Specialize remotecall_pool(remotecall) to wait for the remotecall

Otherwise the worker would prematurely be put back into the pool, causing
oversubscription. Also added a warning about oversubscription to the docstring
for `remote_do(f, ::AbstractWorkerPool)`.

docs/src/_changelog.md

@@ -11,6 +11,10 @@ This documents notable changes in DistributedNext.jl. The format is based on
 
 ### Fixed
 - Fixed a cause of potential hangs when exiting the process ([#16]).
+- Fixed a subtle bug in `remotecall(f, ::AbstractWorkerPool)`, previously the
+  implementation would take a worker out of the pool and immediately put it back
+  in without waiting for the returned [`Future`](@ref). Now it will wait for the
+  `Future` before putting the worker back in the pool ([#20]).
 
 ### Added
 - A watcher mechanism has been added to detect when both the Distributed stdlib

src/workerpool.jl

@@ -135,6 +135,22 @@ function remotecall_pool(rc_f, f, pool::AbstractWorkerPool, args...; kwargs...)
     end
 end
 
+# Specialization for remotecall. We have to wait for the Future it returns
+# before putting the worker back in the pool.
+function remotecall_pool(rc_f::typeof(remotecall), f, pool::AbstractWorkerPool, args...; kwargs...)
+    worker = take!(pool)
+    x = rc_f(f, worker, args...; kwargs...)
+
+    t = Threads.@spawn try
+        wait(x)
+    finally
+        put!(pool, worker)
+    end
+    errormonitor(t)
+
+    return x
+end
+
 # Check if pool is local or remote and forward calls if required.
 # NOTE: remotecall_fetch does it automatically, but this will be more efficient as
 # it avoids the overhead associated with a local remotecall.
@@ -242,6 +258,10 @@ remotecall_fetch(f, pool::AbstractWorkerPool, args...; kwargs...) = remotecall_p
 
 [`WorkerPool`](@ref) variant of `remote_do(f, pid, ....)`. Wait for and take a free worker from `pool` and
 perform a `remote_do` on it.
+
+Note that it's not possible to wait for the result of a `remote_do()` to finish
+so the worker will immediately be put back in the pool (i.e. potentially causing
+oversubscription).
 """
 remote_do(f, pool::AbstractWorkerPool, args...; kwargs...) = remotecall_pool(remote_do, f, pool, args...; kwargs...)
 

test/distributed_exec.jl

@@ -1014,6 +1014,19 @@ f16091b = () -> 1
         @test_throws RemoteException fetch(ref)
     end
 
+    # Test the behaviour of remotecall(f, ::AbstractWorkerPool), this should
+    # keep the worker out of the pool until the underlying remotecall has
+    # finished.
+    remotechan = RemoteChannel(wrkr1)
+    pool = WorkerPool([wrkr1])
+    put_future = remotecall(() -> wait(remotechan), pool)
+    @test !isready(pool)
+    put!(remotechan, 1)
+    wait(put_future)
+    # The task that waits on the future to put it back into the pool runs
+    # asynchronously so we use timedwait() to check when the worker is back in.
+    @test timedwait(() -> isready(pool), 10) === :ok
+
     # Test calling @everywhere from a module not defined on the workers
     LocalBar.bar()
     for p in procs()