Re: [PATCH] coroutine: add libucontext as external library

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Mon, Mar 08, 2021 at 07:26:36PM -0800, Joelle van Dyne wrote:
> iOS does not support ucontext natively for aarch64 and the sigaltstack is
> also unsupported (even worse, it fails silently, see:
> https://openradar.appspot.com/13002712 )
> 
> As a workaround we include a library implementation of ucontext and add it
> as a build option.
> 
> Signed-off-by: Joelle van Dyne <j@getutm.app>
> ---
>  configure                 | 21 ++++++++++++++++++---
>  meson.build               | 12 +++++++++++-
>  util/coroutine-ucontext.c |  9 +++++++++
>  .gitmodules               |  3 +++
>  MAINTAINERS               |  6 ++++++
>  meson_options.txt         |  2 ++
>  subprojects/libucontext   |  1 +
>  7 files changed, 50 insertions(+), 4 deletions(-)
>  create mode 160000 subprojects/libucontext
> 
> diff --git a/configure b/configure
> index 34fccaa2ba..5f225894a9 100755
> --- a/configure
> +++ b/configure
> @@ -1773,7 +1773,7 @@ Advanced options (experts only):
>    --oss-lib                path to OSS library
>    --cpu=CPU                Build for host CPU [$cpu]
>    --with-coroutine=BACKEND coroutine backend. Supported options:
> -                           ucontext, sigaltstack, windows
> +                           ucontext, libucontext, sigaltstack, windows

This approach mixes the concept of the coroutine backend (ucontext,
sigaltstack, etc) with the optional libucontext library dependency.

libucontext is not a coroutine backend. The patch had to introduce
$coroutine_impl in addition to $coroutine in order to work around this.
Let's avoid combining these two independent concepts into
--with-coroutine=.

I suggest treating libucontext as an optional library dependency in
./configure with explicit --enable-libucontext/--disable-libucontext
options. Most of the time neither option will be provided by the user
and ./configure should automatically decide whether libucontext is
needed or not.

> +case $coroutine in
> +libucontext)
> +  git_submodules="${git_submodules} subprojects/libucontext"
> +  mkdir -p libucontext

Why is this mkdir necessary?

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