Fix build on systems with ucontext.h implemented outside the libc

[ismaell]

Description

Check for the libucontext package. The ucontext.h API may be implemented outside the libc.

Without this patch the build seems to succeed but results in under-linking.

• - git commit message contains an appropriate signed-off-by string (see CONTRIBUTING.md for details)

Type of change

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• bug fix - change that fixes an issue
• new feature - change that adds functionality
• tests - change in tests
• infrastructure - change in infrastructure and CI
• documentation - documentation update

Tests

• added - required for new features and some bug fixes
• not needed

Documentation

• updated in # - add PR number
• needs to be updated
• not needed

Breaks backward compatibility

• Yes
• No
• Unknown

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Other information

[pavelkumbrasev]

Hi @ismaell, we have thread-based implementation of resumable tasks. What do you think if we enable such implementation in case where ucontext is absented? (Use libc or thread-based)

https://github.com/oneapi-src/oneTBB/blob/f2af74732ba7b303f23a2f453051a52a63560a75/src/tbb/co_context.h#LL27C5-L27C38

[ismaell]

Ah, that's even better.

[pavelkumbrasev]

Could I ask to do such change in your patch? You simply need to define this macro - __TBB_RESUMABLE_TASKS_USE_THREADS in cmake if ucontext not present.
Also, please add the copyright to cmake file you can copy it from our cmake files.

[pavelkumbrasev]

@ismaell do you have a chance to update PR with proposed changes or we should create separate PR for this?

[brad0]

Could I ask to do such change in your patch? You simply need to define this macro - __TBB_RESUMABLE_TASKS_USE_THREADS in cmake if ucontext not present. Also, please add the copyright to cmake file you can copy it from our cmake files.

Please do. I ran into this trying to build TBB on OpenBSD, which does not have ucontext. Also I see Haiku does not as well.

I tried building mold which has TBB embedded with CFLAGS / CXXFLAGS with __TBB_RESUMABLE_TASKS_USE_THREADS defined and it now built Ok.

To me with how long ucontext has been removed from POSIX, I don't know why TBB doesn't just use the thread-based code path.

#795

[brad0]

Could I ask to do such change in your patch? You simply need to define this macro - __TBB_RESUMABLE_TASKS_USE_THREADS in cmake if ucontext not present. Also, please add the copyright to cmake file you can copy it from our cmake files.

@pavelkumbrasev Can you please try to come up with a diff that checks for ucontext and enables the thread-based implementation if it does not exist? I can help test if you would like.

[pavelkumbrasev]

@brad0 Basically, you could submit a separate PR that implements proposed approach since there is no progress in this PR.
Are you ok with this?

[pavelkumbrasev]

Closing it since #1055 was merged.