Add timeout parameter to wait(::Condition)

base/condition.jl

@@ -125,20 +125,104 @@ proceeding.
 """
 function wait end
 
+# wait with timeout
+#
+# The behavior of wait changes if a timeout is specified. There are
+# three concurrent entities that can interact:
+# 1. Task W: the task that calls wait w/timeout.
+# 2. Task T: the task created to handle a timeout.
+# 3. Task N: the task that notifies the Condition being waited on.
+#
+# Typical flow:
+# - W enters the Condition's wait queue.
+# - W creates T and stops running (calls wait()).
+# - T, when scheduled, waits on a Timer.
+# - Two common outcomes:
+#   - N notifies the Condition.
+#     - W starts running, closes the Timer, sets waiter_left and returns
+#       the notify'ed value.
+#     - The closed Timer throws an EOFError to T which simply ends.
+#   - The Timer expires.
+#     - T starts running and locks the Condition.
+#     - T confirms that waiter_left is unset and that W is still in the
+#       Condition's wait queue; it then removes W from the wait queue,
+#       sets dosched to true and unlocks the Condition.
+#     - If dosched is true, T schedules W with the special :timed_out
+#       value.
+#     - T ends.
+#     - W runs and returns :timed_out.
+#
+# Some possible interleavings:
+# - N notifies the Condition but the Timer expires and T starts running
+#   before W:
+#   - W closing the expired Timer is benign.
+#   - T will find that W is no longer in the Condition's wait queue
+#     (which is protected by a lock) and will not schedule W.
+# - N notifies the Condition; W runs and calls wait on the Condition
+#   again before the Timer expires:
+#   - W sets waiter_left before leaving. When T runs, it will find that
+#     waiter_left is set and will not schedule W.
+#
+# The lock on the Condition's wait queue and waiter_left together
+# ensure proper synchronization and behavior of the tasks involved.
+
 """
-    wait(c::GenericCondition; first::Bool=false)
+    wait(c::GenericCondition; first::Bool=false, timeout::Real=0.0)
 
 Wait for [`notify`](@ref) on `c` and return the `val` parameter passed to `notify`.
 
 If the keyword `first` is set to `true`, the waiter will be put _first_
 in line to wake up on `notify`. Otherwise, `wait` has first-in-first-out (FIFO) behavior.
+
+If `timeout` is specified, cancel the `wait` when it expires and return
+`:timed_out`. The minimum value for `timeout` is 0.001 seconds, i.e. 1
+millisecond.
 """
-function wait(c::GenericCondition; first::Bool=false)
+function wait(c::GenericCondition; first::Bool=false, timeout::Real=0.0)
+    timeout == 0.0 || timeout ≥ 1e-3 || throw(ArgumentError("timeout must be ≥ 1 millisecond"))
+
     ct = current_task()
     _wait2(c, ct, first)
     token = unlockall(c.lock)
+
+    timer::Union{Timer, Nothing} = nothing
+    waiter_left::Union{Threads.Atomic{Bool}, Nothing} = nothing
+    if timeout > 0.0
+        timer = Timer(timeout)
+        waiter_left = Threads.Atomic{Bool}(false)
+        # start a task to wait on the timer
+        t = Task() do
+            try
+                wait(timer)
+            catch e
+                # if the timer was closed, the waiting task has been scheduled; do nothing
+                e isa EOFError && return
+            end
+            dosched = false
+            lock(c.lock)
+            # Confirm that the waiting task is still in the wait queue and remove it. If
+            # the task is not in the wait queue, it must have been notified already so we
+            # don't do anything here.
+            if !waiter_left[] && ct.queue == c.waitq
+                dosched = true
+                Base.list_deletefirst!(c.waitq, ct)
+            end
+            unlock(c.lock)
+            # send the waiting task a timeout
+            dosched && schedule(ct, :timed_out)
+        end
+        t.sticky = false
+        Threads._spawn_set_thrpool(t, :interactive)
+        schedule(t)
+    end
+
     try
-        return wait()
+        res = wait()
+        if timer !== nothing
+            close(timer)
+            waiter_left[] = true
+        end
+        return res
     catch
         q = ct.queue; q === nothing || Base.list_deletefirst!(q::IntrusiveLinkedList{Task}, ct)
         rethrow()

test/channels.jl

@@ -1,6 +1,7 @@
 # This file is a part of Julia. License is MIT: https://julialang.org/license
 
 using Random
+using Base.Threads
 using Base: Experimental
 using Base: n_avail
 
@@ -39,6 +40,22 @@ end
     @test fetch(t) == "finished"
 end
 
+@testset "timed wait on Condition" begin
+    a = Threads.Condition()
+    @test_throws ArgumentError @lock a wait(a; timeout=0.0005)
+    @test @lock a wait(a; timeout=0.1)==:timed_out
+    lock(a)
+    @spawn begin
+        @lock a notify(a)
+    end
+    @test try
+        wait(a; timeout=2)
+        true
+    finally
+        unlock(a)
+    end
+end
+
 @testset "various constructors" begin
     c = Channel()
     @test eltype(c) == Any