We would like to eventually deduplicate blob files.

This supersedes #5495 and #4309. We may be able to revert #5778 afterwards.

Motivation

Especially with Webxdc, there are a lot of duplicate files in the blobs directory, because when you get the same file sent twice then it will be saved twice.

Also, it would be nice to use random filenames because it may happen that the SQL database references a file that doesn't exist anymore, and if the user sends or receives a file with this filename then this new file will accidentally be shown in the place of the removed file.

Prerequisites

• DC Desktop: Don't allow users to edit attachments Attachmentfiles on devices are rw - changes to attachements are not signaled to the user by DeltaChat deltachat-desktop#4352
• Make sure that the actual name of the file isn't used in any UIs, instead dc_msg_get_filename() (C-FFI) or MessageObject.file_name (JsonRPC) needs to be used
• Optional: Make sure that Param::Filename is set to the actual original filename
  • The problem here is that when sending a file, UIs rename the file before passing it to set_file(), and set_file() doesn't have an original_name parameter
  • We could simply ignore the problem because with the current plan (see below) all files will be renamed to a hash anyway, so, in most cases the original name will be free in the blobs dir. Not a super nice solution, but it would work.
  • Implemented in [WIP] File deduplication #6332: Add a function set_file_and_deduplicate(&mut self, path: &str, original_name: &str, mime: Option<&str>) that is similar to set_file() but you can specify the original file name
    • @link2xt says that he thinks that we should do this, and also allow core to rename the file (as opposed to copying it, which set_file() is doing). It should be made to only work on files that already are in the blobs directory, in order to avoid accidentally moving a file that was still needed. Also, it should be allowed to immediately move the file (as opposed to set_file(), which will only rename the file when sending.
    • We need to make sure that things work fine with Preparing messages
      • Either rename the file when sending (i.e. after preparing)
      • Or completely remove Preparing messages. Even on main, when you try to forward Preparing messages, the forwarded message stays Preparing forever, which is another good reason to get rid of them. (Remove prepareMsg (and OUT_PREPARING message state) deltachat-android#3468)

Current plan

TL;DR: Save all files as <hash>, without any extension.

When inserting a file into the blobdir:

• Compute the hash of the file content
  • Use Blake3 for hash because we have blake3 and iroh-blake3 dependencies anyway and iroh devs really like it. It is supposed to be much faster than other cryptographic hashes: https://peergos.org/posts/blake3
• Check if the file <hash> already exists; if yes: use the existing file (and to be safe, check that the content is still correct and overwrite it otherwise). Only if it doesn't exist yet, create it.
• If possible: Set the file permissions to read-only.

Existing files will be kept as they are. Also, the existing set_file() function still won't deduplicate, only the new set_file_and_deduplicate() and when receiving messages.

Alternatives

• Save the hash together with the filename in a SQL table in order to check if the file already exists.
  • Note that right now, guess_msgtype_from_suffix() uses the actual filename on the disk to guess the mime type; this means that we need to be careful if we deduplicate files that have different extensions.
• Save the exact byte size together with the filename in a SQL table in order to check whether the file may already exist. If so, compare the files byte-by-byte.
  • This may be more performant, at least as long as there aren't a lot of files with the exact same size

Open questions

• Is the performance of this solution good enough?
• Should set_file_and_deduplicate() rename the file immediately before returning, asynchronously in the background, or when sending out the message?
  • Right now, it's renaming it immediately ebfore returning