NOTMUCH_STATUS_DUPLICATE_MESSAGE_ID incorrectly interpreted as an error

[elizagamedev]

This affects Database::index_file and Database::remove_message, as these are only two functions documented to return this status. According to the docs for notmuch_database_index_file:

If 'message' is not NULL, then, on successful return
(NOTMUCH_STATUS_SUCCESS or NOTMUCH_STATUS_DUPLICATE_MESSAGE_ID) '*message'
will be initialized to a message object that can be used for things
such as adding tags to the just-added message. The user should call
notmuch_message_destroy when done with the message. On any failure
'*message' will be set to NULL.

So not only is it not an error, it's also causing a memory leak, since the successful return value is discarded.

I think that it would be fine to fix this issue for the next patch release by treating NOTMUCH_STATUS_DUPLICATE_MESSAGE_ID as successful unconditionally. For the next major release, we should perhaps be returning a tuple struct or something for these two functions (and maybe more that might have been missed) so that the caller can still receive this potentially useful status.

[vhdirk]

Ah, quite true. Thanks for noticing.