The first idea of an async I/O interface.

diff --git a/lib/asyncio.h b/lib/asyncio.h
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+++ b/lib/asyncio.h
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+/*
+ *     UCW Library -- Asynchronous I/O
+ *
+ *     (c) 2006 Martin Mares <mj@ucw.cz>
+ *
+ *     This software may be freely distributed and used according to the terms
+ *     of the GNU Lesser General Public License.
+ */
+
+#ifndef _UCW_ASYNCIO_H
+#define _UCW_ASYNCIO_H
+
+#include "lib/semaphore.h"
+#include "lib/clists.h"
+
+/* FIXME: Comment, especially on request ordering */
+
+struct asio_queue {
+  uns max_requests;                    // Maximum number of requests allowed on this queue [user-settable]
+  uns buffer_size;                     // How large buffers do we use [user-settable]
+  uns allocated_requests;
+  uns running_requests;
+  clist idle_list;                     // Recycled requests waiting for get
+  clist done_list;                     // Requests returned from the worker threads
+  sem_t *done_sem;                     // ... and how many of them
+  clist wait_list;                     // Requests available for wait()
+};
+
+enum asio_op {
+  ASIO_FREE,
+  ASIO_READ,
+  ASIO_WRITE,
+  ASIO_BACKGROUND_WRITE,               // Write with no success notification
+};
+
+struct asio_request {
+  cnode n;
+  struct asio_queue *queue;
+  byte *buffer;
+  int fd;
+  enum asio_op op;
+  uns len;
+  int status;
+};
+
+void asio_init(void);
+void asio_cleanup(void);
+
+void asio_init_queue(struct asio_queue *q);                    // Initialize a new queue
+struct asio_request *asio_get(struct asio_queue *q);           // Get an empty request
+void asio_submit(struct asio_request *r);                      // Submit the request
+struct asio_request *asio_wait(struct asio_queue *q);          // Wait for the first finished request, NULL if no more
+void asio_put(struct asio_request *r);                         // Return a finished request for recycling
+struct asio_request *asio_get_bg(struct asio_queue *q);                // Get and if there are no free requests, wait for background writes to finish
+void asio_sync(struct asio_queue *q);                          // Wait for all requests to finish
+
+#endif /* !_UCW_ASYNCIO_H */