Basically, say you had already downloaded a certain image from a certain URL
using pull_hotlinked_images and the onebox. The upload would be stored
by its sha as an upload record. Whenever you linked to the same URL again
in a post (e.g. in our case an og:image on review.discourse) we would
would reuse the original upload record because of the sha1.
However when you turned on secure media this could cause problems as
the first post that uses that upload after secure media is enabled
will set the access control post for the upload to the new post.
Then if the post is deleted every single onebox/link to that same image
URL will fail forever with 403 as the secure-media-uploads URL fails
if the access control post has been deleted.
To fix this when cooking posts and pulling hotlinked images, we only
allow using an original upload by URL if its access control post
matches the current post, and if the original_sha1 is filled in,
meaning it was uploaded AFTER secure media was enabled. otherwise
we just redownload the media again to be safe, as the URL will always
be new then.
### General Changes and Duplication
* We now consider a post `with_secure_media?` if it is in a read-restricted category.
* When uploading we now set an upload's secure status straight away.
* When uploading if `SiteSetting.secure_media` is enabled, we do not check to see if the upload already exists using the `sha1` digest of the upload. The `sha1` column of the upload is filled with a `SecureRandom.hex(20)` value which is the same length as `Upload::SHA1_LENGTH`. The `original_sha1` column is filled with the _real_ sha1 digest of the file.
* Whether an upload `should_be_secure?` is now determined by whether the `access_control_post` is `with_secure_media?` (if there is no access control post then we leave the secure status as is).
* When serializing the upload, we now cook the URL if the upload is secure. This is so it shows up correctly in the composer preview, because we set secure status on upload.
### Viewing Secure Media
* The secure-media-upload URL will take the post that the upload is attached to into account via `Guardian.can_see?` for access permissions
* If there is no `access_control_post` then we just deliver the media. This should be a rare occurrance and shouldn't cause issues as the `access_control_post` is set when `link_post_uploads` is called via `CookedPostProcessor`
### Removed
We no longer do any of these because we do not reuse uploads by sha1 if secure media is enabled.
* We no longer have a way to prevent cross-posting of a secure upload from a private context to a public context.
* We no longer have to set `secure: false` for uploads when uploading for a theme component.
This change both speeds up specs (less strings to allocate) and helps catch
cases where methods in Discourse are mutating inputs.
Overall we will be migrating everything to use #frozen_string_literal: true
it will take a while, but this is the first and safest move in this direction
This reduces the number of scans that the db has to do in the query
to fetch orphan uploads. Futheremore, we were not batching our
records which bloats memory.
