fix(windows): handle null handle.data in libuv callbacks

Add null checks to Windows libuv callbacks to prevent "cast causes
pointer to be null" panics. This is a recurring Windows-specific bug
where libuv callbacks may be invoked with null handle.data due to race
conditions during handle cleanup or event loop processing.

The affected callbacks are:
- PipeReader: onStreamAlloc, onStreamRead, onPipeClose, onTTYClose
- PipeWriter: onPipeClose, onTTYClose
- PathWatcher: uvClosedCallback

When handle.data is null, the callbacks now safely return early instead
of panicking.

Fixes #26574

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/bun.js/node/win_watcher.zig

@@ -217,8 +217,9 @@ pub const PathWatcher = struct {
 
     fn uvClosedCallback(handler: *anyopaque) callconv(.c) void {
         log("onClose", .{});
-        const event = bun.cast(*uv.uv_fs_event_t, handler);
-        const this = bun.cast(*PathWatcher, event.data);
+        const event: *uv.uv_fs_event_t = @ptrCast(@alignCast(handler));
+        // event.data may be null if the handle was already cleaned up or there was a race condition
+        const this: *PathWatcher = @ptrCast(@alignCast(event.data orelse return));
         bun.destroy(this);
     }
 

src/io/PipeReader.zig

@@ -760,7 +760,7 @@ pub const WindowsBufferedReader = struct {
                 return Type.onReaderError(@as(*Type, @ptrCast(@alignCast(this))), err);
             }
             fn loop(this: *anyopaque) *Async.Loop {
-                return Type.loop(@as(*Type, @alignCast(@ptrCast(this))));
+                return Type.loop(@as(*Type, @ptrCast(@alignCast(this))));
             }
         };
         return .{
@@ -955,14 +955,21 @@ pub const WindowsBufferedReader = struct {
     }
 
     fn onStreamAlloc(handle: *uv.Handle, suggested_size: usize, buf: *uv.uv_buf_t) callconv(.c) void {
-        var this = bun.cast(*WindowsBufferedReader, handle.data);
+        // handle.data may be null if the handle was closed (set to self-pointer then deallocated)
+        // or during race conditions in Windows libuv event processing
+        const this: *WindowsBufferedReader = @ptrCast(@alignCast(handle.data orelse {
+            buf.* = uv.uv_buf_t.init("");
+            return;
+        }));
         const result = this.getReadBufferWithStableMemoryAddress(suggested_size);
         buf.* = uv.uv_buf_t.init(result);
     }
 
     fn onStreamRead(handle: *uv.uv_handle_t, nread: uv.ReturnCodeI64, buf: *const uv.uv_buf_t) callconv(.c) void {
-        const stream = bun.cast(*uv.uv_stream_t, handle);
-        var this = bun.cast(*WindowsBufferedReader, stream.data);
+        const stream: *uv.uv_stream_t = @ptrCast(handle);
+        // stream.data may be null if the handle was closed (set to self-pointer then deallocated)
+        // or during race conditions in Windows libuv event processing
+        const this: *WindowsBufferedReader = @ptrCast(@alignCast(stream.data orelse return));
 
         const nread_int = nread.int();
 
@@ -1199,12 +1206,16 @@ pub const WindowsBufferedReader = struct {
     }
 
     fn onPipeClose(handle: *uv.Pipe) callconv(.c) void {
-        const this = bun.cast(*uv.Pipe, handle.data);
+        // handle.data was set to point to the pipe itself before close was called
+        // If null, the handle was already cleaned up or there was a race condition
+        const this: *uv.Pipe = @ptrCast(@alignCast(handle.data orelse return));
         bun.default_allocator.destroy(this);
     }
 
     fn onTTYClose(handle: *uv.uv_tty_t) callconv(.c) void {
-        const this = bun.cast(*uv.uv_tty_t, handle.data);
+        // handle.data was set to point to the tty itself before close was called
+        // If null, the handle was already cleaned up or there was a race condition
+        const this: *uv.uv_tty_t = @ptrCast(@alignCast(handle.data orelse return));
         bun.default_allocator.destroy(this);
     }
 

src/io/PipeWriter.zig

@@ -804,12 +804,16 @@ fn BaseWindowsPipeWriter(
         }
 
         fn onPipeClose(handle: *uv.Pipe) callconv(.c) void {
-            const this = bun.cast(*uv.Pipe, handle.data);
+            // handle.data was set to point to the pipe itself before close was called
+            // If null, the handle was already cleaned up or there was a race condition
+            const this: *uv.Pipe = @ptrCast(@alignCast(handle.data orelse return));
             bun.default_allocator.destroy(this);
         }
 
         fn onTTYClose(handle: *uv.uv_tty_t) callconv(.c) void {
-            const this = bun.cast(*uv.uv_tty_t, handle.data);
+            // handle.data was set to point to the tty itself before close was called
+            // If null, the handle was already cleaned up or there was a race condition
+            const this: *uv.uv_tty_t = @ptrCast(@alignCast(handle.data orelse return));
             bun.default_allocator.destroy(this);
         }