Make Timer(f, ...) tasks match the stickiness of the parent task. Add spawn kwarg. (#56745)

NEWS.md

@@ -71,6 +71,8 @@ New library features
 * `replace(string, pattern...)` now supports an optional `IO` argument to
   write the output to a stream rather than returning a string ([#48625]).
 * `startswith` now supports seekable `IO` streams ([#43055]).
+* `Timer(f, ...)` will now match the stickiness of the parent task when creating timer tasks, which can be overridden
+  by the new `spawn` kwarg. This avoids the issue where sticky tasks i.e. `@async` make their parent sticky ([#56745])
 
 Standard library changes
 ------------------------

base/asyncevent.jl

@@ -268,7 +268,7 @@ end
 
 # timer with repeated callback
 """
-    Timer(callback::Function, delay; interval = 0)
+    Timer(callback::Function, delay; interval = 0, spawn::Union{Nothing,Bool}=nothing)
 
 Create a timer that runs the function `callback` at each timer expiration.
 
@@ -278,6 +278,13 @@ callback is only run once. The function `callback` is called with a single argum
 itself. Stop a timer by calling `close`. The `callback` may still be run one final time, if the timer
 has already expired.
 
+If `spawn` is `true`, the created task will be spawned, meaning that it will be allowed
+to move thread, which avoids the side-effect of forcing the parent task to get stuck to the thread
+it is on. If `spawn` is `nothing` (default), the task will be spawned if the parent task isn't sticky.
+
+!!! compat "Julia 1.12"
+    The `spawn` argument was introduced in Julia 1.12.
+
 # Examples
 
 Here the first number is printed after a delay of two seconds, then the following numbers are
@@ -297,8 +304,9 @@ julia> begin
 3
 ```
 """
-function Timer(cb::Function, timeout::Real; interval::Real=0.0)
-    timer = Timer(timeout, interval=interval)
+function Timer(cb::Function, timeout; spawn::Union{Nothing,Bool}=nothing, kwargs...)
+    sticky = spawn === nothing ? current_task().sticky : !spawn
+    timer = Timer(timeout; kwargs...)
     t = @task begin
         unpreserve_handle(timer)
         while _trywait(timer)
@@ -312,6 +320,7 @@ function Timer(cb::Function, timeout::Real; interval::Real=0.0)
             isopen(timer) || return
         end
     end
+    t.sticky = sticky
     # here we are mimicking parts of _trywait, in coordination with task `t`
     preserve_handle(timer)
     @lock timer.cond begin

test/channels.jl

@@ -555,8 +555,11 @@ end
 # make sure 1-shot timers work
 let a = []
     Timer(t -> push!(a, 1), 0.01, interval = 0)
-    sleep(0.2)
-    @test a == [1]
+    @test timedwait(() -> a == [1], 10) === :ok
+end
+let a = []
+    Timer(t -> push!(a, 1), 0.01, interval = 0, spawn = true)
+    @test timedwait(() -> a == [1], 10) === :ok
 end
 
 # make sure that we don't accidentally create a one-shot timer