Better error type for inactive thread pools (#1787)

* Better error type for inactive thread pools

* Soundness

* Typos

* PR comments

* Defer ELG shutdown

* Remove redundant try

Sources/NIO/NIOThreadPool.swift

@@ -15,6 +15,13 @@
 import Dispatch
 import NIOConcurrencyHelpers
 
+/// Errors that may be thrown when executing work on a `NIOThreadPool`
+public enum NIOThreadPoolError {
+    
+    /// The `NIOThreadPool` was not active.
+    public struct ThreadPoolInactive: Error { }
+}
+
 
 /// A thread pool that should be used if some (kernel thread) blocking work
 /// needs to be performed for which no non-blocking API exists.
@@ -206,6 +213,7 @@ public final class NIOThreadPool {
 }
 
 extension NIOThreadPool {
+    
     /// Runs the submitted closure if the thread pool is still active, otherwise fails the promise.
     /// The closure will be run on the thread pool so can do blocking work.
     ///
@@ -217,7 +225,7 @@ extension NIOThreadPool {
         let promise = eventLoop.makePromise(of: T.self)
         self.submit { shouldRun in
             guard case shouldRun = NIOThreadPool.WorkItemState.active else {
-                promise.fail(ChannelError.ioOnClosedChannel)
+                promise.fail(NIOThreadPoolError.ThreadPoolInactive())
                 return
             }
             do {

Tests/NIOTests/NonBlockingFileIOTest+XCTest.swift

@@ -71,6 +71,7 @@ extension NonBlockingFileIOTest {
                 ("testReadFromEOFDeliversExactlyOneChunk", testReadFromEOFDeliversExactlyOneChunk),
                 ("testReadChunkedFromOffsetFileRegion", testReadChunkedFromOffsetFileRegion),
                 ("testReadManyChunks", testReadManyChunks),
+                ("testThrowsErrorOnUnstartedPool", testThrowsErrorOnUnstartedPool),
            ]
    }
 }

Tests/NIOTests/NonBlockingFileIOTest.swift

@@ -138,7 +138,7 @@ class NonBlockingFileIOTest: XCTestCase {
                                          byteCount: 1,
                                          allocator: self.allocator,
                                          eventLoop: self.eventLoop).wait()) { error in
-                XCTAssertEqual(.ioOnClosedChannel, error as? ChannelError)
+                XCTAssertTrue(error is NIOThreadPoolError.ThreadPoolInactive)
             }
             return [readFH, writeFH]
         }
@@ -915,4 +915,23 @@ class NonBlockingFileIOTest: XCTestCase {
             XCTAssertEqual(numberOfChunks, numberOfCalls.load())
         })
     }
+    
+    func testThrowsErrorOnUnstartedPool() throws {
+        withTemporaryFile(content: "hello, world") { fileHandle, path in
+            let eventLoopGroup = MultiThreadedEventLoopGroup(numberOfThreads: 1)
+            defer {
+                XCTAssertNoThrow(try eventLoopGroup.syncShutdownGracefully())
+            }
+            
+            let expectation = XCTestExpectation(description: "Opened file")
+            let threadPool = NIOThreadPool(numberOfThreads: 1)
+            let fileIO = NonBlockingFileIO(threadPool: threadPool)
+            fileIO.openFile(path: path, eventLoop: eventLoopGroup.next()).whenFailure { (error) in
+                XCTAssertTrue(error is NIOThreadPoolError.ThreadPoolInactive)
+                expectation.fulfill()
+            }
+            
+            self.wait(for: [expectation], timeout: 1.0)
+        }
+    }
 }