std.Build.WebServer: ignore client->server messages on Windows

This is a workaround for a standard library bug which will be resolved
by some upcoming writergate rewrite work. I will open a follow-up issue
before this PR is merged.

Author: mlugg

lib/std/Build/WebServer.zig

@@ -290,9 +290,20 @@ fn serveWebSocket(ws: *WebServer, sock: *std.http.WebSocket) !noreturn {
         copy.* = @atomicLoad(u8, shared, .monotonic);
     }
 
-    // TODO: running `recv` on a separate thread happens to be safe with this particular API. It's
-    // still a horrible hack which should be deleted. Friends don't let friends use threads for IO.
-    _ = try std.Thread.spawn(.{}, recvWebSocketMessages, .{ ws, sock });
+    if (builtin.os.tag == .windows) {
+        // At the time of writing, reading from a socket on Windows on one thread seems to cause
+        // writes to it from another thread to deadlock. That's *probably* because the current path
+        // ends up using `ReadFile` instead of the WSA functions. Once writergate is merged, we'll
+        // be using the proper functions, so should be able to enable this.
+        std.log.warn("implementation currently ignores client messages on Windows; the 'Rebuild' button will do nothing", .{});
+    } else {
+        // TODO: the `std.http.WebSocket` API should include guarantees that receiving messages on
+        // one thread while another sends them is okay. At the time of writing, it is, but it should
+        // be documented as such, because this use case is important! This will become particularly
+        // relevant once we implement `std.Io`, so that concurrent tasks -- potentially on different
+        // threads depending on the `Io` implementation -- become the blessed way to achieve this.
+        _ = try std.Thread.spawn(.{}, recvWebSocketMessages, .{ ws, sock });
+    }
 
     {
         const hello_header: abi.Hello = .{