Ensure that stream_fetch_values!() yields in its loop

Otherwise it may spin (see comments for details). Also refactored it into a
while-loop instead of using a @goto.

Author: JamesWrigley

Diff for: src/stream-fetchers.jl

@@ -2,26 +2,32 @@ struct RemoteFetcher end
 function stream_fetch_values!(::Type{RemoteFetcher}, T, store_ref::Chunk{Store_remote}, buffer::Blocal, id::UInt) where {Store_remote, Blocal}
     thunk_id = STREAM_THUNK_ID[]
     @dagdebug thunk_id :stream "fetching values"
-    @label fetch_values
-    # FIXME: Pass buffer free space
-    # TODO: It would be ideal if we could wait on store.lock, but get unlocked during migration
-    values = MemPool.access_ref(store_ref.handle, id, T, Store_remote, thunk_id) do store, id, T, Store_remote, thunk_id
-        if !isopen(store)
-            throw(InvalidStateException("Stream is closed", :closed))
-        end
-        @dagdebug thunk_id :stream "trying to fetch values at $(myid())"
-        store::Store_remote
-        in_store = store
-        STREAM_THUNK_ID[] = thunk_id
-        values = T[]
-        while !isempty(store, id)
-            value = take!(store, id)::T
-            push!(values, value)
-        end
-        return values
-    end::Vector{T}
-    if isempty(values)
-        @goto fetch_values
+
+    values = T[]
+    while isempty(values)
+        # FIXME: Pass buffer free space
+        # TODO: It would be ideal if we could wait on store.lock, but get unlocked during migration
+        values = MemPool.access_ref(store_ref.handle, id, T, Store_remote, thunk_id) do store, id, T, Store_remote, thunk_id
+            if !isopen(store)
+                throw(InvalidStateException("Stream is closed", :closed))
+            end
+            @dagdebug thunk_id :stream "trying to fetch values at $(myid())"
+            store::Store_remote
+            in_store = store
+            STREAM_THUNK_ID[] = thunk_id
+            values = T[]
+            while !isempty(store, id)
+                value = take!(store, id)::T
+                push!(values, value)
+            end
+            return values
+        end::Vector{T}
+
+        # We explicitly yield in the loop to allow other tasks to run. This
+        # matters on single-threaded instances because MemPool.access_ref()
+        # might not yield when accessing data locally, which can cause this loop
+        # to spin forever.
+        yield()
     end
 
     @dagdebug thunk_id :stream "fetched $(length(values)) values"