Use libpthread primitives for OpenBSD mutexes. #82392
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There has been some discussion as to whether this is the right approach, centered around internal pointers in the underlying mutex structure. After much thought, let's commit this for now. The argument: * While it is true that OpenBSD has futex(2) and in fact is used in the underlying implementations in Dispatch. I'm not confident right now creating robust locking primitives by hand from futexes. It would be ideal to just delegate all of this to Dispatch, but Dispatch doesn't expose a ready-use API for mutexes. Cobbling one together might be possible, but that would take some time and finesse. * While recent versions of C and C++ have language support for mutexes, they are implemented in many cases by libpthread anyway, so there is nothing inherently gained right now by not using the pthread API other than portability. * While the concern for internal pointers for the pthread API is important, the OpenBSD is mitigated by the fact that pthread_mutex_t is pointer-valued on the platform. The concern that this might not always be the case is mitigated by the fact that this commit does not attempt to use the pthread implementation as a catch-all for all platforms. * The alternative is to use MutexUnavailable, but given Foundation has some dependencies on Mutex, this may cause problems elsewhere throughout the port.
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@swift-ci please test. |
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@swift-ci please test. |
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Ping. |
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Ping. |
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The thing you lose out on by using |
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Yes, I think that's fine. Most importantly, I'm concerned with not having an implementation upstream at all, which means Swift won't build on the platform. We can revisit and change to using futex later. |
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Ping. Any other concerns with this? |
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(Please merge on my behalf when ready, I don't have permissions. Thank you!) |
There has been some discussion as to whether this is the right approach, centered around internal pointers in the underlying mutex structure. After much thought, let's commit this for now. The argument:
While it is true that OpenBSD has futex(2) and in fact is used in the underlying implementations in Dispatch. I'm not confident right now creating robust locking primitives by hand from futexes. It would be ideal to just delegate all of this to Dispatch, but Dispatch doesn't expose a ready-use API for mutexes. Cobbling one together might be possible, but that would take some time and finesse.
While recent versions of C and C++ have language support for mutexes, they are implemented in many cases by libpthread anyway, so there is nothing inherently gained right now by not using the pthread API other than portability.
While the concern for internal pointers for the pthread API is important, the OpenBSD is mitigated by the fact that pthread_mutex_t is pointer-valued on the platform. The concern that this might not always be the case is mitigated by the fact that this commit does not attempt to use the pthread implementation as a catch-all for all platforms.
The alternative is to use MutexUnavailable, but given Foundation has some dependencies on Mutex, this may cause problems elsewhere throughout the port.