Saved Wiki content goes through the GitalyClient::WikiService, which calls
StringIO#set_encoding on the input stream. The problem is that this call
mutates the encoding of the given string object to ASCII-88BIT, which
causes problems for models expecting the data to still be in UTF-8.
Freezing the input disables this behavior:
https://github.com/ruby/ruby/blob/v2_4_4/ext/stringio/stringio.c#L1583Closes#50590
It is recommended that we set this to 50:
https://gitlab.com/gitlab-org/gitlab-ce/issues/35098#note_35036746
In this particular issue, the confidence was 42 for Shift JIS,
but in fact that's encoded in UTF-8 just with a single bad
character. In this case, we shouldn't try to treat it as Shift JIS,
but just treat it as UTF-8 and remove invalid bytes.
Treating it like Shift JIS would corrupt the whole data.
Unfortunately, the diff which would cause this could not be
disclosed therefore we can't use it as a test example.
