format_message unescapes already-escaped input before re-escaping, so that
pre-escaped text doesn't get double-escaped. That unescape was three
sequential str.replace calls, which re-scan each other's output:
"&lt;" --(& -> &)--> "<" --(< -> <)--> "<"
The & produced by the first replace pairs with the following "lt;" and
decodes a second time. "&lt;" is the wire form of the literal text
"<", so the text is silently destroyed: Slack receives "<" and renders
"<". Anyone writing about HTML or markup ("&lt;b&gt;" -> "<b>")
loses their literal text, with no error.
re.sub scans left-to-right and never re-scans its own replacements, so a
single pass fixes it. The escape pass on the next line is left untouched --
it is correctly ordered (& first, so the &s it inserts aren't re-escaped).
Only the double-decode cases change; every other input is byte-identical
before and after. This is the same round-trip invariant the neighbouring
test_pre_escaped_{ampersand,lt,gt}_not_double_escaped tests already assert,
extended to the case they miss. Affects the plain mrkdwn path (send,
edit_message) and Block Kit sections, which route section text through
format_message.
briandevans
8e6d1a9a53