More work on the scheduler for windows

- Get a rump derivation goal: hook instance will come later, local
  derivation goal will come after that.

- Start cleaning up the channel / waiting code with an abstraction.

src/libutil/muxable-pipe.hh

@@ -0,0 +1,82 @@
+#pragma once
+///@file
+
+#include "file-descriptor.hh"
+#ifdef _WIN32
+#  include "windows-async-pipe.hh"
+#endif
+
+#ifndef _WIN32
+#  include <poll.h>
+#else
+#  include <ioapiset.h>
+#  include "windows-error.hh"
+#endif
+
+namespace nix {
+
+/**
+ * An "muxable pipe" is a type of pipe supporting endpoints that wait
+ * for events on multiple pipes at once.
+ *
+ * On Unix, this is just a regular anonymous pipe. On Windows, this has
+ * to be a named pipe because we need I/O Completion Ports to wait on
+ * multiple pipes.
+ */
+using MuxablePipe =
+#ifndef _WIN32
+    Pipe
+#else
+    windows::AsyncPipe
+#endif
+    ;
+
+/**
+ * Use pool() (Unix) / I/O Completion Ports (Windows) to wait for the
+ * input side of any logger pipe to become `available'.  Note that
+ * `available' (i.e., non-blocking) includes EOF.
+ */
+struct MuxablePipePollState
+{
+#ifndef _WIN32
+    std::vector<struct pollfd> pollStatus;
+    std::map<int, size_t> fdToPollStatus;
+#else
+    OVERLAPPED_ENTRY oentries[0x20] = {0};
+    ULONG removed;
+    bool gotEOF = false;
+
+#endif
+
+    /**
+     * Check for ready (Unix) / completed (Windows) operations
+     */
+    void poll(
+#ifdef _WIN32
+        HANDLE ioport,
+#endif
+        std::optional<unsigned int> timeout);
+
+    using CommChannel =
+#ifndef _WIN32
+        Descriptor
+#else
+        windows::AsyncPipe *
+#endif
+        ;
+
+    /**
+     * Process for ready (Unix) / completed (Windows) operations,
+     * calling the callbacks as needed.
+     *
+     * @param handleRead callback to be passed read data.
+     *
+     * @param handleEOF callback for when the `MuxablePipe` has closed.
+     */
+    void iterate(
+        std::set<CommChannel> & channels,
+        std::function<void(Descriptor fd, std::string_view data)> handleRead,
+        std::function<void(Descriptor fd)> handleEOF);
+};
+
+}