Add TCP_INFO/TCP_CONNECTION_INFO queries. (#811)

Motivation:

Many users may wish to see the underlying TCP connection state for
important calculations, such as for BDP or RTT numbers. We can make this
available easily enough.

Modifications:

- Added a function to get TCP_INFO on Linux/FreeBSD.
- Added a function to get TCP_CONNECTION_INFO on Darwin.

Result:

Better connection stats

Sources/NIO/SocketOptionProvider.swift

@@ -207,6 +207,30 @@ public extension SocketOptionProvider {
     func getIPv6MulticastLoop() -> EventLoopFuture<CUnsignedInt> {
         return self.unsafeGetSocketOption(level: IPPROTO_IPV6, name: IPV6_MULTICAST_LOOP)
     }
+
+    #if os(Linux) || os(FreeBSD)
+    /// Gets the value of the socket option TCP_INFO.
+    ///
+    /// This socket option cannot be set.
+    ///
+    /// - returns: An `EventLoopFuture` containing the value of the socket option, or
+    ///     any error that occurred while retrieving the socket option.
+    func getTCPInfo() -> EventLoopFuture<tcp_info> {
+        return self.unsafeGetSocketOption(level: IPPROTO_TCP, name: TCP_INFO)
+    }
+    #endif
+
+    #if os(macOS) || os(iOS) || os(watchOS) || os(tvOS)
+    /// Gets the value of the socket option TCP_CONNECTION_INFO.
+    ///
+    /// This socket option cannot be set.
+    ///
+    /// - returns: An `EventLoopFuture` containing the value of the socket option, or
+    ///     any error that occurred while retrieving the socket option.
+    func getTCPConnectionInfo() -> EventLoopFuture<tcp_connection_info> {
+        return self.unsafeGetSocketOption(level: IPPROTO_TCP, name: TCP_CONNECTION_INFO)
+    }
+    #endif
 }
 
 

Tests/NIOTests/SocketOptionProviderTest+XCTest.swift

@@ -38,6 +38,8 @@ extension SocketOptionProviderTest {
                 ("testIpv6MulticastIf", testIpv6MulticastIf),
                 ("testIPv6MulticastHops", testIPv6MulticastHops),
                 ("testIPv6MulticastLoop", testIPv6MulticastLoop),
+                ("testTCPInfo", testTCPInfo),
+                ("testTCPConnectionInfo", testTCPConnectionInfo),
            ]
    }
 }

Tests/NIOTests/SocketOptionProviderTest.swift

@@ -241,4 +241,26 @@ final class SocketOptionProviderTest: XCTestCase {
             XCTAssertNotEqual($0, 0)
         }.wait())
     }
+
+    func testTCPInfo() throws {
+        // This test only runs on Linux and FreeBSD.
+        #if os(Linux) || os(FreeBSD)
+        let channel = self.clientChannel! as! SocketOptionProvider
+        let tcpInfo = try assertNoThrowWithValue(channel.getTCPInfo().wait())
+
+        // We just need to sanity check something here to ensure that the data is vaguely reasonable.
+        XCTAssertEqual(tcpInfo.tcpi_state, UInt8(TCP_ESTABLISHED))
+        #endif
+    }
+
+    func testTCPConnectionInfo() throws {
+        // This test only runs on Darwin.
+        #if os(macOS) || os(iOS) || os(watchOS) || os(tvOS)
+        let channel = self.clientChannel! as! SocketOptionProvider
+        let tcpConnectionInfo = try assertNoThrowWithValue(channel.getTCPConnectionInfo().wait())
+
+        // We just need to sanity check something here to ensure that the data is vaguely reasonable.
+        XCTAssertEqual(tcpConnectionInfo.tcpi_state, UInt8(TSI_S_ESTABLISHED))
+        #endif
+    }
 }