When I run this session, I get a result that's definitely wrong. This result came from a custom build of pandoc, but pandoc.org/try shows the same thing.

$ pandoc -f commonmark_x -t html5
[^foo]:
    bar [^bar]

[^foo]

[^bar]: baz
^D
<p><a href="#fn1" class="footnote-ref" id="fnref1"
role="doc-noteref"><sup>1</sup></a></p>
<section id="footnotes" class="footnotes footnotes-end-of-document"
role="doc-endnotes">
<hr />
<ol>
<li id="fn1"><p>bar <a href="#fn1" class="footnote-ref" id="fnref1"
role="doc-noteref"><sup>1</sup></a><a href="#fnref1"
class="footnote-back" role="doc-backlink">↩︎</a></p></li>
</ol>
</section>

However, commonmark-cli gives a correct result (in the sense that github and commonmark-cli are in alignment):

$ stack run -- --extension=footnotes
[^foo]:
    bar [^bar]

[^foo]

[^bar]: baz
^D
<p><sup class="footnote-ref"><a href="#fn-foo" id="fnref-foo">1</a></sup></p>
<section class="footnotes">
<div class="footnote" id="fn-foo">
<div class="footnote-number">
<a href="#fnref-foo">1</a>
</div>
<div class="footnote-contents">
<p>bar <sup class="footnote-ref"><a href="#fn-bar" id="fnref-bar">2</a></sup></p>
</div>
</div>
<div class="footnote" id="fn-bar">
<div class="footnote-number">
<a href="#fnref-bar">2</a>
</div>
<div class="footnote-contents">
<p>baz</p>
</div>
</div>
</section>

I would expect Pandoc to do one of these things:

1. Throw an error message and refuse to output anything.
2. Treat the nested footnote as plain text, just like pandoc -f markdown does.
3. Work the same way commonmark-cli and github do. I understand that Pandoc has no interest in supporting this, since it's bad typography and unsupported by some of Pandoc's output formats, so the first two options would make more sense.

[jgm]

pandoc -f commonmark_x -t native gives:

[ Para
    [ Note
        [ Para [ Str "bar" , Space , Note [ Para [ Str "baz" ] ] ] ]
    ]
]

which seems right. It's pandoc's HTML writer that is unable to handle this gracefully. So, we could address this there, or we could modify commonmark-pandoc so that it detects nested notes and emits

[ Para
    [ Note
        [ Para [ Str "bar" , Space , Str "[^bar]" ] ]
    ]
]

Or both, of course. One reason to address it in pandoc's HTML writer is that other readers may allow the nested notes to be created.

[notriddle]

First problem: you'd have to fix a lot of writers.

$ pandoc -f commonmark+footnotes -t commonmark+footnotes
[^1]

[^1]: bar [^2]
$ pandoc -f commonmark+footnotes -t latex
\footnote{bar \footnotemark{}}
$ pandoc -f commonmark+footnotes -t rst
 [1]_

.. [1]
   bar  [2]_

Second problem: Pandoc duplicates the footnote when I refer to it more than once. This isn't a problem in Pandoc-flavored Markdown, but with recursive footnotes, you probably want a recursion limit.

$ cat checker.py 
print("[^lol1]")
print("[^lol8]: lol")
for a in range(1, 8):
    print(f"[^lol{a}]:")
    for b in range(10):
        print(f"    [^lol{(a + 1)}]")
$ python checker.py > test-in.md && du -h test-in.md
12K	test-in.md
$ cgmemtime -- pandoc -f commonmark+footnotes < test-in.md > test-out.md

user: 167.129 s
sys:  003.155 s
wall: 170.746 s
child_RSS_high:    6619136 KiB
group_mem_high:    6540472 KiB

[jgm]

Pushed a fix that gives

[^foo]:
    bar [^bar]

[^foo]

[^bar]: baz
^D
[ Para [ Note [ Para [ Str "bar" , Space , Str "" ] ] ] ]

I think that's good enough.