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diff --git a/sntp/libevent/whatsnew-2.0.txt b/sntp/libevent/whatsnew-2.0.txt new file mode 100644 index 000000000000..3561fcb94c6f --- /dev/null +++ b/sntp/libevent/whatsnew-2.0.txt @@ -0,0 +1,609 @@ +What's New In Libevent 2.0 so far: + +1. Meta-issues + +1.1. About this document + + This document describes the key differences between Libevent 1.4 and + Libevent 2.0, from a user's point of view. It was most recently + updated based on features in git master as of August 2010. + + NOTE: I am very sure that I missed some thing on this list. Caveat + haxxor. + +1.2. Better documentation + + There is now a book-in-progress that explains how to use Libevent and its + growing pile of APIs. As of this writing, it covers everything except the + http and rpc code. Check out the latest draft at + http://www.wangafu.net/~nickm/libevent-book/ . + +2. New and Improved Event APIs + + Many APIs are improved, refactored, or deprecated in Libevent 2.0. + + COMPATIBILITY: + + Nearly all existing code that worked with Libevent 1.4 should still + work correctly with Libevent 2.0. However, if you are writing new code, + or if you want to port old code, we strongly recommend using the new APIs + and avoiding deprecated APIs as much as possible. + + Binaries linked against Libevent 1.4 will need to be recompiled to link + against Libevent 2.0. This is nothing new; we have never been good at + preserving binary compatibility between releases. We'll try harder in the + future, though: see 2.1 below. + +2.1. New header layout for improved forward-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's + wrong with the old functions, and what functions you want to start using + instead of the old ones. Some of these functions might be removed in a + future release. 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 or + 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, are + willing to rebuild their software whenever they want to link it + against a new version of Libevent, and are willing to risk their code + breaking if and when data structures change. + + 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. + + COMPATIBILITY: + + 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-compatible, harder-to-mess-up 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 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" event_base + 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(). + + (For people who do really want to allocate a struct event on the + stack, or put one inside another structure, you can still use + event2/event_compat.h.) + + So in the case where old code would look like this: + + #include <event.h> + ... + struct event *ev = malloc(sizeof(struct event)); + /* This call will cause a buffer overrun if you compile with one version + of Libevent and link dynamically against another. */ + event_set(ev, fd, EV_READ, cb, NULL); + /* If you forget this call, your code will break in hard-to-diagnose + ways in the presence of multiple event bases. */ + event_set_base(ev, base); + + New code will look more like this: + + #include <event2/event.h> + ... + struct event *ev; + ev = event_new(base, fd, EV_READ, cb, NULL); + +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 + issues, or so on), you can replace them by calling + event_set_mem_functions. It takes replacements for malloc(), + free(), and realloc(). + + If you're going to use this facility, you need to call it _before_ + Libevent does any memory allocation; otherwise, Libevent may allocate some + memory with malloc(), and free it with the free() function you provide. + + You can disable this feature when you are building Libevent by passing + the --disable-malloc-replacement argument to configure. + +2.4. 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); + + Supported features are documented in event2/event.h + +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: + struct event *ev; + struct timeval tv; + ev = event_new(base, fd, EV_READ|EV_PERSIST, cb, NULL); + tv.tv_sec = 1; + tv.tv_usec = 0; + event_add(ev, &tv); + + The callback 'cb' will be invoked whenever fd is ready to read, OR whenever + a second has passed since the last invocation of cb. + +2.7. 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. + +2.8. evthread_* functions for thread-safe structures. + + Libevent structures can now be built with locking support. This code + makes it safe to add, remove, and activate events on an event base from a + different thread. (Previously, if you wanted to write multithreaded code + with Libevent, you could only an event_base or its events in one thread at + a time.) + + 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_core. These functions are + not in libevent_core.) + + If you want threading support and you're using Windows, you can just + call evthread_use_windows_threads(). + + If you are using some locking system besides Windows and pthreads, You + can enable this on a per-event-base level by writing functions to + implement mutexes, conditions, and thread IDs, and passing them to + evthread_set_lock_callbacks and related functions in event2/thread.h. + + Once locking functions are enabled, every new event_base is created with a + lock. You can prevent a single event_base from being built with a lock + disabled by using the EVENT_BASE_FLAG_NOLOCK flag in its + event_config. If an event_base is created with a lock, it is safe to call + event_del, event_add, and event_active on its events from any thread. The + event callbacks themselves are still all executed from the thread running + the event loop. + + To make an evbuffer or a bufferevent object threadsafe, call its + *_enable_locking() function. + + The HTTP api is not currently threadsafe. + + To build Libevent with threading support disabled, pass + --disable-thread-support to the configure script. + +2.9. Edge-triggered events on some backends. + + With some backends, it's now possible to add the EV_ET flag to an event + in order to request that the event's semantics be edge-triggered. Right + now, epoll and kqueue support this. + + The corresponding event_config feature is EV_FEATURE_ET; see 2.4 for more + information. + +2.10. Better support for huge numbers of timeouts + + The heap-based priority queue timer implementation for Libevent 1.4 is good + for randomly distributed timeouts, but suboptimal if you have huge numbers + of timeouts that all expire in the same amount of time after their + creation. The new event_base_init_common_timeout() logic lets you signal + that a given timeout interval will be very common, and should use a linked + list implementation instead of a priority queue. + +2.11. Improved debugging support + + It's been pretty easy to forget to delete all your events before you + re-initialize them, or otherwise put Libevent in an internally inconsistent + state. You can tell libevent to catch these and other common errors with + the new event_enable_debug_mode() call. Just invoke it before you do + any calls to other libevent functions, and it'll catch many common + event-level errors in your code. + +2.12. Functions to access all event fields + + So that you don't have to access the struct event fields directly, Libevent + now provides accessor functions to retrieve everything from an event that + you set during event_new() or event_assign(). + +3. Backend-specific and performance improvements. + +3.1. Change-minimization on O(1) backends + + With previous versions of Libevent, if you called event_del() and + event_add() repeatedly on a single event between trips to the backend's + dispatch function, the backend might wind up making unnecessary calls or + passing unnecessary data to the kernel. The new backend logic batches up + redundant adds and deletes, and performs no more operations than necessary + at the kernel level. + + This logic is on for the kqueue backend, and available (but off by + default) for the epoll backend. To turn it on for the epoll backend, + set the EVENT_BASE_FLAG_EPOLL_USE_CHANGELIST flag in the + event_base_cofig, or set the EVENT_EPOLL_USE_CHANGELIST environment + variable. Doing this with epoll may result in weird bugs if you give + any fds closed by dup() or its variants. + +3.2. Improved notification on Linux + + When we need to wake the event loop up from another thread, we use + an epollfd to do so, instead of a socketpair. This is supposed to be + faster. + +3.3. Windows: better support for everything + + Bufferevents on Windows can use a new mechanism (off-by-default; see below) + to send their data via Windows overlapped IO and get their notifications + via the IOCP API. This should be much faster than using event-based + notification. + + Other functions throughout the code have been fixed to work more + consistently with Windows. Libevent now builds on Windows using either + mingw, or using MSVC (with nmake). Libevent works fine with UNICODE + defined, or not. + + Data structures are a little smarter: our lookups from socket to pending + event are now done with O(1) hash tables rather than O(lg n) red-black + trees. + + Unfortunately, the main Windows backend is still select()-based: from + testing the IOCP backends on the mailing list, it seems that there isn't + actually a way to tell for certain whether a socket is writable with IOCP. + Libevent 2.1 may add a multithreaded WaitForMultipleEvents-based + backend for better performance with many inactive sockets and better + integration with Windows events. + +4. Improvements to evbuffers + + Libevent has long had an "evbuffer" implementation to wrap access to an + input or output memory buffer. In previous versions, the implementation + was very inefficient and lacked some desirable features. We've made many + improvements in Libevent 2.0. + +4.1. Chunked-memory internal representation + + Previously, each evbuffer was a huge chunk of memory. When we ran out of + space in an evbuffer, we used realloc() to grow the chunk of memory. When + data was misaligned, we used memmove to move the data back to the front + of the buffer. + + Needless to say, this is a terrible interface for networked IO. + + Now, evbuffers are implemented as a linked list of memory chunks, like + most Unix kernels use for network IO. (See Linux's skbuf interfaces, + or *BSD's mbufs). Data is added at the end of the linked list and + removed from the front, so that we don't ever need realloc huge chunks + or memmove the whole buffer contents. + + To avoid excessive calls to read and write, we use the readv/writev + interfaces (or WSASend/WSARecv on Windows) to do IO on multiple chunks at + once with a single system call. + + COMPATIBILITY NOTE: + The evbuffer struct is no longer exposed in a header. The code here is + too volatile to expose an official evbuffer structure, and there was never + any means provided to create an evbuffer except via evbuffer_new which + heap-allocated the buffer. + + If you need access to the whole buffer as a linear chunk of memory, the + EVBUFFER_DATA() function still works. Watch out, though: it needs to copy + the buffer's contents in a linear chunk before you can use it. + +4.2. 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 + containing NUL characters), is now deprecated. The preferred + function is evbuffer_readln(), which supports a variety of + line-ending styles, and which can return the number of characters in + the line returned. + + You can also call evbuffer_search_eol() to find the end of a line + in an evbuffer without ever extracting the line. + +4.3. Support for file-based IO in evbuffers. + + You can now add chunks of a file into a evbuffer, and Libevent will have + your OS use mapped-memory functionality, sendfile, or splice to transfer + the data without ever copying it to userspace. On OSs where this is not + supported, Libevent just loads the data. + + There are probably some bugs remaining in this code. On some platforms + (like Windows), it just reads the relevant parts of the file into RAM. + +4.4. Support for zero-copy ("scatter/gather") writes in evbuffers. + + You can add a piece of memory to an evbuffer without copying it. + Instead, Libevent adds a new element to the evbuffer's linked list of + chunks with a pointer to the memory you supplied. You can do this + either with a reference-counted chunk (via evbuffer_add_reference), or + by asking Libevent for a pointer to its internal vectors (via + evbuffer_reserve_space or evbuffer_peek()). + +4.5. Multiple callbacks per evbuffer + + Previously, you could only have one callback active on an evbuffer at a + time. In practice, this meant that if one part of Libevent was using an + evbuffer callback to notice when an internal evbuffer was reading or + writing data, you couldn't have your own callback on that evbuffer. + + Now, you can now use the evbuffer_add_cb() function to add a callback that + does not interfere with any other callbacks. + + The evbuffer_setcb() function is now deprecated. + +4.6. New callback interface + + Previously, evbuffer callbacks were invoked with the old size of the + buffer and the new size of the buffer. This interface could not capture + operations that simultaneously filled _and_ drained a buffer, or handle + cases where we needed to postpone callbacks until multiple operations were + complete. + + Callbacks that are set with evbuffer_setcb still use the old API. + Callbacks added with evbuffer_add_cb() use a new interface that takes a + pointer to a struct holding the total number of bytes drained read and the + total number of bytes written. See event2/buffer.h for full details. + +4.7. Misc new evbuffer features + + You can use evbuffer_remove() to move a given number of bytes from one + buffer to another. + + The evbuffer_search() function lets you search for repeated instances of + a pattern inside an evbuffer. + + You can use evbuffer_freeze() to temporarily suspend drains from or adds + to a given evbuffer. This is useful for code that exposes an evbuffer as + part of its public API, but wants users to treat it as a pure source or + sink. + + There's an evbuffer_copyout() that looks at the data at the start of an + evbuffer without doing a drain. + + You can have an evbuffer defer all of its callbacks, so that rather than + being invoked immediately when the evbuffer's length changes, they are + invoked from within the event_loop. This is useful when you have a + complex set of callbacks that can change the length of other evbuffers, + and you want to avoid having them recurse and overflow your stack. + +5. Bufferevents improvements + + Libevent has long included a "bufferevents" structure and related + functions that were useful for generic buffered IO on a TCP connection. + This is what Libevent uses for its HTTP implementation. In addition to + the improvements that they get for free from the underlying evbuffer + implementation above, there are many new features in Libevent 2.0's + evbuffers. + +5.1. New OO implementations + + The "bufferevent" structure is now an abstract base type with multiple + implementations. This should not break existing code, which always + allocated bufferevents with bufferevent_new(). + + Current implementations of the bufferevent interface are described below. + +5.2. bufferevent_socket_new() replaces bufferevent_new() + + Since bufferevents that use a socket are not the only kind, + bufferevent_new() is now deprecated. Use bufferevent_socket_new() + instead. + +5.3. Filtered bufferevent IO + + You can use bufferevent_filter_new() to create a bufferevent that wraps + around another bufferevent and transforms data it is sending and + receiving. See test/regress_zlib.c for a toy example that uses zlib to + compress data before sending it over a bufferevent. + +5.3. Linked pairs of bufferevents + + 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. + |