From 86d526a0648a884dd966b52142c620f2f5180c28 Mon Sep 17 00:00:00 2001 From: Nick Mathewson Date: Fri, 23 Jan 2009 01:04:10 +0000 Subject: [PATCH] Start writing more of whatsnew-xx.txt, which will become whatsnew-2.0.txt svn:r1041 --- whatsnew-xx.txt | 175 ++++++++++++++++++++++++++++++++++++++++++++---- 1 file changed, 163 insertions(+), 12 deletions(-) diff --git a/whatsnew-xx.txt b/whatsnew-xx.txt index f6a9750d..c7290426 100644 --- a/whatsnew-xx.txt +++ b/whatsnew-xx.txt @@ -1,23 +1,84 @@ -What's New In Libevent SVN: +What's New In Libevent 2.0 so far: 0. About this document This document describes the key differences between Libevent 1.4 and - Libevent ???, from a user's point of view. It was most recently + Libevent 2.0, from a user's point of view. It was most recently updated based on features in subversion trunk as of 27 Dec 2007. - NOTE 1: As of this writing, we haven't decided whether the trunk - version of svn will turn into a 1.5 or 2.0. - - NOTE 2: If any features or fixes get backported from trunk to 1.4, + NOTE 1: If any features or fixes get backported from trunk to 1.4, they should get moved from here into whatsnew-14.txt, since they will no longer be differences between 1.4 and this version. -1. Packaging Issues. - 2. New and Improved APIs -2.1. Overrideable allocation functions +2.1. New header layout for improved compatibility + + Libevent 2.0 has a new header layout to make it easier for programmers to + write good, well-supported libevent code. The new headers are divided + into three types. + + There are *regular headers*, like event2/event.h. These headers contain + the functions that most programmers will want to use. + + There are *backward compatibility headers*, like event2/event_compat.h. + These headers contain declarations for deprecated functions from older + versions of Libevent. Documentation in these headers should suggest what + functions you want to start using instead of the old ones. New programs + should generally not include these headers. + + Finally, there are *structure headers*, like event2/event_struct.h. + These headers contain definitions of some structures that Libevent has + historically exposed. Exposing them caused problems in the past, since + programs that were compiled to work with one version of libevent would + often stop working with another version that changed the size of layout + of some object. We've moving them into separate headers so that + programmers can know that their code is not depending on any unstable + aspect of the Libvent ABI. New programs should generally not include + these headers unless they really know what they are doing, and are + willing to rebuild their software whenever they want to link it against a + new version of libevent. + + Functionality that once was located in event.h is now more subdivided. + The core event logic is now in event2/event.h. The "evbuffer" functions + for low-level buffer manipulation are in event2/buffer.h. The + "bufferevent" functions for higher-level buffered IO are in + event2/bufferevent.h. + + All of the old headers (event.h, evdns.h, evhttp.h, evrpc.h, and + evutil.h) will continue to work by including the corresponding new + headers. Old code should not be broken by this change. + +2.2. New thread-safe, binary-compatibile APIs + + Some aspects of the historical Libevent API have encouraged + non-threadsafe code, or forced code built against one version of Libevent + to no longer build with another. The problems with now-deprecated APIs + fell into two categories: + + 1) Dependence on the "current" event_base. In an application with + multiple event_bases, Libevent previously had a notion of the + "current" event_base. New events were linked to use this base, and + the caller needed to explicitly reattach them to another base. + This was horribly error-prone. + + Functions like "event_set" that worked with the "current" are now + deprecated but still available (see 2.1). There are new functions + like "event_assign" that take an explicit event_base argument when + setting up a structure. Using these functions will help prevent + errors in your applications, and to be more threadsafe. + + 2) Structure dependence. Applications needed to allocate 'struct + event' themselves, since there was no function in Libevent to do it + for them. But since the size and contents of struct event can + change between libevent versions, this created binary-compatibility + nightmares. All structures of this kind are now isolated in + _struct.h header (see 2.1), and there are new allocate-and- + initialize functions you can use instead of the old initialize-only + functions. For example, instead of malloc and event_set, you + can use event_new(). + +2.3. Overrideable allocation functions If you want to override the allocation functions used by libevent (for example, to use a specialized allocator, or debug memory @@ -25,7 +86,42 @@ What's New In Libevent SVN: event_set_mem_functions. It takes replacements for malloc(), free(), and realloc(). -2.2. More flexible readline support +2.X. Configurable event_base creation + + Older versions of Libevent would always got the fastest backend + available, unless you reconfigured their behavior with the environment + variables EVENT_NOSELECT, EVENT_NOPOLL, and so forth. This was annoying + to programmers who wanted to pick a backend explicitly without messing + with the environment. + + Also, despite our best efforts, not every backend supports every + operation we might like. Some features (like edge-triggered events, or + working with non-socket file descriptors) only work with some operating + systems' fast backends. Previously, programmers who cared about this + needed to know which backends supported what. This tended to get quite + ungainly. + + There is now an API to choose backends, either by name or by feature. + Here is an example: + + struct event_config_t *config; + struct event_base *base; + + /* Create a new configuration object. */ + config = event_config_new(); + /* We don't want to use the "select" method. */ + event_config_avoid_method(config, "select"); + /* We want a method that can work with non-socket file descriptors */ + event_config_require_features(config, EV_FEATURE_FDS); + + base = event_base_new_with_config(config); + if (!base) { + /* There is no backend method that does what we want. */ + exit(1); + } + event_config_free(config); + +2.4. More flexible readline support The old evbuffer_readline() function (which accepted any sequence of CR and LF characters as a newline, and which couldn't handle lines @@ -34,15 +130,70 @@ What's New In Libevent SVN: line-ending styles, and which can return the number of characters in the line returned. -2.3. Socket is now an abstract type +2.5. Socket is now an abstract type All APIs that formerly accepted int as a socket type now accept "evutil_socket_t". On Unix, this is just an alias for "int" as before. On Windows, however, it's an alias for SOCKET, which can be wider than int on 64-bit platforms. +2.6. Timeouts and persistent events work together. + + Previously, it wasn't useful to set a timeout on a persistent event: + the timeout would trigger once, and never again. This is not what + applications tend to want. Instead, applications tend to want every + triggering of the event to re-set the timeout. So now, if you set + up an event like this: + XXXX + +2.X. kqueue event ordering consistency + +2.X. Multiple events allowed per fd + + Older versions of Libevent allowed at most one EV_READ event and at most + one EV_WRITE event per socket, per event base. This restriction is no + longer present. + +2.X. evthread_* functions for thread-safe structures. + + Libevent structures can now be built with locking support. You can + enable this on a per-event-base level by writing functions to implement + mutexes and thread IDs, and passing them to evthread_set_locking_callback + and evthread_set_id_callback. This makes it safe to add, remove, + and activate events on an event base from a different thread. + + If you want threading support and you're using pthreads, you can just + call evthread_use_pthreads(). (You'll need to link against the + libevent_pthreads library in addition to libevent.) + + If you want threading support and you're using Windows, you can just + call evthread_use_windows_threads(). + +2.X. bufferevent_setfd/cb + +2.X. Bufferevent IO filters (????) + +2.X. Edge-triggered events on some backends. + 3. Big bugfixes +3.X. Win32 bufferevents work + 4. Big performance improvements -5. Removed code and features +4.X. Faster windows backend(s) + +4.X. Faster evbuffer implementation + +4.X. Generic notify support + +5. Extras improvements + +5.X. DNS: IPv6 nameservers + +5.X. DNS: 0x20 hack support + +5.X. DNS: Better security. + + +6. Removed/Deprecated code and features -- 2.40.0