[Distributed] Worker local race condition between put! and fetch for Futures (JuliaLang#42339)

* add local_lock to Future, use it in fetch and put!
* add corrections to the remote/clientref logic
* add memory ordering guarantees
* serialize a (unlocked) copy of the future to avoid problems with the lock

Co-authored-by: Jameson Nash <vtjnash@gmail.com>
Co-authored-by: Takafumi Arakaki <aka.tkf@gmail.com>

Author: 3 people

stdlib/Distributed/src/remotecall.jl

@@ -26,12 +26,13 @@ mutable struct Future <: AbstractRemoteRef
     where::Int
     whence::Int
     id::Int
-    v::Union{Some{Any}, Nothing}
+    lock::ReentrantLock
+    @atomic v::Union{Some{Any}, Nothing}
 
     Future(w::Int, rrid::RRID, v::Union{Some, Nothing}=nothing) =
-        (r = new(w,rrid.whence,rrid.id,v); return test_existing_ref(r))
+        (r = new(w,rrid.whence,rrid.id,ReentrantLock(),v); return test_existing_ref(r))
 
-    Future(t::NTuple{4, Any}) = new(t[1],t[2],t[3],t[4])  # Useful for creating dummy, zeroed-out instances
+    Future(t::NTuple{4, Any}) = new(t[1],t[2],t[3],ReentrantLock(),t[4])  # Useful for creating dummy, zeroed-out instances
 end
 
 """
@@ -69,10 +70,17 @@ function test_existing_ref(r::AbstractRemoteRef)
     found = getkey(client_refs, r, nothing)
     if found !== nothing
         @assert r.where > 0
-        if isa(r, Future) && found.v === nothing && r.v !== nothing
-            # we have recd the value from another source, probably a deserialized ref, send a del_client message
-            send_del_client(r)
-            found.v = r.v
+        if isa(r, Future)
+            # this is only for copying the reference from Future to RemoteRef (just created)
+            fv_cache = @atomic :acquire found.v
+            rv_cache = @atomic :monotonic r.v
+            if fv_cache === nothing && rv_cache !== nothing
+                # we have recd the value from another source, probably a deserialized ref, send a del_client message
+                send_del_client(r)
+                @lock found.lock begin
+                    @atomicreplace found.v nothing => rv_cache
+                end
+            end
         end
         return found::typeof(r)
     end
@@ -91,8 +99,9 @@ function finalize_ref(r::AbstractRemoteRef)
                     send_del_client_no_lock(r)
                 else
                     # send_del_client only if the reference has not been set
-                    r.v === nothing && send_del_client_no_lock(r)
-                    r.v = nothing
+                    v_cache = @atomic :monotonic r.v
+                    v_cache === nothing && send_del_client_no_lock(r)
+                    @atomic :monotonic r.v = nothing
                 end
                 r.where = 0
             finally
@@ -201,7 +210,8 @@ isready(f)  # will not block
 ```
 """
 function isready(rr::Future)
-    rr.v === nothing || return true
+    v_cache = @atomic rr.v
+    v_cache === nothing || return true
 
     rid = remoteref_id(rr)
     return if rr.where == myid()
@@ -354,26 +364,33 @@ end
 
 channel_type(rr::RemoteChannel{T}) where {T} = T
 
-serialize(s::ClusterSerializer, f::Future) = serialize(s, f, f.v === nothing)
-serialize(s::ClusterSerializer, rr::RemoteChannel) = serialize(s, rr, true)
-function serialize(s::ClusterSerializer, rr::AbstractRemoteRef, addclient)
-    if addclient
+function serialize(s::ClusterSerializer, f::Future)
+    v_cache = @atomic f.v
+    if v_cache === nothing
         p = worker_id_from_socket(s.io)
-        (p !== rr.where) && send_add_client(rr, p)
+        (p !== f.where) && send_add_client(f, p)
     end
+    fc = Future((f.where, f.whence, f.id, v_cache)) # copy to be used for serialization (contains a reset lock)
+    invoke(serialize, Tuple{ClusterSerializer, Any}, s, fc)
+end
+
+function serialize(s::ClusterSerializer, rr::RemoteChannel)
+    p = worker_id_from_socket(s.io)
+    (p !== rr.where) && send_add_client(rr, p)
     invoke(serialize, Tuple{ClusterSerializer, Any}, s, rr)
 end
 
 function deserialize(s::ClusterSerializer, t::Type{<:Future})
-    f = invoke(deserialize, Tuple{ClusterSerializer, DataType}, s, t)
-    f2 = Future(f.where, RRID(f.whence, f.id), f.v) # ctor adds to client_refs table
+    fc = invoke(deserialize, Tuple{ClusterSerializer, DataType}, s, t) # deserialized copy
+    f2 = Future(fc.where, RRID(fc.whence, fc.id), fc.v) # ctor adds to client_refs table
 
     # 1) send_add_client() is not executed when the ref is being serialized
     #    to where it exists, hence do it here.
     # 2) If we have received a 'fetch'ed Future or if the Future ctor found an
     #    already 'fetch'ed instance in client_refs (Issue #25847), we should not
     #    track it in the backing RemoteValue store.
-    if f2.where == myid() && f2.v === nothing
+    f2v_cache = @atomic f2.v
+    if f2.where == myid() && f2v_cache === nothing
         add_client(remoteref_id(f2), myid())
     end
     f2
@@ -567,7 +584,7 @@ end
 
 Wait for a value to become available for the specified [`Future`](@ref).
 """
-wait(r::Future) = (r.v !== nothing && return r; call_on_owner(wait_ref, r, myid()); r)
+wait(r::Future) = (v_cache = @atomic r.v; v_cache !== nothing && return r; call_on_owner(wait_ref, r, myid()); r)
 
 """
     wait(r::RemoteChannel, args...)
@@ -584,11 +601,41 @@ Further calls to `fetch` on the same reference return the cached value. If the r
 is an exception, throws a [`RemoteException`](@ref) which captures the remote exception and backtrace.
 """
 function fetch(r::Future)
-    r.v !== nothing && return something(r.v)
-    v = call_on_owner(fetch_ref, r)
-    r.v = Some(v)
+    v_cache = @atomic r.v
+    v_cache !== nothing && return something(v_cache)
+
+    if r.where == myid()
+        rv, v_cache = @lock r.lock begin
+            v_cache = @atomic :monotonic r.v
+            rv = v_cache === nothing ? lookup_ref(remoteref_id(r)) : nothing
+            rv, v_cache
+        end
+
+        if v_cache !== nothing
+            return something(v_cache)
+        else
+            v_local = fetch(rv.c)
+        end
+    else
+        v_local = call_on_owner(fetch_ref, r)
+    end
+
+    v_cache = @atomic r.v
+
+    if v_cache === nothing # call_on_owner case
+        v_old, status = @lock r.lock begin
+            @atomicreplace r.v nothing => Some(v_local)
+        end
+        # status == true - when value obtained through call_on_owner
+        # status == false - any other situation: atomicreplace fails, because by the time the lock is obtained cache will be populated
+        # why? local put! performs caching and putting into channel under r.lock
+
+        # for local put! use the cached value, for call_on_owner cases just take the v_local as it was just cached in r.v
+        v_cache = status ? v_local : v_old
+    end
+
     send_del_client(r)
-    v
+    something(v_cache)
 end
 
 fetch_ref(rid, args...) = fetch(lookup_ref(rid).c, args...)
@@ -612,12 +659,30 @@ A `put!` on an already set `Future` throws an `Exception`.
 All asynchronous remote calls return `Future`s and set the
 value to the return value of the call upon completion.
 """
-function put!(rr::Future, v)
-    rr.v !== nothing && error("Future can be set only once")
-    call_on_owner(put_future, rr, v, myid())
-    rr.v = Some(v)
-    rr
+function put!(r::Future, v)
+    if r.where == myid()
+        rid = remoteref_id(r)
+        rv = lookup_ref(rid)
+        isready(rv) && error("Future can be set only once")
+        @lock r.lock begin
+            put!(rv, v) # this notifies the tasks waiting on the channel in fetch
+            set_future_cache(r, v) # set the cache before leaving the lock, so that the notified tasks already see it cached
+        end
+        del_client(rid, myid())
+    else
+        @lock r.lock begin # same idea as above if there were any local tasks fetching on this Future
+            call_on_owner(put_future, r, v, myid())
+            set_future_cache(r, v)
+        end
+    end
+    r
 end
+
+function set_future_cache(r::Future, v)
+    _, ok = @atomicreplace r.v nothing => Some(v)
+    ok || error("internal consistency error detected for Future")
+end
+
 function put_future(rid, v, caller)
     rv = lookup_ref(rid)
     isready(rv) && error("Future can be set only once")

stdlib/Distributed/test/distributed_exec.jl

@@ -350,6 +350,9 @@ function test_regular_io_ser(ref::Distributed.AbstractRemoteRef)
         v = getfield(ref2, fld)
         if isa(v, Number)
             @test v === zero(typeof(v))
+        elseif fld == :lock
+            @test v isa ReentrantLock
+            @test !islocked(v)
         elseif v !== nothing
             error(string("Add test for field ", fld))
         end