Provide more fine-grained connection requirements

[rfc2822]

• Normal sync over mobile Internet → covered by NetworkType.CONNECTED
• Normal sync over WiFi only → covered by NetWorkType.UNMETERED if "Only sync over WiFi" is set
• Normal sync over private WiFi
• Normal sync over public WiFi with login (don't sync until user has logged in)
• Sync with connection in private LAN without validated Internet
• Sync with Internet connection where Android processes are blocked by firewall so they can't validate the connection
• Sync with "always-on" VPN (like Blokada) → currently filtering VPNs, but seems that this isn't a good solution because there are real VPNs without validated Internet
• Sync with real VPN on top of validated Internet
• Shorten version hash of vcard4android library #345
  • Birthdays not synced from Nextcloud 27 to Android 12/13 #385
• If WiFi is used: limit WiFi to specific SSIDs (the SSID filter could also be used even if all connection types are allowed)

Further info:

• mapping of WorkManager → JobScheduler: https://github.com/aosp-mirror/platform_frameworks_support/blob/a9ac247af2afd4115c3eb6d16c05bc92737d6305/work/workmanager/src/main/java/androidx/work/impl/background/systemjob/SystemJobInfoConverter.java#L134

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Because this is starting to overwhelm me, I tried to create a table for the current state (v4.3.6.1):

SCENARIO	WWW	Server reachable	Account Setting	Checking	sync starting
metered or wifi	yes	yes	-	NetworkType.CONNECTED + INTERNET VALIDATED	yes ✅
specific WiFi SSIDs only	yes	yes	Added SSIDs	INTERNET VALIDATED	yes ✅
public WiFi after login	yes	yes		?	yes ✅
public WiFi before login	no	no	-	?	no ✅
WiFi only	yes	yes	Enabled "Only sync over WiFi"	NetworkType.UNMETERED	yes ✅
blocked validation	yes	yes	Disabled validation requirement	INTERNET	yes ✅
VPN	yes	yes	Enabled "VPN Connectivity" (Ignore VPNs)	INTERNET VALIDATED NOT_VPN	yes ✅
VPN	yes	yes	Disabled "VPN Connectivity" (VPN counts)	INTERNET VALIDATED	yes ✅
VPN	no	no	Enabled "VPN Connectivity" (Ignore VPNs)	INTERNET VALIDATED NOT_VPN	no ✅
VPN	no	no	Disabled "VPN Connectivity" (VPN counts)	INTERNET VALIDATED	yes :( ❗ It's needed for the below to work
VPN + blocked validation	yes	yes	Disabled "VPN Connectivity" (VPN counts)	INTERNET VALIDATED	yes ✅ (see https://github.com/bitfireAT/davx5/issues/345)
VPN without internet	no	yes	N/A	N/A	N/A
LAN without internet	no	yes	N/A	N/A	N/A

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We should also consider whether potential settings should be account-specific or app-wide.

For VPN we may need to distinguish between "sync with and without VPN" and "sync only over VPN".

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Android 16 introduced local-network-permission:
https://developer.android.com/about/versions/16/behavior-changes-16#local-network-permission

[nutpantz]

manual sync should be listed and the preferred action regardless of anything else is an attempted sync expected fail or not.
manual sync should not be queued waiting for INTERNET VALIDATED

[rfc2822]

Since WorkManager 2.10, we can now specify a NetworkRequest as a WorkManager constraint. This allows to request specific WiFi SSIDs or network types etc.

[devvv4ever]

We often get inquiries from people that have a NAS that is not available the whole day. They want to limit the sync to a specific time frame to avoid the connection error messages.

[mbnoimi]

I'm really frustrated of not planning to fix "Sync with connection in private LAN without validated Internet" issue.
I run my network over intranet so there is no way to connect to the internet which makes DavX⁵ useless in my case :(

[hoyalu]

Could we add "localhost" as the lowest requirement? Alternatively just "no requirement"/"no validated network" as others have suggested? It makes no sense to me DAVx5 requires an internet connection (or a VPN, connected or not) to sync with a Radicale server on the very same device.

See [https://forum.syncthing.net/t/how-to-sync-contacts-and-calendars-between-desktop-and-android-without-root/21326] for the use case I'm referring to. Since DecSyncCC is seemingly on its way to abandonware, this solution is the only reliable one I know for users who want to avoid a central server. I expect quite a few people to adopt it for this reason.

Of course one could argue that DAVx5 was made for setups with a central server. But why not support even more flexibility without much effort? I can't write a PR, but I can see that an option not to require anything is not overly complex (just skip the check), even if prone to produce more errors. Could be tagged "If you don't know what this option does, you don't need it" or similar. And would make some frustrated DAVx5 users happy again. :-)