Revert "parser: Ensure whitespace is stripped for long headers"

This reverts commit 841f966b8d54b2f51ab1c498eed6e5391f2546a9.

In July 2018, we received a report of OzLabs patchwork mangling
emails that have subjects containing words with internal commas,
like "Insert DT binding for foo,bar" (#197).

Stephen took a look and came up with the comment this reverts. Quoting
the commit message:

    RFC2822 states that long headers can be wrapped using CRLF followed by
    WSP [1]. For example:

        Subject: Foo bar,
         baz

    Should be parsed as:

        Foo bar,baz

As it turns out, this is not the case. Journey with me to
section 2.2.3 of RFC 2822:

2.2.3. Long Header Fields

   Each header field is logically a single line of characters comprising
   the field name, the colon, and the field body.  For convenience
   however, and to deal with the 998/78 character limitations per line,
   the field body portion of a header field can be split into a multiple
   line representation; this is called "folding".  The general rule is
   that wherever this standard allows for folding white space (not
   simply WSP characters), a CRLF may be inserted before any WSP.  For
   example, the header field:

           Subject: This is a test

   can be represented as:

           Subject: This
            is a test

So the issue with the example in the reverted commit is that there is no
folding white space in "bar,baz", so it's not valid to split it.

These are valid:

        Subject: Foo bar,baz
        Subject: Foo
	 bar,baz

but splitting "bar,baz" into "bar,\n baz" is not valid.

What then is correct unfolding behaviour? Quoting the RFC again:

   The process of moving from this folded multiple-line representation
   of a header field to its single line representation is called
   "unfolding". Unfolding is accomplished by simply removing any CRLF
   that is immediately followed by WSP.  Each header field should be
   treated in its unfolded form for further syntactic and semantic
   evaluation.

In other words, the unfolding rule requires you to strip the CRLF, but
it does not permit you to strip the WSP. Indeed, if "bar,\n baz" is
received, the correct unfolding is "bar, baz".

If you do strip the WSP, you end up mashing words together, such as in
https://patchwork.ozlabs.org/patch/1097852/

So revert the commit, restoring original behaviour, but keep a corrected
version of the test.

This presents a big question though: how did Rob's email up with a
mangled subject line?

To answer this question, you end up having to learn about OzLabs
Patchwork and how it differs from Patchwork the project.

OzLabs Patchwork (patchwork.ozlabs.org) is an installation of Patchwork.
Part of what makes it so useful for so many projects is a little
intervening layer that can massage some mail to make it end up in the
right project. Email that lands in the device tree project is an example
of email that goes through this process.  I only learned about this
today and I haven't looked in any detail at precisely what is done to
the mail. The script is not part of the Patchwork project.

This intervening filter is a Python script that runs - and this is an
important detail - in Python 2.7.

Ignoring all the details, the filter basically operates in a pipe
between the mail program and patchwork's parsemail, like

   (mail from system) | filter.py | parsemail

At it's very simplest, filter.py acts as follows:

   import email
   import sys
   mail = email.parse_from_file(sys.stdin)
   sys.stdout.write(mail.as_string())

Fascinatingly, if you take Rob's email from #197 and put it through this
process, you can see that it is getting mangled:

Before:
Subject: [PATCH v2 3/4] dt-bindings: sound: wm8994: document wlf,csnaddr-pd property

After:
Subject: [PATCH v2 3/4] dt-bindings: sound: wm8994: document wlf,
 csnaddr-pd property

You can see that python27 has incorrectly wrapped the header, breaking
where there is not a foldable space. Python3 does not have this issue.

To summarise:

- part of the magic of OzLabs PW is a filter to make sure mail gets to
  the right place. This isn't part of the Patchwork project and so is
  usually invisible to patchwork developers.

- the filter is written in python27. The email module in py27 has a bug
  that incorrectly breaks subjects around commas within words.

- patchwork correctly unfolds those broken subjects with a space after
  the comma.

- the extra space was interpreted as a bug in patchwork, leading to a
  misinterpretation of the spec to strip out the whitespace that was
  believed to be in error.

- that broke other wrapped subjects.

To solve this, revert the commit and I'll work with jk to get the filter
script into py3 compatibility. (Given that py27 sunsets in ~7mo, trying
to fix it is not worth it.)

Closes: #273
Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Stephen Finucane <stephen@that.guru>
[stephenfin: Use a new release note instead of editing the original one]

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