Alex Vandiver fe25517295 email_notifications: Handle empty rendered_messages. ...

The transforms called from `build_message_payload` use
`lxml.html.fromstring` to parse (and stringify, and re-parse) the HTML
generated by Markdown.  However, this function fails if it is passed
an empty document.  "empty" is broader than just the empty string; it
also includes any document made entirely out of control characters,
spaces, unpaired surrogates, U+FFFE, or U+FFFF, and so forth.  These
documents would fail to parse, and raise a ParserError.

Using `lxml.html.fragment_fromstring` handles these cases, but does by
wrapping the contents in a <div> every time it is called.  As such,
replacing each `fromstring` with `fragment_fromstring` would nest
another layer of `<div>`.

Instead of each of the helper functions re-parsing, modifying, and
stringifying the HTML, parse it once with `fragment_fromstring` and
pass around the parsed document to each helper, which modifies it
in-place.  This adds one outer `<div>`, requiring minor changes to
tests and the prepend-sender functions.

The modification to add the sender is left using BeautifulSoup, as
that sort of transform is much less readable, and more fiddly, in raw
lxml.

Partial fix for #19559.

2021-08-25 15:50:55 -07:00

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