From 03c5f7dd2259ed8dfc9f008e0391088863c094cd Mon Sep 17 00:00:00 2001
From: Ryan Lovelett <ryan@lovelett.me>
Date: Wed, 7 Feb 2018 21:57:43 -0500
Subject: [PATCH] Create a scaffold implementation of
 FileHandle#waitForDataInBackgroundAndNotify

This change alone is still insufficient. It needs a few details to be
addressed.

1. From the docs it says the function must be called from a thread that
has an active run loop. How does one go about checking if a thread has
an active run-loop? Would that be an assert or precondition or something
else entirely?

2. From the docs it says the function will post the
NSFileHandleDataAvailable notification on the current thread. Presumably
that means the thread that was used to call
waitForDataInBackgroundAndNotify in the first place. How do you specify
the thread when posting a notification?

3. Tests. Are there any suggestions for tests?
---
 Foundation/FileHandle.swift         | 15 ++++++++++++++-
 TestFoundation/TestFileHandle.swift | 18 ++++++++++++++++++
 2 files changed, 32 insertions(+), 1 deletion(-)

diff --git a/Foundation/FileHandle.swift b/Foundation/FileHandle.swift
index 2f31d44e72..663c13fb43 100644
--- a/Foundation/FileHandle.swift
+++ b/Foundation/FileHandle.swift
@@ -8,6 +8,7 @@
 //
 
 import CoreFoundation
+import Dispatch
 
 #if os(OSX) || os(iOS)
 import Darwin
@@ -345,7 +346,19 @@ extension FileHandle {
     }
 
     open func waitForDataInBackgroundAndNotify() {
-        NSUnimplemented()
+        let global = DispatchQueue.global(qos: .background)
+        let channel = DispatchIO(type: .stream, fileDescriptor: fileDescriptor, queue: global) { _ in }
+
+        // The desire is to trigger the handler blocka as soon as there is any data in the channel.
+        channel.setLimit(lowWater: 1)
+
+        channel.read(offset: 0, length: Int.max, queue: global) { (_, _, error) in
+            var notification = Notification(name: .NSFileHandleDataAvailable, object: nil, userInfo: nil)
+            if error != 0 {
+                notification.userInfo = ["NSFileHandleError": NSNumber(value: error)]
+            }
+            NotificationCenter.default.post(notification)
+        }
     }
     
     open var readabilityHandler: ((FileHandle) -> Void)? {
diff --git a/TestFoundation/TestFileHandle.swift b/TestFoundation/TestFileHandle.swift
index 1b389ff0f7..8776a38145 100644
--- a/TestFoundation/TestFileHandle.swift
+++ b/TestFoundation/TestFileHandle.swift
@@ -21,6 +21,7 @@ class TestFileHandle : XCTestCase {
             ("test_constants", test_constants),
             ("test_pipe", test_pipe),
             ("test_nullDevice", test_nullDevice),
+            ("test_waitForDataInBackgroundAndNotify", test_waitForDataInBackgroundAndNotify),
         ]
     }
 
@@ -64,4 +65,21 @@ class TestFileHandle : XCTestCase {
         fh.seek(toFileOffset: 0)
         XCTAssertEqual(fh.readDataToEndOfFile().count, 0)
     }
+
+    func test_waitForDataInBackgroundAndNotify() {
+        let expect = expectation(description: "Receiving asynchronous data from pipe")
+        let pipe = Pipe()
+        pipe.fileHandleForReading.waitForDataInBackgroundAndNotify()
+
+        NotificationCenter.default.addObserver(forName: .NSFileHandleDataAvailable, object: nil, queue: nil) { (notification) in
+            XCTAssertEqual(notification.name, .NSFileHandleDataAvailable)
+            XCTAssertNil(notification.userInfo)
+            expect.fulfill()
+        }
+
+        let data = Data(bytes: [0x48, 0x65, 0x6c, 0x6c, 0x6f, 0x2c, 0x20, 0x57, 0x6f, 0x72, 0x6c, 0x64, 0x21])
+        pipe.fileHandleForWriting.write(data)
+
+        waitForExpectations(timeout: 2)
+    }
 }