SR-3002: DispatchData.enumerateBytes on Linux leaks the block

To avoid leaking the block parameter of enumerateBytes,
we need to directly call CDispatch.dispatch_data_apply
(not _swift_dispatch_data_apply). However, we need to tell
the compiler that the internal closure that translates between
the C and Swift layers will not cause the captured block
parameter to escape.  If SR-2313 were implemented, we could use
withoutActuallyEscaping.  Since withoutActuallyEscaping is not available,
we instead use unsafeBitCast to bypass the compiler's analysis.

Signed-off-by: Daniel A. Steffen <dsteffen@apple.com>

Author: dgrove-oss

src/swift/Data.swift

@@ -91,11 +91,13 @@ public struct DispatchData : RandomAccessCollection {
 	public func enumerateBytes(
 		block: @noescape (_ buffer: UnsafeBufferPointer<UInt8>, _ byteIndex: Int, _ stop: inout Bool) -> Void)
 	{
-		_swift_dispatch_data_apply(__wrapped.__wrapped) { (_, offset: Int, ptr: UnsafeRawPointer, size: Int) in
+		// FIXME: When SR-2313 (withoutActuallyEscaping) is implemented, use it to replace unsafeBitCast
+		let nonEscapingBlock = unsafeBitCast(block, to: _enumerateBytesBlock.self)
+		_ = CDispatch.dispatch_data_apply(__wrapped.__wrapped) { (_, offset: Int, ptr: UnsafeRawPointer, size: Int) in
 			let bytePtr = ptr.bindMemory(to: UInt8.self, capacity: size)
 			let bp = UnsafeBufferPointer(start: bytePtr, count: size)
 			var stop = false
-			block(bp, offset, &stop)
+			nonEscapingBlock(bp, offset, &stop)
 			return !stop
 		}
 	}
@@ -274,10 +276,7 @@ public struct DispatchDataIterator : IteratorProtocol, Sequence {
 	internal var _position: DispatchData.Index
 }
 
-typealias _swift_data_applier = @convention(block) (dispatch_data_t, Int, UnsafeRawPointer, Int) -> Bool
-
-@_silgen_name("_swift_dispatch_data_apply")
-internal func _swift_dispatch_data_apply(_ data: dispatch_data_t, _ block: _swift_data_applier)
+typealias _enumerateBytesBlock = (_ buffer: UnsafeBufferPointer<UInt8>, _ byteIndex: Int, _ stop: inout Bool) -> Void
 
 @_silgen_name("_swift_dispatch_data_empty")
 internal func _swift_dispatch_data_empty() -> dispatch_data_t

src/swift/DispatchStubs.cc

@@ -141,12 +141,6 @@ _swift_dispatch_block_testcancel(dispatch_block_t block) {
   return dispatch_block_testcancel(block);
 }
 
-SWIFT_CC(swift) DISPATCH_RUNTIME_STDLIB_INTERFACE
-extern "C" bool
-_swift_dispatch_data_apply(dispatch_data_t data, bool (^applier)(dispatch_data_t, size_t, const void *, size_t)) {
-  return dispatch_data_apply(data, applier);
-}
-
 SWIFT_CC(swift) DISPATCH_RUNTIME_STDLIB_INTERFACE
 extern "C" void
 _swift_dispatch_async(dispatch_queue_t queue, dispatch_block_t block) {