Dart sockets don't respect iOS's VPN

[gaaclarke]

Originally filed for Flutter: flutter/flutter#41500

The issue reports that using Dart to access resources over VPN doesn't work.

I looked through Dart's source code and it appears to be using posix sockets. Apple's documentation recommends against that: In iOS, POSIX networking is discouraged because it does not activate the cellular radio or on-demand VPN. Thus, as a general rule, you should separate the networking code from any common data processing functionality and rewrite the networking code using higher-level APIs.

source: https://developer.apple.com/library/archive/documentation/NetworkingInternet/Conceptual/NetworkingTopics/Articles/UsingSocketsandSocketStreams.html

We should implement a socket implementation based on CFSocket.

[gaaclarke]

cc @zichangg

[zichangg]

I think this should be added.
But I'm not sure whether VM is able to distinguish ios from mac. We only have a mac implementation so far. @a-siva @rmacnak-google

[a-siva]

We do have HOST_OS_IOS defined when TARGET_OS_IPHONE is defined, see runtime/platform/globals.h

[rmacnak-google]

We do distinguish between iOS and Mac, it's just that the implementations are so similar we put them in the same files (unlike how we have separated files for Android and Linux) that have a small number of ifdefs.

[gaaclarke]

@zichangg The CoreNetworking framework exists for iOS and macosx. If you change iOS and macosx to use it, it will be easier on you since you'll be able to test it locally instead of having to test on the simulator / device all the time.

[dnfield]

CFSocket should be available on both iOS and macOS.

An alternative to this is #39104, but this is likely easier to implement and should not require any breakages in the API surface.

[zichangg]

Right. That should be easier for testing.

If we move to CFSocket, it will be another set of implementation which won't use our kqueue-based eventhandler. It is basically rewriting whole socket with higher level CFSocket and CFSocketCreateRunLoopSource .

[zichangg]

Looked at some docs and code examples. CFSocket can be embedded into our eventhandler system.
Unfortunately, this link explicitly says

In iOS, using sockets directly using POSIX functions or CFSocket does not automatically activate the device’s cellular modem or on-demand VPN.

What @dnfield proposed might be a good choice.

Since the only problem is activation of modem and VPN. Is it possible to turn them on programmatically for this case?