Xapian provides an interator-based interface to all search results.
So it was natural to make notmuch_messages_t be iterator-based as
well. Which we did originally.
But we ran into a problem when we added two APIs, (_get_replies and
_get_toplevel_messages), that want to return a messages iterator
that's *not* based on a Xapian search result. My original compromise
was to use notmuch_message_list_t as the basis for all returned
messages iterators in the public interface.
This had the problem of introducing extra latency at the beginning
of a search for messages, (the call would block while iterating over
all results from Xapian, converting to a message list).
In this commit, we remove that initial conversion and instead provide
two alternate implementations of notmuch_messages_t (one on top of a
Xapian iterator and one on top of a message list).
With this change, I tested a "notmuch search" returning *many* results
as previously taking about 7 seconds before results started appearing,
and now taking only 2 seconds.
In our scheme it's illegal for any message to refer to itself, (nor
would it be useful for anything anyway). Cut these self-references off
at the source, before they trip up any internal errors.
Previously, the notmuch_messages_t object was a linked list built on
top of a linked-list node with the odd name of notmuch_message_list_t.
Now, we've got much more sane naming with notmuch_message_list_t being
a list built on a linked-list node named notmuch_message_node_t. And
now the public notmuch_messages_t object is a separate iterator based
on notmuch_message_node_t. This means the interfaces for the new
notmuch_message_list_t object are now made available to the library
internals.
The new object is simply a linked-list of notmuch_message_t objects,
(unlike the old object which contained a couple of Xapian iterators).
This works now by the query code immediately iterator over all results
and creating notmuch_message_t objects for them, (rather than waiting
to create the objects until the notmuch_messages_get call as we did
earlier).
The point of this change is to allow other instances of lists of
messages, (such as in notmuch_thread_t), that are not directly related
to Xapian search results.
