Extend `MvboxMove` meaning (and maybe rename `Config::OnlyFetchMvbox` to `OnlyScanSpam` later)

[iequidoo]

SentboxWatch is to be removed: #7178 . Then if we look at imap::should_ignore_folder(), OnlyFetchMvbox actually keeps scanning "Spam" (so the current naming isn't quite correct). Mvbox ("DeltaChat") is watched anyway if it exists (e.g. for chatmail it doesn't).

MvboxMove ("Move automatically to DeltaChat folder") makes no sense with OnlyFetchMvbox enabled: useful messages from "Spam" are moved anyway to "DeltaChat" (see imap::spam_target_folder_cfg()) and Inbox messages aren't fetched and moved. Then with OnlyScanSpam, MvboxMove could mean "Move Inbox and all scanned folders' messages to DeltaChat", so if it's enabled, Inbox is watched too, so the user can configure fetching only from "Inbox", "DeltaChat" and "Spam" which is unachievable currently and helps e.g. for the Gmail case which has many virtual folders and the whole folder scanning process is really slow (and useless for most users): #7178 (comment). This may also be useful if the user has another messenger, e.g. a Delta Chat fork who moves messages to "ZetaChat": this folder shouldn't be scanned, but Inbox should be. Mvbox is watched anyway, this should just be made clear to users (if not already).

For existing users accidentally having both options enabled "MvboxMove" should be disabled in a migration to not cause watching Inbox. EDIT: This isn't possible at all in the UIs currently, so the migration isn't needed. Also implementation in Core doesn't break any UI, so the UIs may be tweaked later.

[link2xt]

and helps e.g. for the Gmail case which has many virtual folders and the whole folder scanning process is really slow

Virtual folders should not be scanned, messages from virtual folders should never be fetched:

core/src/imap.rs

Lines 151 to 157 in 1cc7ce6

	/// Virtual folders.
	///
	/// On Gmail there are virtual folders marked as \\All, \\Important and \\Flagged.
	/// Delta Chat ignores these folders because the same messages can be fetched
	/// from the real folder and the result of moving and deleting messages via
	/// virtual folder is unclear.
	Virtual,

If on Gmail messages are fetched from the virtual Sent folder, then it's a bug that needs its own issue and a fix preventing this unconditionally without any action from the user. There should be no configuration in which virtual folders are even selected.

[link2xt]

For existing users accidentally having both options enabled "MvboxMove" should be disabled in a migration to not cause watching Inbox. EDIT: This isn't possible at all in the UIs currently, so the migration isn't needed. Also implementation in Core doesn't break any UI, so the UIs may be tweaked later.

What is not possible in the UIs? I tried and I can enable both "Move automatically to DeltaChat Folder" and "Only Fetch from DeltaChat Folder" on Android at the same time.

[iequidoo]

True, virtual folders shouldn't be even selected, but they are scanned (i see this for my Gmail account). I coudn't understand yet why, maybe virtual folders are not marked as NameAttribute::Flagged, in this case it may be an imap_proto bug. But this issue overall is about allowing a configuration in which only Inbox, Mvbox and "Spam" are scanned, skipping all unknown folders that may be created by other software.

What is not possible in the UIs? I tried and I can enable both "Move automatically to DeltaChat Folder" and "Only Fetch from DeltaChat Folder" on Android at the same time.

Then this may be a DC Android bug, in Desktop this isn't possible. Anyway, as far as i can see from the code, this configuration is not meaningful currently, i.e. MvboxMove does nothing.

[link2xt]

Renaming Config enum keys is not an option, this will require migration and result in a different configuration on downgrade or if backup is imported from a newer version to older one. Also UI code changes will be needed and any bots that toggled this option will need updates. This creates a lot of problems with no gain. Also creates confusion, even makes it more difficult to understand the issue description now because you refer to OnlyFetchMvbox as OnlyScanSpam.

So currently we have 4 configurations and at least Android allows all of them:

1. OnlyFetchMvbox is false, MvboxMove is false - default for chatmail
2. OnlyFetchMvbox is false, MvboxMove is true - default for non-chatmail
3. OnlyFetchMvbox is true, MvboxMove is false - option that was added for a very special case reported on the forum and implemented in Flub/watch mvbox only #3028, see also Add OnlyFetchMvbox option #3014
4. OnlyFetchMvbox is true, MvboxMove is true - meaningless configuration. Should work as OnlyFetchMvbox = false and MvboxMove = true. Desktop UI shows it this way, Android does not, but you can configure it this way in both UIs.

Desktop allows to configure the meaningless variant, it just displays it differently visually. If you turn OnlyFetchMvbox off, true value of MvboxMove is displayed and it is saved while OnlyFetchMvbox is true. Changing how this configuration variant works will result in the behavior changing for users that have it configured this way while currently it works the same as configuration variant 3.

I wish we did not introduce OnlyFetchMvbox at a single user request in the first place.

As far as I understand the actual reason for this issue is that you have a lot of folders on Gmail, scanning all folders takes a long time and you want to skip scanning all folders and only scan inbox, mvbox and spam. If so, then:

1. Fixing skipping of virtual folders might actually solve the issue.
2. If not, then I'd consider removing scanning of all folders other then spam altogether. Scanning of all folders was introduced for the reason described in Improve onboarding by scanning all folders from time to time #1994 and the reason "Some providers also have special "priority" folders by default." for scanning non-spam folders is very weak, may be even theoretical. Likely was a mistake to introduce scanning of all folders at that time as well.

[iequidoo]

2. If not, then I'd consider removing scanning of all folders other then spam altogether.

I'd vote for this change then. If scanning of all folders is impossible to disable (what i suggest in this issue; renaming OnlyFetchMvbox isn't that important and can't be done easily anyway), it's better to be removed at all. Then the issue with detection of virtual folders disappears as well.

[link2xt]

Replacing it with #7190