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Event lib support as it has been isn't scaling well, at the low level libevent and libev headers have a namespace conflict so they can't both be built into the same image, and at the distro level, binding all the event libs to libwebsockets.so makes a bloaty situation for packaging, lws will drag in all the event libs every time. This patch implements the plan discussed here https://github.com/warmcat/libwebsockets/issues/1980 and refactors the event lib support so they are built into isolated plugins and bound at runtime according to what the application says it wants to use. The event lib plugins can be packaged individually so that only the needed sets of support are installed (perhaps none of them if the user code is OK with the default poll() loop). And dependent user code can mark the specific event loop plugin package as required so pieces are added as needed. The eventlib-foreign example is also refactored to build the selected lib support isolated. A readme is added detailing the changes and how to use them. https://libwebsockets.org/git/libwebsockets/tree/READMEs/README.event-libs.md
81 lines
3.2 KiB
Markdown
81 lines
3.2 KiB
Markdown
# lws event library support
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## v4.0 and below
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Before v4.1, lws allowed selecting some event library support for inclusion
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in the libwebsockets library
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Option|Feature
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---|---
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`LWS_WITH_GLIB`|glib
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`LWS_WITH_LIBEVENT`|libevent
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`LWS_WITH_LIBUV`|libuv
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`LWS_WITH_LIBEV`|libev
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The user code can select by `info->options` flags at runtime which event loop
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it wants to use.
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The only restriction is that libev and libevent can't coexist, because their
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header namespace conflicts.
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## v4.1 and above
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Lws continues to support the old way described above, but there's an additional
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new cmake option that decides how they are built if any are selected,
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`LWS_WITH_EVLIB_PLUGINS`.
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The old behaviour is set by `LWS_WITH_EVLIB_PLUGINS=0`, for UNIX platforms, this
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is set to 1 by default. This causes the enabled event lib support to each be built into
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its own dynamically linked plugin, and lws will bring in the requested support alone
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at runtime after seeing the `info->options` flags requested by the user code.
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This has two main benefits, first the conflict around building libevent and libev
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together is removed, they each build isolated in their own plugin; the libwebsockets
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core library build doesn't import any of their headers (see below for exception).
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And second, for distro packaging, the event lib support plugins can be separately
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packaged, and apps take dependencies on the specific event lib plugin package, which
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itself depends on the libwebsockets core library. This allows just the needed
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dependencies for the packageset without forcing everything to bring everything in.
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Separately, lws itself has some optional dependencies on libuv, if you build lwsws
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or on Windows you want plugins at all. CMake will detect these situations and
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select to link the lws library itself to libuv if so as well, independent of whatever
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is happening with the event lib support.
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## evlib plugin install
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The produced plugins are named
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event lib|plugin name
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---|---
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glib|`libwebsockets-evlib_glib.so`
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event|`libwebsockets-evlib_event.so`
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uv|`libwebsockets-evlib_uv.so`
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ev|`libwebsockets-evlib_ev.so`
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The evlib plugins are installed alongside libwebsockets.so/.a into the configured
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library dir, it's often `/usr/local/lib/` by default on linux.
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Lws looks for them at runtime using the build-time-configured path.
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## Component packaging
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The canonical package name is `libwebsockets`, the recommended way to split the
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packaging is put the expected libs and pkgconfig in `libwebsockets` or `libwebsockets-core`,
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the latter is followed by the provided cmake, and produce an additional package per build
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event library plugin, named, eg `libwebsockets-evlib_glib`, which has a dependency on
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`libwebsockets[-core]`.
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Applications that use the default event loop can directly require `libwebsockets[-core]`,
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and application packages that need specific event loop support can just require, eg,
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`libwebsockets-evlib_glib`, which will bring that in and the core lws pieces in one step.
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There is then no problem with multiple apps requiring different event libs, they will
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bring in all the necessary pieces which will not conflict either as packages or at
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runtime.
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## `LWS_WITH_DISTRO_RECOMMENDED`
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The cmake helper config `LWS_WITH_DISTRO_RECOMMENDED` is adapted to build all the
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event libs with the event lib plugin support enabled.
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