[SR-5761] corelibs-foundation: fix concurrency/fd ownership in MultiHandle.swift

[weissi]

Previous ID	SR-5761
Radar	None
Original Reporter	@weissi
Type	Bug

Additional Detail from JIRA

Votes	0
Component/s	Foundation
Labels	Bug
Assignee	None
Priority	Medium

md5: 7e7084911c5c1527e92b7224cdfe2621

Issue Description:

libcurl requires you to call all call related to one handle on the same thread and MultiHandle.swift violates that in a few places, eg (incomplete list):

• calls CFURLSessionMultiHandleRemoveHandle handles in deinit but we can't know the thread of where that's called

• calls some CFURLSessionMultiHandle* functions on the initial callers thread and others on self.queue

• calls others within commandQueue.async (fixed by https://github.com/apple/swift-corelibs-foundation/pull/1188/files )

On top of that, the ownership of the file descriptor that we get from libcurl in this bit

        try! CFURLSession_multi_setopt_sf(rawHandle, CFURLSessionMultiOptionSOCKETFUNCTION) { (easyHandle: CFURLSessionEasyHandle, socket: CFURLSession_socket_t, what: Int32, userdata: UnsafeMutableRawPointer?, socketptr: UnsafeMutableRawPointer?) -> Int32 in
            guard let handle = URLSession._MultiHandle.from(callbackUserData: userdata) else { fatalError() }
            return handle.register(socket: socket, for: easyHandle, what: what, socketSourcePtr: socketptr)
            }.asError()

isn't correctly maintained. In deinit (which is wrong in itself) we just do this:

        deinit {
            // C.f.: <https://curl.haxx.se/libcurl/c/curl_multi_cleanup.html>
            easyHandles.forEach {
                try! CFURLSessionMultiHandleRemoveHandle(rawHandle, $0.rawHandle).asError()
            }
            try! CFURLSessionMultiHandleDeinit(rawHandle).asError()
        }

which will probably close(2) the file descriptor. That is only legal however if all the DisatchSources have run their cancellation handler.

So for any fd/DispatchSource we have, we'll need something like this

let fd = /* fd from curl */
let fdGroup = DispatchGroup()
fdGroup.notify(queue: curlQueue) {
  CFURLSessionMultiHandleRemoveHandle(...)
}

and for every source we create, we need this

fdGroup.enter()
let src = DispatchSource.makeReadSource(fd: fd, ...)
src.setCancellationHandler {
  fdGroup.leave()
}

that'll make sure that every existing DispatchSource for a given file descriptor has fully deregistered before we allow curl to close it.

I haven't worked out all the details but there's quite a bit of work. I can do this after my holiday, this ticket to remind me.