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/windows/muxable-pipe.cc

@@ -0,0 +1,70 @@
+#include <ioapiset.h>
+#include "windows-error.hh"
+
+#include "logging.hh"
+#include "util.hh"
+#include "muxable-pipe.hh"
+
+namespace nix {
+
+void MuxablePipePollState::poll(HANDLE ioport, std::optional<unsigned int> timeout)
+{
+    /* We are on at least Windows Vista / Server 2008 and can get many
+       (countof(oentries)) statuses in one API call. */
+    if (!GetQueuedCompletionStatusEx(
+            ioport, oentries, sizeof(oentries) / sizeof(*oentries), &removed, timeout ? *timeout : INFINITE, false)) {
+        windows::WinError winError("GetQueuedCompletionStatusEx");
+        if (winError.lastError != WAIT_TIMEOUT)
+            throw winError;
+        assert(removed == 0);
+    } else {
+        assert(0 < removed && removed <= sizeof(oentries) / sizeof(*oentries));
+    }
+}
+
+void MuxablePipePollState::iterate(
+    std::set<MuxablePipePollState::CommChannel> & channels,
+    std::function<void(Descriptor fd, std::string_view data)> handleRead,
+    std::function<void(Descriptor fd)> handleEOF)
+{
+    auto p = channels.begin();
+    while (p != channels.end()) {
+        decltype(p) nextp = p;
+        ++nextp;
+        for (ULONG i = 0; i < removed; i++) {
+            if (oentries[i].lpCompletionKey == ((ULONG_PTR) ((*p)->readSide.get()) ^ 0x5555)) {
+                printMsg(lvlVomit, "read %s bytes", oentries[i].dwNumberOfBytesTransferred);
+                if (oentries[i].dwNumberOfBytesTransferred > 0) {
+                    std::string data{
+                        (char *) (*p)->buffer.data(),
+                        oentries[i].dwNumberOfBytesTransferred,
+                    };
+                    handleRead((*p)->readSide.get(), data);
+                }
+
+                if (gotEOF) {
+                    handleEOF((*p)->readSide.get());
+                    nextp = channels.erase(p); // no need to maintain `channels`?
+                } else {
+                    BOOL rc = ReadFile(
+                        (*p)->readSide.get(), (*p)->buffer.data(), (*p)->buffer.size(), &(*p)->got, &(*p)->overlapped);
+                    if (rc) {
+                        // here is possible (but not obligatory) to call
+                        // `handleRead` and repeat ReadFile immediately
+                    } else {
+                        windows::WinError winError("ReadFile(%s, ..)", (*p)->readSide.get());
+                        if (winError.lastError == ERROR_BROKEN_PIPE) {
+                            handleEOF((*p)->readSide.get());
+                            nextp = channels.erase(p); // no need to maintain `channels` ?
+                        } else if (winError.lastError != ERROR_IO_PENDING)
+                            throw winError;
+                    }
+                }
+                break;
+            }
+        }
+        p = nextp;
+    }
+}
+
+}