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Implement Condition.waitTimeout #31278

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andrewrk merged 3 commits from lukasl/zig:condition-timeout into master 2026年04月17日 19:02:43 +02:00
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I'd have preferred if vtable.futexWait returned error.Timeout, since all the OS-level APIs provide it. However, if I keep the vtable untouched, I had to determine the timeout case by post-checking the deadline. It's fine functionally, but one extra syscall that be avoided at cost of changing the vtable and all the futex implementations.

Don't worry, I've used no LLM for this, I took the time to hand-write this.

I'd have preferred if `vtable.futexWait` returned `error.Timeout`, since all the OS-level APIs provide it. However, if I keep the vtable untouched, I had to determine the timeout case by post-checking the deadline. It's fine functionally, but one extra syscall that be avoided at cost of changing the vtable and all the futex implementations. Don't worry, I've used no LLM for this, I took the time to hand-write this.

I'd have preferred if vtable.futexWait returned error.Timeout, since all the OS-level APIs provide it. However, if I keep the vtable untouched, I had to determine the timeout case by post-checking the deadline. It's fine functionally, but one extra syscall that be avoided at cost of changing the vtable and all the futex implementations.

@mlugg can I get your opinion on this? If I recall correctly, we chose the current futexWait API (without error.Timeout) together, but my memory is fuzzy on the reasoning there. I'm inclined to agree with @lukasl here and make that change along with merging this patch.

> I'd have preferred if `vtable.futexWait` returned `error.Timeout`, since all the OS-level APIs provide it. However, if I keep the vtable untouched, I had to determine the timeout case by post-checking the deadline. It's fine functionally, but one extra syscall that be avoided at cost of changing the vtable and all the futex implementations. @mlugg can I get your opinion on this? If I recall correctly, we chose the current `futexWait` API (without `error.Timeout`) together, but my memory is fuzzy on the reasoning there. I'm inclined to agree with @lukasl here and make that change along with merging this patch.
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I believe the problem is that error.Timeout cannot be reliable in either direction:

  • We cannot guarantee precision of timing with these primitives, because OSes may provide an uncustomisable or unspecified clock, so "false positive" timeouts are possible.
  • Spurious wakeups are allowed, which may race with the timeout, so the timeout might have elapsed when you see a spurious wake-up, resulting in a "false negative" timeout. This could mean you would never see error.Timeout even if the condition you are waiting for is never met.

My memory is fuzzy, but I believe we actually discussed Condition.waitTimeout in particular and decided that we didn't see a clear use case for it, but I could be misremembering.

I would suggest taking a look through the commit history, because I think I probably discussed this in a commit message? I say that because I'm pretty sure error.Timeout actually was possible at one point, but we agreed to remove it.

I believe the problem is that `error.Timeout` cannot be reliable in either direction: * We cannot guarantee precision of timing with these primitives, because OSes may provide an uncustomisable or unspecified clock, so "false positive" timeouts are possible. * Spurious wakeups are allowed, which may race with the timeout, so the timeout might have elapsed when you see a spurious wake-up, resulting in a "false negative" timeout. **This could mean you would never see `error.Timeout` even if the condition you are waiting for is never met.** My memory is fuzzy, but I believe we actually discussed `Condition.waitTimeout` in particular and decided that we didn't see a clear use case for it, but I could be misremembering. I would suggest taking a look through the commit history, because I think I probably discussed this in a commit message? I say that because I'm pretty sure `error.Timeout` actually *was* possible at one point, but we agreed to remove it.
andrewrk force-pushed condition-timeout from c47559b4c2
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2026年04月16日 23:53:20 +02:00 Compare

My memory is fuzzy, but I believe we actually discussed Condition.waitTimeout in particular and decided that we didn't see a clear use case for it, but I could be misremembering.

To be clear, you're talking about specifically there not being a use case for error.Timeout, right? Because there's clearly a use case for waiting on a Condition with a timeout.

> My memory is fuzzy, but I believe we actually discussed `Condition.waitTimeout` in particular and decided that we didn't see a clear use case for it, but I could be misremembering. To be clear, you're talking about specifically there not being a use case for `error.Timeout`, right? Because there's clearly a use case for waiting on a Condition with a timeout.

I think having error.Timeout on Condition.waitTimeout makes sense because as implemented, even if futexes have spurious wakeups, it has the property that if it returns successfully, a signal is consumed, whereas if it returns with error.Canceled or error.Timeout , a signal was not consumed.

I think having `error.Timeout` on `Condition.waitTimeout` makes sense because as implemented, even if futexes have spurious wakeups, it has the property that if it returns successfully, a signal is consumed, whereas if it returns with `error.Canceled` or `error.Timeout` , a signal was not consumed.
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