std.posix: unlock nonblocking connect() use case

Match Windows `WSAEALREADY` with Linux `EALREADY`
return gracefully when socket reports it is connected

Author: iamtimmy

lib/std/net/test.zig

@@ -404,3 +404,38 @@ test "non-blocking tcp server" {
     const msg = buf[0..len];
     try testing.expect(mem.eql(u8, msg, "hello from server\n"));
 }
+
+test "non-blocking tcp client" {
+    if (builtin.os.tag == .wasi) return error.SkipZigTest;
+
+    if (builtin.os.tag == .windows) {
+        _ = try std.os.windows.WSAStartup(2, 2);
+    }
+    defer {
+        if (builtin.os.tag == .windows) {
+            std.os.windows.WSACleanup() catch unreachable;
+        }
+    }
+
+    // create a blocking server that accepts a single connection then ignores it
+    const listenAddress = std.net.Address.initIp4(.{ 127, 0, 0, 1 }, 0);
+    var localhost = try listenAddress.listen(.{});
+    defer localhost.deinit();
+
+    // Address.listen calls getsockname to fill localhost.listen_address with a port
+    const address = localhost.listen_address;
+
+    const tpe: u32 = std.posix.SOCK.STREAM | std.posix.SOCK.NONBLOCK | std.posix.SOCK.CLOEXEC;
+    const protocol = std.posix.IPPROTO.TCP;
+    const socket = try std.posix.socket(address.any.family, tpe, protocol);
+    defer std.posix.close(socket);
+
+    // first call returns .WouldBlock like usual
+    try testing.expectError(error.WouldBlock, std.posix.connect(socket, &address.any, address.getOsSockLen()));
+
+    var con = try localhost.accept();
+    defer con.stream.close();
+
+    // another call after accept returns returns gracefully
+    try std.posix.connect(socket, &address.any, address.getOsSockLen());
+}

lib/std/posix.zig

@@ -4274,6 +4274,7 @@ pub fn connect(sock: socket_t, sock_addr: *const sockaddr, len: socklen_t) Conne
         const rc = windows.ws2_32.connect(sock, sock_addr, @intCast(len));
         if (rc == 0) return;
         switch (windows.ws2_32.WSAGetLastError()) {
+            .WSAEISCONN => return,
             .WSAEADDRINUSE => return error.AddressInUse,
             .WSAEADDRNOTAVAIL => return error.AddressNotAvailable,
             .WSAECONNREFUSED => return error.ConnectionRefused,
@@ -4284,9 +4285,9 @@ pub fn connect(sock: socket_t, sock_addr: *const sockaddr, len: socklen_t) Conne
             => return error.NetworkUnreachable,
             .WSAEFAULT => unreachable,
             .WSAEINVAL => unreachable,
-            .WSAEISCONN => unreachable,
             .WSAENOTSOCK => unreachable,
             .WSAEWOULDBLOCK => return error.WouldBlock,
+            .WSAEALREADY => return error.ConnectionPending,
             .WSAEACCES => unreachable,
             .WSAENOBUFS => return error.SystemResources,
             .WSAEAFNOSUPPORT => return error.AddressFamilyNotSupported,
@@ -4298,6 +4299,7 @@ pub fn connect(sock: socket_t, sock_addr: *const sockaddr, len: socklen_t) Conne
     while (true) {
         switch (errno(system.connect(sock, sock_addr, len))) {
             .SUCCESS => return,
+            .ISCONN => return,
             .ACCES => return error.AccessDenied,
             .PERM => return error.PermissionDenied,
             .ADDRINUSE => return error.AddressInUse,
@@ -4310,7 +4312,6 @@ pub fn connect(sock: socket_t, sock_addr: *const sockaddr, len: socklen_t) Conne
             .CONNRESET => return error.ConnectionResetByPeer,
             .FAULT => unreachable, // The socket structure address is outside the user's address space.
             .INTR => continue,
-            .ISCONN => unreachable, // The socket is already connected.
             .HOSTUNREACH => return error.NetworkUnreachable,
             .NETUNREACH => return error.NetworkUnreachable,
             .NOTSOCK => unreachable, // The file descriptor sockfd does not refer to a socket.