Condition variables are subtle and error-prone, and this example
demonstrates exactly the sorts of problems that they introduce.
Unfortunately, we're stuck with them for the foreseeable future.
As previously implemented, this example was racy: since the callback
passed to context.AfterFunc did not lock the mutex before calling
Broadcast, it was possible for the Broadcast to occur before the
goroutine was parked in the call to Wait, causing in a missed wakeup
resulting in deadlock.
The example also had a more insidious problem: it was not safe for
multiple goroutines to call waitOnCond concurrently, but the whole
point of using a sync.Cond is generally to synchronize concurrent
goroutines. waitOnCond must use Broadcast to ensure that it wakes up
the target goroutine, but the use of Broadcast in this way would
produce spurious wakeups for all of the other goroutines waiting on
the same condition variable. Since waitOnCond did not recheck the
condition in a loop, those spurious wakeups would cause waitOnCond
to spuriously return even if its own ctx was not yet done.
Fixing the aforementioned bugs exposes a final problem, inherent to
the use of condition variables in this way. This one is a performance
problem: for N concurrent calls to waitOnCond, the resulting CPU cost
is at least O(N²). This problem cannot be addressed without either
reintroducing one of the above bugs or abandoning sync.Cond in the
example entirely. Given that this example was already published in Go
1.21, I worry that Go users may think that it is appropriate to use a
sync.Cond in conjunction with context.AfterFunc, so I have chosen to
retain the Cond-based example and document its pitfalls instead of
removing or replacing it entirely.
I described this class of bugs and performance issues — and suggested
some channel-based alternatives — in my GopherCon 2018 talk,
“Rethinking Classical Concurrency Patterns”. The section on condition
variables starts on slide 37. (https://youtu.be/5zXAHh5tJqQ?t=679)
Fixes#62180.
For #20491.
Change-Id: If987cd9d112997c56171a7ef4fccadb360bb79bc
Reviewed-on: https://go-review.googlesource.com/c/go/+/521596
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
Add an AfterFunc function, which registers a function to run after
a context has been canceled.
Add support for contexts that implement an AfterFunc method, which
can be used to avoid the need to start a new goroutine watching
the Done channel when propagating cancellation signals.
Fixes#57928
Change-Id: If0b2cdcc4332961276a1ff57311338e74916259c
Reviewed-on: https://go-review.googlesource.com/c/go/+/482695
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Damien Neil <dneil@google.com>
Reviewed-by: Sameer Ajmani <sameer@golang.org>
Tests in package context cannot depend directly on package testing due to an import cycle.
We resolved this by having test functions in package context_test (x_test.go) forward to
test functions in package context (context_test.go). This is fragile, since it's easy
to add a test to context_test.go and forget to add the forwarding function, and tests
written in this way cannot easily use testing package features like t.Run for subtests.
It turns out that only four test functions actually use unexported members of package
context. This CL moves all except those four to x_test.go and makes them regular tests.
It also updates TestCause to use t.Run and t.Parallel to parallelize its test cases.
It also adds documentation indicating when tests should be added to each file.
Change-Id: Ic60bae32a7a44e07831b5388c9af219d53ba9af3
Reviewed-on: https://go-review.googlesource.com/c/go/+/480375
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Sameer Ajmani <sameer@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
ExampleWithDeadline and ExampleWithTimeout used an arbitrary 1-second
timeout for a “blocked” select case, which could fail if the test
goroutine happens to be descheduled for over a second, or perhaps if
an NTP synchronization happens to jump by a second at just the right
time.
Either case is plausible, especially on a heavily-loaded or slow
machine (as is often the case for builders for unusual ports).
Instead of an arbitrary timeout, use a “ready” channel that is never
actually ready.
Fixes#57594.
Change-Id: I9ff68f50b041a3382e7b267c28c5259e886a9d23
Reviewed-on: https://go-review.googlesource.com/c/go/+/460999
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Sameer Ajmani <sameer@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
In CL 223019, I reduced the short timeout in the testLayers helper to
be even shorter than it was. That exposed a racy (time-dependent)
select later in the function, which failed in one of the slower
builders (android-386-emu).
Also streamline the test to make it easier to test with a very high -count flag:
- Run tests that sleep for shortDuration in parallel to reduce latency.
- Use shorter durations in examples to reduce test running time.
- Avoid mutating global state (in package math/rand) in testLayers.
After this change (but not before it),
'go test -run=TestLayersTimeout -count=100000 context' passes on my workstation.
Fixes#38161
Change-Id: Iaf4abe7ac308b2100d8828267cda9f4f8ae4be82
Reviewed-on: https://go-review.googlesource.com/c/go/+/226457
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Though there is variation in the spelling of canceled,
cancellation is always spelled with a double l.
Reference: https://www.grammarly.com/blog/canceled-vs-cancelled/
Change-Id: I240f1a297776c8e27e74f3eca566d2bc4c856f2f
Reviewed-on: https://go-review.googlesource.com/c/go/+/170060
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Bryan C. Mills
Damien Neil
Sameer Ajmani
Josh Bleecher Snyder
Ioannis Georgoulas
Kunpei Sakai
Jaana Burcu Dogan
Carlos C