Re: [RFC2 Patch 5/5] lib: iterator API for message properties

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+From: Daniel Kahn Gillmor <dkg@fifthhorseman.net>\r
+To: David Bremner <david@tethera.net>, notmuch@notmuchmail.org\r
+Subject: Re: [RFC2 Patch 5/5] lib: iterator API for message properties\r
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+On Tue 2016-05-31 21:12:21 -0400, David Bremner <david@tethera.net> wrote:\r
+> I was thinking a bit about how to dump/restore these.\r
+>\r
+> The most upwardly compatible way that i thought of is something like\r
+>\r
+> #= msg-id key=val key=val\r
+>\r
+> i.e. duplicate the msg-id for messages with properties\r
+>\r
+> This would be ignored by old notmuch-restore.\r
+>\r
+> Otherwise, maybe something like\r
+>\r
+> msg-id -- +tag +tag # key=val key=val\r
+>\r
+> I'm not sure. this might crash old notmuch-restore.\r
+>\r
+> How important is backward compatibility, and how important is minimizing\r
+> dump size? It's a bit hard to predict the things people might use\r
+> message properties for, but for thread surgery, I would expect a small\r
+> number of messages with properties.\r
+\r
+The other concern is our conception of how properties are unset/removed,\r
+right?\r
+\r
+With tags, it's possible to include -blah to remove the tag "blah".  how\r
+do we remove/clear/overwrite these tags?  what about using +key=val or\r
+-key=val to set/unset certain key/value combinations, and a value-less\r
+key= to remove all values matching a given key?\r
+\r
+alternately:\r
+\r
+ key=val (clears all values for "key", and sets a new value "val")\r
+ key+=val (appends a value "val" for "key")\r
+ key-=val (removes any "key" set to "val")\r
+ key= (clears all values for "key"\r
+\r
+---------\r
+\r
+However we resolve this particular decision, it'd be nice to have a\r
+stable, sane story about backward compatibility going forward, so that\r
+we don't have to worry about it in the future.\r
+\r
+For example, each dump file could start with a line like:\r
+\r
+  #version 1\r
+\r
+and notmuch restore would assume that without "#version n" as the first\r
+line, it's version 0.  then notmuch restore could decline to parse dump\r
+files of a version that it doesn't know about.\r
+\r
+Alternately, we could have the first line be something like:\r
+\r
+   #features config properties\r
+\r
+and if the first line is not #features, then we assume that no features\r
+are in place -- but if restore sees features it doesn't know about, it\r
+can offer to proceed while warning the user that we might miss something\r
+(or that something might break).\r
+\r
+\r
+Thanks for working on this, David!  I think this is going to be really\r
+useful!\r
+\r
+    --dkg\r