[reddit] implement Reddit Mobile share links

[ghost]

Reddit Mobile app has implemented a new share URL format relatively recently (somewhere around August), which looks like this:

https://www.reddit.com/r/analog/s/hKrTTvFVwZ

This is an obfuscated personalized tracking URL which redirects to a submission:

https://www.reddit.com/r/analog/comments/179exao/photographing_the_recent_annular_eclipse_with_a/?share_id=okPOBW_OzrDClMSiZSLjV&utm_content=1&utm_medium=ios_app&utm_name=ioscss&utm_source=share&utm_term=1

This PR adds a minimal redirect extractor for this URL format.

[mikf]

Nice. Thanks for this.
You should run make to update docs/supportedsites.md for this new extractor.

I wonder if it would somehow be possible to translate a share ID like /s/hKrTTvFVwZ to its submission ID /comments/179exao without a round-trip to Reddit.

[ghost]

I wonder if it would somehow be possible to translate a share ID like /s/hKrTTvFVwZ to its submission ID /comments/179exao without a round-trip to Reddit.

So do I. The share button generates a new URL every time (for instance, for this submission it also generated /r/analog/s/wtyoA8OBdx and /r/analog/s/kKWXLI8d0W), and it doesn't seem like they have anything obvious in common:

base62 decode

>>> for frag in ["hKrTTvFVwZ", "wtyoA8OBdx", "kKWXLI8d0W"]:
...     format(base62.decode(frag), "b")
...
'100000100100001100101110110010111010111111001001001011100011'
'101100010000110101110100111111000101101101001000111011011001'
'100010110100001101111001011011000110001111000111010000111100'

(it should be noted that each share URL seems to be unique to subreddit; if you replace the subreddit with another, it will redirect to that subreddit itself)

So I think probably not.

[ghost]

Update: starting today(?), Reddit seems to 403 these links quite aggressively. Tests still work, so it's probably only relevant to logged-in clients.

$ gallery-dl https://www.reddit.com/r/analog/s/hKrTTvFVwZ
[reddit][error] HttpError: '403 Blocked' for 'https://www.reddit.com/r/analog/s/hKrTTvFVwZ'

[Hrxn]

A 403? Uh, probably just intermittent Reddit server issues..

[ghost]

Nevermind! These mobile share links seem to need browser cookies to work properly now. I set those up and they do work again. So, no further action needed at this time, I guess.

@Hrxn It looks more like an anti-crawl measure, tbh. Reddit also 403s curl requests without cookies for mobile share links now, and I don't think they did at the time this PR was submitted.