push_outputs ended with
else pending_ = std::move(result);
and the retry path calls it as push_outputs(std::move(*pending_), …), so on
that path `result` is the parked tuple itself. The assignment was a
self-move-assignment. std::tuple's is elementwise, and libstdc++'s
std::vector does not guard against self-move: _M_move_assign swaps its data
into a temporary, which is then destroyed. The vector ends up empty.
So the first park was clean — the argument there is a local temporary — and
the second erased the payload. The value was still delivered, still in
order, still counted, just empty. Downstream cannot distinguish that from a
frame on which the node genuinely found nothing, which is why it would never
surface as an error: in scene-actor-extraction it reads as "no faces in this
frame" and the run completes with a quietly wrong answer.
Scope, stated precisely because I first got it wrong: this needs a node with
*two or more* outputs. With one output the only thing that resubmits a
parked node is that output's own space callback, which by definition fires
when there is room, so the retry always succeeds and never reassigns. With
two, output A draining resubmits the node while output B is still full — the
retry skips A (already delivered, tracked in pending_done_) and fails on B,
and that is the reassignment that eats B's payload.
Every node in the scene-actor-extraction pipeline currently has exactly one
output, and the fanout is a separate class that does not use pending_, so
this is latent there rather than active. It is reachable by any multi-output
node under backpressure, which the library supports and documents.
Verified in both directions: on 6a4f45f the parked payload arrives with size
0; here it arrives intact. The test drives raw channels rather than consumer
nodes so each step is forced rather than raced, and both channels are
capacity 1 — Channel fires the space callback only on the full->not-full
edge, so a roomy channel A would never resubmit the node and the retry would
never happen at all.
a8cfe73 appended queued/wake_pending/total_exec_ms to the NodeSnapshot
aggregate in an order no call site used. Every node type fills the aggregate
positionally and all of them supply total_exec_ms as the element straight
after queue_wait_ms — but the struct declared the two bools there. So the
exec total landed in `queued`, `queued` landed in `wake_pending`, and
`wake_pending` landed in total_exec_ms.
GCC reported it as -Wnarrowing (bool to double), 88 times, once per node
instantiation across the test build. The build carried on.
This is worse than a cosmetic mix-up, because all three fields were added
specifically to diagnose a wedge. A wedged pipeline reported total_exec_ms
as 0.0 or 1.0, and `queued` as "has this node ever run" — true for every
node that had, including ones asleep with nothing to do. The web debug JSON
served the same values. Anyone reading them to find the stalled node would
have been pointed at the wrong one.
Moves total_exec_ms above the two bools to match every call site, and notes
in the struct why the order is load-bearing.
Verified in both directions: on a8cfe73 the new case reports frames=8
total=0 queued=true; here total >= ema and both flags are false.
push_blocking parked a scheduler worker inside the push. Nodes own a
private single-thread pool, so the parked thread was the only one that
could drain that node's own input — hold-and-wait, and under sustained
backpressure four nodes of a five-node chain slept in nanosleep at once.
channel.hpp already warned about this for sentinels; it applies just as
much to data pushes.
The scheduler was purely input-driven: on_input_ready() wakes a node when
input arrives, with no counterpart for "my output has room". Lacking that
signal, blocking the thread was the only way to handle a full output.
This adds the missing half.
- Channel::try_push + has_space + set_space_callback; the callback fires
from both pop() and try_pop_now().
- PoolNode/PoolObjectNode keep a one-slot pending_ buffer with per-element
done flags, so a retry cannot duplicate an already-accepted element. One
slot suffices because queued_ admits at most one fire_once per node.
- The re-check after clearing queued_ closes the lost-wakeup race where a
space callback fires while the flag is still up and is swallowed.
Two bugs surfaced once nodes actually parked, both fixed here:
- pop_one reports an *empty* channel as ChannelClosedError, which is also
the node's "upstream finished, self-stop" signal. A node woken by output
space with empty inputs therefore killed itself. fire_once now releases
the worker when its inputs are not ready rather than falling through.
- The drained-park path resubmitted unconditionally instead of via
on_input_ready(), firing nodes with nothing to read.
compute_priority is now output-aware: mean output fill is deducted from
mean input fill, mapped as 0.5·(1 + in - out). Input fill alone asks only
"how much work is waiting for me"; a node whose outputs are already full
cannot deliver, so running it just parks it again and wastes the slot
while the node that would drain that channel waits behind it. The
scheduler now favours whoever is furthest downstream of a bottleneck.
Also adds a network-level error listener. A node's exception was discarded
at the node boundary and survived only as a Closed event, which reports
that a node stopped but not why — that missing detail is what made the
above slow to diagnose. INode::set_network_error_callback plus
StaticNetwork::set_error_handler forward it to the application.
Tests: 121/121. test_backpressure_deadlock drives a five-node chain with
capacity-2 channels against a slow sink and fails on the old code. The
four test_pool_node overflow tests now assert parking rather than the
removed drop-and-report behaviour.
Known-incomplete: a rare hang remains, roughly 1 run in 20 against a 300s
timeout, down from every run failing. Committed because the fix is a large
strict improvement and the residual case needs its own reproduction.
dtourolle