async-append: New library to allow asynchronous appending to a log file.

[sliver-openvswitch.git] / lib / async-append.h

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+/* Copyright (c) 2013 Nicira, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at:
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef ASYNC_APPEND_H
+#define ASYNC_APPEND_H 1
+
+#include <stddef.h>
+
+/* This module defines a simple, abstract interface to asynchronous file I/O.
+ * It is currently used only for logging.  Thus, for now the interface only
+ * supports appending to a file.  Multiple implementations are possible
+ * depending on the operating system's degree and form of support for
+ * asynchronous I/O.
+ *
+ * The comments below document the requirements on any implementation.
+ *
+ * Thread-safety
+ * =============
+ *
+ * Only a single thread may use a given 'struct async_append' at one time.
+ */
+
+/* Enables using asynchronous I/O.  Some implementations may treat this as a
+ * no-op.
+ *
+ * Before this function is called, the POSIX aio implementation uses ordinary
+ * synchronous I/O because some POSIX aio libraries actually use threads
+ * internally, which has enough cost and robustness implications that it's
+ * better to use asynchronous I/O only when it has real expected benefits.
+ *
+ * Must be called while the process is still single-threaded.  May forbid the
+ * process from subsequently forking. */
+void async_append_enable(void);
+
+/* Creates and returns a new asynchronous appender for file descriptor 'fd',
+ * which the caller must have opened in append mode (O_APPEND).
+ *
+ * This function must always succeed.  If the system is for some reason unable
+ * to support asynchronous I/O on 'fd' then the library must fall back to
+ * syncrhonous I/O. */
+struct async_append *async_append_create(int fd);
+
+/* Destroys 'ap', without closing its underlying file descriptor. */
+void async_append_destroy(struct async_append *ap);
+
+/* Appends the 'size' bytes of 'data' to 'ap', asynchronously if possible. */
+void async_append_write(struct async_append *ap,
+                        const void *data, size_t size);
+
+/* Blocks until all data asynchronously written to 'ap' with
+ * async_append_write() has been committed to the point that it will be written
+ * to disk barring an operating system or hardware failure. */
+void async_append_flush(struct async_append *ap);
+
+#endif /* async-append.h */