Improve magnet link handling

[akater]

Interactive transmission-add is quite broken for magnet links at the moment.

Arguably, magnet links are almost always provided via kill ring, or are fed into transmission-add non-interactively. Non-interactive use shows no issues. (When I (transmission-add "magnet:?xt=...") from repl, torrent is added properly.)

However, when called interactively, the user is prompted: “Add torrent [...]: ”. Suppose user now removes whatever is written in input field and inserts magnet link from kill ring.

Debugger shows
transmission-add("/path/to/some/dir/magnet:?xt=..." nil)
with some directory (default-directory?) prefixed for no reason.

In addition, the prompt itself depends on a variety of sources including the last killed item in kill ring, for reasons not entirely clear to me but likely not very well thought through. When a magnet link is last killed, it is shown in the prompt while the input field contains a directory path which is extremely confusing. (Is transmission going to start this magnet download into this directory? — Alas, it is not.) Furthermore, on my Emacs (26.0.90), if 'not' is last killed, a 'notes.org' is added to the prompt, which is some extremely far-fetching guess that certainly has nothing to do with anything transmission.

I propose a patch that attempts to guess if magnet-link is provided via one of the transmission-torrent-functions (in fact it might be better to search by means of one hook only, this is debatable). If something resembling magnet link is found, it is (attempted to be) added automatically. The prompt is not modified in this case. The latter might seem silly, as the effect is instantaneous but 1) it just doesn't feel right; 2) modifying prompt only makes sense for filenames while transmission-add's docstring claims to accept much more than filenames or magnet links, so it's likely that more has to yet be done in this regard.

The patch does not fix misguided guessing (see the not / notes.org example above) but does not make it worse either.

[akater]

It might be wiser to merely alter the input mechanism. But I have to admit, the goals of interactive transmission-add in its present form, are obscure to me, and design is baffling at best and flawed at worst. Adding magnet link interactively seems to be impossible at the moment. This pull request tries to make it at least possible while covering what seems to be a majority of use cases, and don't change anything else. I'm not saying it's good but I'd rather offer a working solution than simply open an issue.

[holomorph]

Providing a recipe would be helpful: at the very least an actual magnet link that causes the issue. The only problem I have is with with magnets that aren't hexified, so the slashes are treated as paths. Also see #2. Alternatively, in older Emacsen, the pasted '?' is treated as an attempt at completion, which is troublesome.

As for the design, an abnormal hook is run, the first result--if any--from any function in the hook is used as the default in the prompt. Every result from the abnormal hook is made available to the prompt, so you can cycle through them with M-p and M-n.

[akater]

The only problem I have is with with magnets that aren't hexified, so the slashes are treated as paths.

An example of offending link (no explicit slashes):
magnet:?xt=urn:btih:86cfeead6c4c7f7a89d370c9a2c7235ce5b17657&dn=N+Stephan+Kinsella+-+Against+Intellectual+Property+ebook&tr=udp%3A%2F%2Ftracker.leechers-paradise.org%3A6969&tr=udp%3A%2F%2Fzer0day.ch%3A1337&tr=udp%3A%2F%2Fopen.demonii.com%3A1337&tr=udp%3A%2F%2Ftracker.coppersurfer.tk%3A6969&tr=udp%3A%2F%2Fexodus.desync.com%3A6969

Note: I normally use ido (ido-mode is always on). However, helm-find-files is activated sometimes, in particular during interactive use of transmission-add.

[rien333]

What is the current way you're supposed to add magnets? Calling the LISP function? I've discovered that all TUI interfaces to transmission suck, and this seems pretty decent and has a set of intuitive shortcuts, but giving that basically all mayor torrenting is done with magnets not having decent magnet link support makes this a pain to use.