Fix a memory leak in DispatchData.withUnsafeBytes

[ahoppen]

DispatchData.withUnsafeBytes created a new dispatch_data_t by calling dispatch_data_create_map. I assume that the intention was that this memory was freed when data is destroyed, based on the presence of _fixLifetime(data) but data was just a plain dispatch_data_t C struct, that doesn’t have any cleanup operations associated with it when destroyed.

To fix the leak, wrap the dispatch_data_t in a DispatchData, which takes over the ownership of the dispatch_data_t and releases it when data gets destroyed.

Alternatively, _fixLifetime could have been replaced by _swift_dispatch_release(unsafeBitCast(data, to: dispatch_object_t.self)) but I think using DispatchData is the cleaner solution.

---

For future reference, a minimal reproducer for this issue was

import Dispatch
import Foundation

func run() {
  let queue = DispatchQueue(label: "jsonrpc-queue", qos: .userInitiated)

  let receiveIO = DispatchIO(
    type: .stream,
    fileDescriptor: FileHandle.standardInput.fileDescriptor,
    queue: queue,
    cleanupHandler: { _ in }
  )

  receiveIO.setLimit(lowWater: 1)
  receiveIO.setLimit(highWater: Int.max)

  receiveIO.read(offset: 0, length: Int.max, queue: queue) { done, data, errorCode in
    guard let data, !data.isEmpty else {
      return
    }

    data.withUnsafeBytes { (pointer: UnsafePointer<UInt8>) in }
  }
}

try await Task.sleep(for: .seconds(2))
print("start")
run()
try await Task.sleep(for: .seconds(10))

Then run it as

seq 1 100000000 | .build/debug/repro

and attach heaptrack to it during the initial 2s waiting period by running

heaptrack --pid -$(pidof repro)

Heaptrack will list multiple GB worth of leaks without this fix.

[ahoppen]

@swift-ci Please test

[ahoppen]

@swift-ci please test

[ahoppen]

@swift-ci please test

[azharudd]

@swift-ci Please test Linux platform

[shahmishal]

@swift-ci please test

[jessezamora]

Will this be included in 6.1? Or does is need to be cherry picked into the release/6.1 branch now?

[ahoppen]

6.1 has not branched yet, so this will be in Swift 6.1