610 lines
26 KiB
Plaintext
610 lines
26 KiB
Plaintext
What's New In Libevent 2.0 so far:
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1. Meta-issues
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1.1. About this document
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This document describes the key differences between Libevent 1.4 and
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Libevent 2.0, from a user's point of view. It was most recently
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updated based on features in git master as of August 2010.
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NOTE: I am very sure that I missed some thing on this list. Caveat
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haxxor.
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1.2. Better documentation
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There is now a book-in-progress that explains how to use Libevent and its
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growing pile of APIs. As of this writing, it covers everything except the
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http and rpc code. Check out the latest draft at
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http://www.wangafu.net/~nickm/libevent-book/ .
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2. New and Improved Event APIs
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Many APIs are improved, refactored, or deprecated in Libevent 2.0.
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COMPATIBILITY:
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Nearly all existing code that worked with Libevent 1.4 should still
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work correctly with Libevent 2.0. However, if you are writing new code,
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or if you want to port old code, we strongly recommend using the new APIs
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and avoiding deprecated APIs as much as possible.
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Binaries linked against Libevent 1.4 will need to be recompiled to link
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against Libevent 2.0. This is nothing new; we have never been good at
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preserving binary compatibility between releases. We'll try harder in the
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future, though: see 2.1 below.
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2.1. New header layout for improved forward-compatibility
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Libevent 2.0 has a new header layout to make it easier for programmers to
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write good, well-supported libevent code. The new headers are divided
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into three types.
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There are *regular headers*, like event2/event.h. These headers contain
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the functions that most programmers will want to use.
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There are *backward compatibility headers*, like event2/event_compat.h.
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These headers contain declarations for deprecated functions from older
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versions of Libevent. Documentation in these headers should suggest what's
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wrong with the old functions, and what functions you want to start using
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instead of the old ones. Some of these functions might be removed in a
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future release. New programs should generally not include these headers.
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Finally, there are *structure headers*, like event2/event_struct.h.
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These headers contain definitions of some structures that Libevent has
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historically exposed. Exposing them caused problems in the past,
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since programs that were compiled to work with one version of Libevent
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would often stop working with another version that changed the size or
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layout of some object. We've moving them into separate headers so
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that programmers can know that their code is not depending on any
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unstable aspect of the Libvent ABI. New programs should generally not
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include these headers unless they really know what they are doing, are
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willing to rebuild their software whenever they want to link it
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against a new version of Libevent, and are willing to risk their code
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breaking if and when data structures change.
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Functionality that once was located in event.h is now more subdivided.
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The core event logic is now in event2/event.h. The "evbuffer" functions
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for low-level buffer manipulation are in event2/buffer.h. The
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"bufferevent" functions for higher-level buffered IO are in
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event2/bufferevent.h.
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COMPATIBILITY:
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All of the old headers (event.h, evdns.h, evhttp.h, evrpc.h, and
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evutil.h) will continue to work by including the corresponding new
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headers. Old code should not be broken by this change.
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2.2. New thread-safe, binary-compatible, harder-to-mess-up APIs
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Some aspects of the historical Libevent API have encouraged
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non-threadsafe code, or forced code built against one version of Libevent
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to no longer build with another. The problems with now-deprecated APIs
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fell into two categories:
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1) Dependence on the "current" event_base. In an application with
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multiple event_bases, Libevent previously had a notion of the
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"current" event_base. New events were linked to this base, and
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the caller needed to explicitly reattach them to another base.
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This was horribly error-prone.
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Functions like "event_set" that worked with the "current" event_base
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are now deprecated but still available (see 2.1). There are new
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functions like "event_assign" that take an explicit event_base
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argument when setting up a structure. Using these functions will help
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prevent errors in your applications, and to be more threadsafe.
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2) Structure dependence. Applications needed to allocate 'struct
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event' themselves, since there was no function in Libevent to do it
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for them. But since the size and contents of struct event can
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change between libevent versions, this created binary-compatibility
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nightmares. All structures of this kind are now isolated in
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_struct.h header (see 2.1), and there are new allocate-and-
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initialize functions you can use instead of the old initialize-only
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functions. For example, instead of malloc and event_set, you
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can use event_new().
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(For people who do really want to allocate a struct event on the
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stack, or put one inside another structure, you can still use
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event2/event_compat.h.)
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So in the case where old code would look like this:
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#include <event.h>
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...
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struct event *ev = malloc(sizeof(struct event));
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/* This call will cause a buffer overrun if you compile with one version
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of Libevent and link dynamically against another. */
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event_set(ev, fd, EV_READ, cb, NULL);
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/* If you forget this call, your code will break in hard-to-diagnose
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ways in the presence of multiple event bases. */
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event_set_base(ev, base);
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New code will look more like this:
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#include <event2/event.h>
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...
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struct event *ev;
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ev = event_new(base, fd, EV_READ, cb, NULL);
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2.3. Overrideable allocation functions
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If you want to override the allocation functions used by libevent
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(for example, to use a specialized allocator, or debug memory
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issues, or so on), you can replace them by calling
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event_set_mem_functions. It takes replacements for malloc(),
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free(), and realloc().
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If you're going to use this facility, you need to call it _before_
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Libevent does any memory allocation; otherwise, Libevent may allocate some
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memory with malloc(), and free it with the free() function you provide.
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You can disable this feature when you are building Libevent by passing
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the --disable-malloc-replacement argument to configure.
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2.4. Configurable event_base creation
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Older versions of Libevent would always got the fastest backend
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available, unless you reconfigured their behavior with the environment
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variables EVENT_NOSELECT, EVENT_NOPOLL, and so forth. This was annoying
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to programmers who wanted to pick a backend explicitly without messing
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with the environment.
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Also, despite our best efforts, not every backend supports every
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operation we might like. Some features (like edge-triggered events, or
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working with non-socket file descriptors) only work with some operating
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systems' fast backends. Previously, programmers who cared about this
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needed to know which backends supported what. This tended to get quite
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ungainly.
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There is now an API to choose backends, either by name or by feature.
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Here is an example:
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struct event_config_t *config;
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struct event_base *base;
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/* Create a new configuration object. */
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config = event_config_new();
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/* We don't want to use the "select" method. */
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event_config_avoid_method(config, "select");
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/* We want a method that can work with non-socket file descriptors */
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event_config_require_features(config, EV_FEATURE_FDS);
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base = event_base_new_with_config(config);
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if (!base) {
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/* There is no backend method that does what we want. */
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exit(1);
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}
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event_config_free(config);
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Supported features are documented in event2/event.h
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2.5. Socket is now an abstract type
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All APIs that formerly accepted int as a socket type now accept
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"evutil_socket_t". On Unix, this is just an alias for "int" as
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before. On Windows, however, it's an alias for SOCKET, which can
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be wider than int on 64-bit platforms.
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2.6. Timeouts and persistent events work together.
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Previously, it wasn't useful to set a timeout on a persistent event:
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the timeout would trigger once, and never again. This is not what
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applications tend to want. Instead, applications tend to want every
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triggering of the event to re-set the timeout. So now, if you set
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up an event like this:
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struct event *ev;
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struct timeval tv;
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ev = event_new(base, fd, EV_READ|EV_PERSIST, cb, NULL);
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tv.tv_sec = 1;
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tv.tv_usec = 0;
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event_add(ev, &tv);
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The callback 'cb' will be invoked whenever fd is ready to read, OR whenever
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a second has passed since the last invocation of cb.
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2.7. Multiple events allowed per fd
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Older versions of Libevent allowed at most one EV_READ event and at most
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one EV_WRITE event per socket, per event base. This restriction is no
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longer present.
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2.8. evthread_* functions for thread-safe structures.
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Libevent structures can now be built with locking support. This code
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makes it safe to add, remove, and activate events on an event base from a
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different thread. (Previously, if you wanted to write multithreaded code
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with Libevent, you could only an event_base or its events in one thread at
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a time.)
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If you want threading support and you're using pthreads, you can just
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call evthread_use_pthreads(). (You'll need to link against the
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libevent_pthreads library in addition to libevent_core. These functions are
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not in libevent_core.)
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If you want threading support and you're using Windows, you can just
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call evthread_use_windows_threads().
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If you are using some locking system besides Windows and pthreads, You
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can enable this on a per-event-base level by writing functions to
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implement mutexes, conditions, and thread IDs, and passing them to
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evthread_set_lock_callbacks and related functions in event2/thread.h.
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Once locking functions are enabled, every new event_base is created with a
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lock. You can prevent a single event_base from being built with a lock
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disabled by using the EVENT_BASE_FLAG_NOLOCK flag in its
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event_config. If an event_base is created with a lock, it is safe to call
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event_del, event_add, and event_active on its events from any thread. The
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event callbacks themselves are still all executed from the thread running
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the event loop.
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To make an evbuffer or a bufferevent object threadsafe, call its
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*_enable_locking() function.
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The HTTP api is not currently threadsafe.
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To build Libevent with threading support disabled, pass
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--disable-thread-support to the configure script.
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2.9. Edge-triggered events on some backends.
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With some backends, it's now possible to add the EV_ET flag to an event
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in order to request that the event's semantics be edge-triggered. Right
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now, epoll and kqueue support this.
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The corresponding event_config feature is EV_FEATURE_ET; see 2.4 for more
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information.
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2.10. Better support for huge numbers of timeouts
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The heap-based priority queue timer implementation for Libevent 1.4 is good
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for randomly distributed timeouts, but suboptimal if you have huge numbers
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of timeouts that all expire in the same amount of time after their
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creation. The new event_base_init_common_timeout() logic lets you signal
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that a given timeout interval will be very common, and should use a linked
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list implementation instead of a priority queue.
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2.11. Improved debugging support
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It's been pretty easy to forget to delete all your events before you
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re-initialize them, or otherwise put Libevent in an internally inconsistent
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state. You can tell libevent to catch these and other common errors with
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the new event_enable_debug_mode() call. Just invoke it before you do
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any calls to other libevent functions, and it'll catch many common
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event-level errors in your code.
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2.12. Functions to access all event fields
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So that you don't have to access the struct event fields directly, Libevent
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now provides accessor functions to retrieve everything from an event that
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you set during event_new() or event_assign().
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3. Backend-specific and performance improvements.
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3.1. Change-minimization on O(1) backends
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With previous versions of Libevent, if you called event_del() and
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event_add() repeatedly on a single event between trips to the backend's
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dispatch function, the backend might wind up making unnecessary calls or
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passing unnecessary data to the kernel. The new backend logic batches up
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redundant adds and deletes, and performs no more operations than necessary
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at the kernel level.
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This logic is on for the kqueue backend, and available (but off by
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default) for the epoll backend. To turn it on for the epoll backend,
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set the EVENT_BASE_FLAG_EPOLL_USE_CHANGELIST flag in the
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event_base_cofig, or set the EVENT_EPOLL_USE_CHANGELIST environment
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variable. Doing this with epoll may result in weird bugs if you give
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any fds closed by dup() or its variants.
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3.2. Improved notification on Linux
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When we need to wake the event loop up from another thread, we use
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an epollfd to do so, instead of a socketpair. This is supposed to be
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faster.
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3.3. Windows: better support for everything
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Bufferevents on Windows can use a new mechanism (off-by-default; see below)
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to send their data via Windows overlapped IO and get their notifications
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via the IOCP API. This should be much faster than using event-based
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notification.
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Other functions throughout the code have been fixed to work more
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consistently with Windows. Libevent now builds on Windows using either
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mingw, or using MSVC (with nmake). Libevent works fine with UNICODE
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defined, or not.
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Data structures are a little smarter: our lookups from socket to pending
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event are now done with O(1) hash tables rather than O(lg n) red-black
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trees.
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Unfortunately, the main Windows backend is still select()-based: from
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testing the IOCP backends on the mailing list, it seems that there isn't
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actually a way to tell for certain whether a socket is writable with IOCP.
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Libevent 2.1 may add a multithreaded WaitForMultipleEvents-based
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backend for better performance with many inactive sockets and better
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integration with Windows events.
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4. Improvements to evbuffers
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Libevent has long had an "evbuffer" implementation to wrap access to an
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input or output memory buffer. In previous versions, the implementation
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was very inefficient and lacked some desirable features. We've made many
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improvements in Libevent 2.0.
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4.1. Chunked-memory internal representation
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Previously, each evbuffer was a huge chunk of memory. When we ran out of
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space in an evbuffer, we used realloc() to grow the chunk of memory. When
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data was misaligned, we used memmove to move the data back to the front
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of the buffer.
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Needless to say, this is a terrible interface for networked IO.
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Now, evbuffers are implemented as a linked list of memory chunks, like
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most Unix kernels use for network IO. (See Linux's skbuf interfaces,
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or *BSD's mbufs). Data is added at the end of the linked list and
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removed from the front, so that we don't ever need realloc huge chunks
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or memmove the whole buffer contents.
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To avoid excessive calls to read and write, we use the readv/writev
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interfaces (or WSASend/WSARecv on Windows) to do IO on multiple chunks at
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once with a single system call.
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COMPATIBILITY NOTE:
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The evbuffer struct is no longer exposed in a header. The code here is
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too volatile to expose an official evbuffer structure, and there was never
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any means provided to create an evbuffer except via evbuffer_new which
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heap-allocated the buffer.
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If you need access to the whole buffer as a linear chunk of memory, the
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EVBUFFER_DATA() function still works. Watch out, though: it needs to copy
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the buffer's contents in a linear chunk before you can use it.
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4.2. More flexible readline support
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The old evbuffer_readline() function (which accepted any sequence of
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CR and LF characters as a newline, and which couldn't handle lines
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containing NUL characters), is now deprecated. The preferred
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function is evbuffer_readln(), which supports a variety of
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line-ending styles, and which can return the number of characters in
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the line returned.
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You can also call evbuffer_search_eol() to find the end of a line
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in an evbuffer without ever extracting the line.
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4.3. Support for file-based IO in evbuffers.
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You can now add chunks of a file into a evbuffer, and Libevent will have
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your OS use mapped-memory functionality, sendfile, or splice to transfer
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the data without ever copying it to userspace. On OSs where this is not
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supported, Libevent just loads the data.
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There are probably some bugs remaining in this code. On some platforms
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(like Windows), it just reads the relevant parts of the file into RAM.
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4.4. Support for zero-copy ("scatter/gather") writes in evbuffers.
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You can add a piece of memory to an evbuffer without copying it.
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Instead, Libevent adds a new element to the evbuffer's linked list of
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chunks with a pointer to the memory you supplied. You can do this
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either with a reference-counted chunk (via evbuffer_add_reference), or
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by asking Libevent for a pointer to its internal vectors (via
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evbuffer_reserve_space or evbuffer_peek()).
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4.5. Multiple callbacks per evbuffer
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Previously, you could only have one callback active on an evbuffer at a
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time. In practice, this meant that if one part of Libevent was using an
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evbuffer callback to notice when an internal evbuffer was reading or
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writing data, you couldn't have your own callback on that evbuffer.
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Now, you can now use the evbuffer_add_cb() function to add a callback that
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does not interfere with any other callbacks.
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The evbuffer_setcb() function is now deprecated.
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4.6. New callback interface
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Previously, evbuffer callbacks were invoked with the old size of the
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buffer and the new size of the buffer. This interface could not capture
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operations that simultaneously filled _and_ drained a buffer, or handle
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cases where we needed to postpone callbacks until multiple operations were
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complete.
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Callbacks that are set with evbuffer_setcb still use the old API.
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Callbacks added with evbuffer_add_cb() use a new interface that takes a
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pointer to a struct holding the total number of bytes drained read and the
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total number of bytes written. See event2/buffer.h for full details.
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4.7. Misc new evbuffer features
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You can use evbuffer_remove() to move a given number of bytes from one
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buffer to another.
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The evbuffer_search() function lets you search for repeated instances of
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a pattern inside an evbuffer.
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You can use evbuffer_freeze() to temporarily suspend drains from or adds
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to a given evbuffer. This is useful for code that exposes an evbuffer as
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part of its public API, but wants users to treat it as a pure source or
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sink.
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There's an evbuffer_copyout() that looks at the data at the start of an
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evbuffer without doing a drain.
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You can have an evbuffer defer all of its callbacks, so that rather than
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being invoked immediately when the evbuffer's length changes, they are
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invoked from within the event_loop. This is useful when you have a
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complex set of callbacks that can change the length of other evbuffers,
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and you want to avoid having them recurse and overflow your stack.
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5. Bufferevents improvements
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Libevent has long included a "bufferevents" structure and related
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functions that were useful for generic buffered IO on a TCP connection.
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This is what Libevent uses for its HTTP implementation. In addition to
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the improvements that they get for free from the underlying evbuffer
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implementation above, there are many new features in Libevent 2.0's
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evbuffers.
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5.1. New OO implementations
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The "bufferevent" structure is now an abstract base type with multiple
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implementations. This should not break existing code, which always
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allocated bufferevents with bufferevent_new().
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Current implementations of the bufferevent interface are described below.
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5.2. bufferevent_socket_new() replaces bufferevent_new()
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Since bufferevents that use a socket are not the only kind,
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bufferevent_new() is now deprecated. Use bufferevent_socket_new()
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instead.
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5.3. Filtered bufferevent IO
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You can use bufferevent_filter_new() to create a bufferevent that wraps
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around another bufferevent and transforms data it is sending and
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receiving. See test/regress_zlib.c for a toy example that uses zlib to
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compress data before sending it over a bufferevent.
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5.3. Linked pairs of bufferevents
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You can use bufferevent_pair_new() to produce two linked
|
|
bufferevents. This is like using socketpair, but doesn't require
|
|
system-calls.
|
|
|
|
5.4. SSL support for bufferevents with OpenSSL
|
|
|
|
There is now a bufferevent type that supports SSL/TLS using the
|
|
OpenSSL library. The code for this is build in a separate
|
|
library, libevent_openssl, so that your programs don't need to
|
|
link against OpenSSL unless they actually want SSL support.
|
|
|
|
There are two ways to construct one of these bufferevents, both
|
|
declared in <event2/bufferevent_ssl.h>. If you want to wrap an
|
|
SSL layer around an existing bufferevent, you would call the
|
|
bufferevent_openssl_filter_new() function. If you want to do SSL
|
|
on a socket directly, call bufferevent_openssl_socket_new().
|
|
|
|
5.5. IOCP support for bufferevents on Windows
|
|
|
|
There is now a bufferevents backend that supports IOCP on Windows.
|
|
Supposedly, this will eventually make Windows IO much faster for
|
|
programs using bufferevents. We'll have to see; the code is not
|
|
currently optimized at all. To try it out, call the
|
|
event_base_start_iocp() method on an event_base before contructing
|
|
bufferevents.
|
|
|
|
This is tricky code; there are probably some bugs hiding here.
|
|
|
|
5.6. Improved connect support for bufferevents.
|
|
|
|
You can now create a bufferevent that is not yet connected to any
|
|
host, and tell it to connect, either by address or by hostname.
|
|
|
|
The functions to do this are bufferevent_socket_connect and
|
|
bufferevent_socket_connect_hostname.
|
|
|
|
5.7. Rate-limiting for bufferevents
|
|
|
|
If you need to limit the number of bytes read/written by a single
|
|
bufferevent, or by a group of them, you can do this with a new set of
|
|
bufferevent rate-limiting calls.
|
|
|
|
6. Other improvements
|
|
|
|
6.1. DNS improvements
|
|
|
|
6.1.1. DNS: IPv6 nameservers
|
|
|
|
The evdns code now lets you have nameservers whose addresses are IPv6.
|
|
|
|
6.1.2. DNS: Better security
|
|
|
|
Libevent 2.0 tries harder to resist DNS answer-sniping attacks than
|
|
earlier versions of evdns. See comments in the code for full details.
|
|
|
|
Notably, evdns now supports the "0x20 hack" to make it harder to
|
|
impersonate a DNS server. Additionally, Libevent now uses a strong
|
|
internal RNG to generate DNS transaction IDs, so you don't need to supply
|
|
your own.
|
|
|
|
6.1.3. DNS: Getaddrinfo support
|
|
|
|
There's now an asynchronous getaddrinfo clone, evdns_getaddrinfo(),
|
|
to make the results of the evdns functions more usable. It doesn't
|
|
support every feature of a typical platform getaddrinfo() yet, but it
|
|
is quite close.
|
|
|
|
There is also a blocking evutil_getaddrinfo() declared in
|
|
event2/util.h, to provide a getaddrinfo() implementation for
|
|
platforms that don't have one, and smooth over the differences in
|
|
various platforms implementations of RFC3493.
|
|
|
|
Bufferevents provide bufferevent_connect_hostname(), which combines
|
|
the name lookup and connect operations.
|
|
|
|
6.1.4. DNS: No more evdns globals
|
|
|
|
Like an event base, evdns operations are now supposed to use an evdns_base
|
|
argument. This makes them easier to wrap for other (more OO) languages,
|
|
and easier to control the lifetime of. The old evdns functions will
|
|
still, of course, continue working.
|
|
|
|
6.2. Listener support
|
|
|
|
You can now more easily automate setting up a bound socket to listen for
|
|
TCP connections. Just use the evconnlistener_*() functions in the
|
|
event2/listener.h header.
|
|
|
|
The listener code supports IOCP on Windows if available.
|
|
|
|
6.3. Secure RNG support
|
|
|
|
Network code very frequently needs a secure, hard-to-predict random number
|
|
generator. Some operating systems provide a good C implementation of one;
|
|
others do not. Libevent 2.0 now provides a consistent implementation
|
|
based on the arc4random code originally from OpenBSD. Libevent (and you)
|
|
can use the evutil_secure_rng_*() functions to access a fairly secure
|
|
random stream of bytes.
|
|
|
|
6.4. HTTP
|
|
|
|
The evhttp uriencoding and uridecoding APIs have updated versions
|
|
that behave more correctly, and can handle strings with internal NULs.
|
|
|
|
The evhttp query parsing and URI parsing logic can now detect errors
|
|
more usefully. Moreover, we include an actual URI parsing function
|
|
(evhttp_uri_parse()) to correctly parse URIs, so as to discourage
|
|
people from rolling their own ad-hoc parsing functions.
|
|
|
|
There are now accessor functions for the useful fields of struct http
|
|
and friends; it shouldn't be necessary to access them directly any
|
|
more.
|
|
|
|
Libevent now lets you declare support for all specified HTTP methods,
|
|
including OPTIONS, PATCH, and so on. The default list is unchanged.
|
|
|
|
Numerous evhttp bugs also got fixed.
|
|
|
|
7. Infrastructure improvements
|
|
|
|
7.1. Better unit test framework
|
|
|
|
We now use a unit test framework that Nick wrote called "tinytest".
|
|
The main benefit from Libevent's point of view is that tests which
|
|
might mess with global state can all run each in their own
|
|
subprocess. This way, when there's a bug that makes one unit test
|
|
crash or mess up global state, it doesn't affect any others.
|
|
|
|
7.2. Better unit tests
|
|
|
|
Despite all the code we've added, our unit tests are much better than
|
|
before. Right now, iterating over the different backends on various
|
|
platforms, I'm getting between 78% and 81% test coverage, compared
|
|
with less than 45% test coverage in Libevent 1.4.
|
|
|