gh-124452: Fix header mismatches when folding/unfolding with email message (#125919)

The header-folder of the new email API has a long standing known buglet where
if the first token is longer than max_line_length, it puts that token on the next
line.  It turns out there is also a *parsing* bug when parsing such a header:
the space prefixing that first, non-empty line gets preserved and tacked on to
the start of the header value, which is not the expected behavior per the RFCs.
The bug arises from the fact that the parser assumed that there would be at
least one token on the line with the header, which is going to be true for
probably every email producer other than the python email library with its
folding buglet.  Clearly, though, this is a case that needs to be handled
correctly.  The fix is simple: strip the blanks off the start of the whole
value, not just the first physical line of the value.

Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>
Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

1 files changed, 4 insertions, 0 deletions

diff --git a/Misc/NEWS.d/next/Library/2024-10-24-10-49-47.gh-issue-124452.eqTRgx.rst b/Misc/NEWS.d/next/Library/2024-10-24-10-49-47.gh-issue-124452.eqTRgx.rst
new file mode 100644
index 0000000..b0d6379
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2024-10-24-10-49-47.gh-issue-124452.eqTRgx.rst
@@ -0,0 +1,4 @@
+Fix an issue in :meth:`email.policy.EmailPolicy.header_source_parse` and

+:meth:`email.policy.Compat32.header_source_parse` that introduced spurious

+leading whitespaces into header values when the header includes a newline

+character after the header name delimiter (``:``) and before the value.