don't hold references to unowned DispatchData objects (SR-3628)

[weissi]

In DispatchIO.read, the dispatch_data_t object received from dispatch_io_read will be released once the block passed to dispatch_io_read returns.

In the Swift overlay (src/swift/IO.swift) the code marks that data object correctly as "borrowedData"

public func read(offset: off_t, length: Int, queue: DispatchQueue, ioHandler: @escaping (_ done: Bool, _ data: DispatchData?, _ error: Int32) -> Void) {
	dispatch_io_read(self.__wrapped, offset, length, queue.__wrapped) { (done: Bool, data: dispatch_data_t?, error: Int32) in
		ioHandler(done, data.flatMap { DispatchData(borrowedData: $0) }, error)
	}
}

that however leads to use-after-frees if the Swift DispatchData is kept around for longer.

The change I made will retain the underlying dispatch_data_t if it's "borrowed" which I believe is correct and fixes SR-3628.

[dgrove-oss]

borrowedData was added to Data to compensate for memory leaks that I think were related to iteration. I will go back and look at that PR / SR to see how this proposed change interacts with that.

[dgrove-oss]

ok. This change makes sense to me.

When I "fixed" the over-release problem of SR-2656 by introducing borrowedData: in #175 I didn't account for when the __DispatchData that was borrowing the dispatch_data_t outlived the "real" owner. I was too focused on trying to avoid the overhead of an "extra" retain/release operation. But, it really can't be avoided (as shown by the test case for this bug).

[MadCoder]

that owned tag should really be transferOwnership: Bool but the patch LGTM and I should have realized when we reviewed the earlier patch :/

[MadCoder]

@swift-ci please test

[weissi]

build seems healthy to me but then failed because of

Build timed out (after 166 minutes). Marking the build as failed.

[weissi]

@swift-ci please test

[weissi]

@MadCoder are there any special permissions required to '@swift-ci please this'?

[MadCoder]

@dgrove-oss I'm not 100% sure, do we need this for another branch than master? we want that for whatever the next swift 3 update will be.

[dgrove-oss]

We should get it into the swift-3.1-branch. I don't think it is worth putting into the swift-3.0-branch.
@weissi can you easily do the PR to the swift-3.1 branch or would you like me to take care of it?

[weissi]

@dgrove-oss sure, on it!

[weissi]

@dgrove-oss #201