Remove not on eventLoop precondition for NIOPipeBootstrap (#1977)

Motivation:

Currently you cannot create a NIOPipeBootstrap when you are on
an event loop due to a precondition check. This check seeks to prevent
consumers of the API from passing in a file descriptor thats referencing
a file on disk or on the network.

The method at hand 'validateFileDescriptorIsNotAFile' uses fstat, which potentially
could block the event loop especially if the fd is referencing a file over the network.
This check however prevents certain use cases of SwiftNIO, where the downsides
of a consumer of this api blocking their own event loop do not weigh in against preventing
an entire use case from SwiftNIO.

Modifications:

Removed the precondition

Result:

After this change it will be possible to feed file descriptors into
the bootstrap which potentially block the current event loop.
A potential (portable) replacement for fstat still has to be found in order to solve this problem completely.

Sources/NIOPosix/Bootstrap.swift

@@ -1023,8 +1023,6 @@ public final class NIOPipeBootstrap {
     }
 
     private func validateFileDescriptorIsNotAFile(_ descriptor: CInt) throws {
-        precondition(MultiThreadedEventLoopGroup.currentEventLoop == nil,
-                     "limitation in SwiftNIO: cannot bootstrap PipeChannel on EventLoop")
         var s: stat = .init()
         try withUnsafeMutablePointer(to: &s) { ptr in
             try Posix.fstat(descriptor: descriptor, outStat: ptr)

Tests/NIOPosixTests/BootstrapTest+XCTest.swift

@@ -39,6 +39,7 @@ extension BootstrapTest {
                 ("testPreConnectedSocketSetsChannelOptionsBeforeChannelInitializer", testPreConnectedSocketSetsChannelOptionsBeforeChannelInitializer),
                 ("testDatagramBootstrapSetsChannelOptionsBeforeChannelInitializer", testDatagramBootstrapSetsChannelOptionsBeforeChannelInitializer),
                 ("testPipeBootstrapSetsChannelOptionsBeforeChannelInitializer", testPipeBootstrapSetsChannelOptionsBeforeChannelInitializer),
+                ("testPipeBootstrapInEventLoop", testPipeBootstrapInEventLoop),
                 ("testServerBootstrapAddsAcceptHandlerAfterServerChannelInitialiser", testServerBootstrapAddsAcceptHandlerAfterServerChannelInitialiser),
                 ("testClientBootstrapValidatesWorkingELGsCorrectly", testClientBootstrapValidatesWorkingELGsCorrectly),
                 ("testClientBootstrapRejectsNotWorkingELGsCorrectly", testClientBootstrapRejectsNotWorkingELGsCorrectly),

Tests/NIOPosixTests/BootstrapTest.swift

@@ -389,6 +389,32 @@ class BootstrapTest: XCTestCase {
         })
     }
 
+    func testPipeBootstrapInEventLoop() {
+        let testGrp = DispatchGroup()
+        testGrp.enter()
+
+        let eventLoop = self.group.next()
+
+        eventLoop.execute {
+            do {
+                let pipe = Pipe()
+                let readHandle = NIOFileHandle(descriptor: pipe.fileHandleForReading.fileDescriptor)
+                let writeHandle = NIOFileHandle(descriptor: pipe.fileHandleForWriting.fileDescriptor)
+                _ = NIOPipeBootstrap(group: self.group)
+                    .withPipes(inputDescriptor: try readHandle.takeDescriptorOwnership(), outputDescriptor: try writeHandle.takeDescriptorOwnership())
+                    .flatMap({ channel in
+                        channel.close()
+                    }).always({ _ in
+                        testGrp.leave()
+                    })
+            } catch {
+                XCTFail("Failed to bootstrap pipechannel in eventloop: \(error)")
+                testGrp.leave()
+            }
+        }
+        testGrp.wait()
+    }
+
     func testServerBootstrapAddsAcceptHandlerAfterServerChannelInitialiser() {
         // It's unclear if this is the right solution, see https://github.com/apple/swift-nio/issues/1392
         let group = MultiThreadedEventLoopGroup(numberOfThreads: 1)