std.net: enable forcing non-blocking mode for accept

Justification: It is common for non-CPU bound short routines to do
non-blocking accept to eliminate unnecessary delays before subscribing
to data, for example in hardware integration tests.

lib/std/net.zig

@@ -1871,6 +1871,7 @@ pub const StreamServer = struct {
     kernel_backlog: u31,
     reuse_address: bool,
     reuse_port: bool,
+    force_nonblocking: bool,
 
     /// `undefined` until `listen` returns successfully.
     listen_address: Address,
@@ -1888,6 +1889,9 @@ pub const StreamServer = struct {
 
         /// Enable SO.REUSEPORT on the socket.
         reuse_port: bool = false,
+
+        /// Force non-blocking mode.
+        force_nonblocking: bool = false,
     };
 
     /// After this call succeeds, resources have been acquired and must
@@ -1898,6 +1902,7 @@ pub const StreamServer = struct {
             .kernel_backlog = options.kernel_backlog,
             .reuse_address = options.reuse_address,
             .reuse_port = options.reuse_port,
+            .force_nonblocking = options.force_nonblocking,
             .listen_address = undefined,
         };
     }
@@ -1911,9 +1916,11 @@ pub const StreamServer = struct {
     pub fn listen(self: *StreamServer, address: Address) !void {
         const nonblock = if (std.io.is_async) os.SOCK.NONBLOCK else 0;
         const sock_flags = os.SOCK.STREAM | os.SOCK.CLOEXEC | nonblock;
+        var use_sock_flags: u32 = sock_flags;
+        if (self.force_nonblocking) use_sock_flags |= os.SOCK.NONBLOCK;
         const proto = if (address.any.family == os.AF.UNIX) @as(u32, 0) else os.IPPROTO.TCP;
 
-        const sockfd = try os.socket(address.any.family, sock_flags, proto);
+        const sockfd = try os.socket(address.any.family, use_sock_flags, proto);
         self.sockfd = sockfd;
         errdefer {
             os.closeSocket(sockfd);
@@ -1963,8 +1970,8 @@ pub const StreamServer = struct {
         /// The system-wide limit on the total number of open files has been reached.
         SystemFdQuotaExceeded,
 
-        /// Not enough free memory.  This often means that the memory allocation  is  limited
-        /// by the socket buffer limits, not by the system memory.
+        /// Not enough free memory. This often means that the memory allocation
+        /// is limited by the socket buffer limits, not by the system memory.
         SystemResources,
 
         /// Socket is not listening for new connections.
@@ -1972,6 +1979,9 @@ pub const StreamServer = struct {
 
         ProtocolFailure,
 
+        /// Socket is in non-blocking mode and there is no connection to accept.
+        WouldBlock,
+
         /// Firewall rules forbid connection.
         BlockedByFirewall,
 
@@ -2007,9 +2017,8 @@ pub const StreamServer = struct {
                 .stream = Stream{ .handle = fd },
                 .address = accepted_addr,
             };
-        } else |err| switch (err) {
-            error.WouldBlock => unreachable,
-            else => |e| return e,
+        } else |err| {
+            return err;
         }
     }
 };

lib/std/net/test.zig

@@ -340,3 +340,27 @@ fn generateFileName(base_name: []const u8) ![]const u8 {
     _ = std.fs.base64_encoder.encode(&sub_path, &random_bytes);
     return std.fmt.allocPrint(testing.allocator, "{s}-{s}", .{ sub_path[0..], base_name });
 }
+
+test "non-blocking tcp server" {
+    if (builtin.os.tag == .wasi) return error.SkipZigTest;
+
+    const localhost = try net.Address.parseIp("127.0.0.1", 0);
+    var server = net.StreamServer.init(.{ .force_nonblocking = true });
+    defer server.deinit();
+    try server.listen(localhost);
+
+    const accept_err = server.accept();
+    try testing.expectError(error.WouldBlock, accept_err);
+
+    const socket_file = try net.tcpConnectToAddress(server.listen_address);
+    defer socket_file.close();
+
+    var client = try server.accept();
+    const stream = client.stream.writer();
+    try stream.print("hello from server\n", .{});
+
+    var buf: [100]u8 = undefined;
+    const len = try socket_file.read(&buf);
+    const msg = buf[0..len];
+    try testing.expect(mem.eql(u8, msg, "hello from server\n"));
+}