mda|learn|watch: support dropUniqueUnsubscribe config

List-Unsubscribe headers with unique identifiers (such as those
generated by our examples/unsubscribe.milter) should not
end up in public archives.  Add a new config knob to strip
List-Unsubscribe headers if they have the
`List-Unsubscribe-Post: List-Unsubscribe=One-Click'
header.

Unfortunately, this breaks DKIM signatures if the signature
covers either of these List-Unsubscribe* headers.  However,
breaking DKIM is the lesser evil compared to any archive reader
being able to stop archival by an independent archivist.

As much as I would like this to be the default, it probably
affects few users at the moment since very few mailing lists
use unique identifiers in List-Unsubscribe (but that number
has grown, recently).

4 files changed, 58 insertions, 2 deletions

diff --git a/Documentation/public-inbox-config.pod b/Documentation/public-inbox-config.pod
index 871ac6c5..1ef7f46f 100644
--- a/Documentation/public-inbox-config.pod
+++ b/Documentation/public-inbox-config.pod
@@ -196,6 +196,23 @@ and the path may be "/dev/null" or any empty file.
 Multiple files may be specified and will be included in the
 order specified.
 
+=item publicinboxImport.dropUniqueUnsubscribe
+
+Drop C<List-Unsubscribe> headers if the message also includes
+the C<List-Unsubscribe-Post: List-Unsubscribe=One-Click> header
+to signal MUAs to support an instantaneous unsubscribe.  This
+is strongly recommended for users creating their own public
+archives of mailing lists they subscribe to, otherwise any
+archive reader can unsubscribe the archivist.
+
+This may break DKIM signatures if the C<List-Unsubscribe*>
+headers are signed, but breaking DKIM signatures is the
+lesser evil compared to allowing any reader to unsubscribe
+the archivist.
+
+This affects L<public-inbox-mda(1)>, L<public-inbox-watch(1)>,
+and L<public-inbox-learn(1)>
+
 =item publicinboxmda.spamcheck
 
 This may be set to C<none> to disable the use of SpamAssassin
diff --git a/Documentation/public-inbox-learn.pod b/Documentation/public-inbox-learn.pod
index f776df6b..b08e4bc8 100644
--- a/Documentation/public-inbox-learn.pod
+++ b/Documentation/public-inbox-learn.pod
@@ -73,6 +73,25 @@ Default: ~/.public-inbox/config
 
 =back
 
+=head1 CONFIGURATION
+
+These configuration knobs should be used in the
+L<public-inbox-config(5)> file.
+
+=over 8
+
+=item publicinboxImport.dropUniqueUnsubscribe
+
+=item publicinbox.<name>.address
+
+=item publicinbox.<name>.listid
+
+=item publicinboxmda.spamcheck
+
+See L<public-inbox-config(5)> for descriptions of these options
+
+=back
+
 =head1 CONTACT
 
 Feedback welcome via plain-text mail to L<mailto:meta@public-inbox.org>
diff --git a/Documentation/public-inbox-mda.pod b/Documentation/public-inbox-mda.pod
index 93cb0e9c..edc90287 100644
--- a/Documentation/public-inbox-mda.pod
+++ b/Documentation/public-inbox-mda.pod
@@ -68,6 +68,22 @@ Default: ~/.public-inbox/emergency/
 
 =back
 
+=head1 CONFIGURATION
+
+Various configuration knobs should be used in the
+L<public-inbox-config(5)> file.
+
+=over 8
+
+=item publicinboxImport.dropUniqueUnsubscribe
+
+=item publicinbox.<name>.address
+
+=item publicinbox.<name>.listid
+
+See L<public-inbox-config(5)> for descriptions of these options
+
+=back
 
 =head1 CONTACT
 
@@ -78,7 +94,7 @@ L<http://4uok3hntl7oi7b4uf4rtfwefqeexfzil2w6kgk2jn5z2f764irre7byd.onion/meta/>
 
 =head1 COPYRIGHT
 
-Copyright 2013-2021 all contributors L<mailto:meta@public-inbox.org>
+Copyright all contributors L<mailto:meta@public-inbox.org>
 
 License: AGPL-3.0+ L<https://www.gnu.org/licenses/agpl-3.0.txt>
 
diff --git a/Documentation/public-inbox-watch.pod b/Documentation/public-inbox-watch.pod
index febda0b1..7c21f7ce 100644
--- a/Documentation/public-inbox-watch.pod
+++ b/Documentation/public-inbox-watch.pod
@@ -66,6 +66,10 @@ L<public-inbox-config(5)> file.
 
 =over 8
 
+=item publicinboxImport.dropUniqueUnsubscribe
+
+See L<public-inbox-config(5)/publicinboxImport.dropUniqueUnsubscribe>
+
 =item publicinbox.<name>.watch
 
 A location to watch.  public-inbox 1.5.0 and earlier only supported
@@ -201,7 +205,7 @@ L<http://4uok3hntl7oi7b4uf4rtfwefqeexfzil2w6kgk2jn5z2f764irre7byd.onion/meta/>
 
 =head1 COPYRIGHT
 
-Copyright 2016-2021 all contributors L<mailto:meta@public-inbox.org>
+Copyright all contributors L<mailto:meta@public-inbox.org>
 
 License: AGPL-3.0+ L<https://www.gnu.org/licenses/agpl-3.0.txt>