`process_validation_metrics()` crashes on None-filled sparse `reward_extra_info` keys (metrics emitted for only some samples)

System Info

Summary

When a reward_extra_info key is emitted for only some validation samples (a sparse key), main_ppo_sync.PPOTrainer._validate null-fills it to a uniform schema, but process_validation_metrics then crashes computing np.mean([..., None, ...]) over the per-uid group. Any reward function that conditionally adds an extra-info key makes validation crash as soon as one sample omits it.

Environment

• verl 0.8.0 (I confirmed this is an issue in the latest repo version as well, except some files moved around)
• Trainer: verl.trainer.main_ppo_sync.PPOTrainer (sync / TransferQueue path)
• trainer.validation_data_dir set (validation generation dump enabled)

Information

• The official example scripts
• My own modified scripts

Tasks

• An officially supported task in the examples folder (such as GLUE/SQuAD, ...)
• My own task or dataset (give details below)

Reproduction

Root cause

The two halves are individually reasonable but incompatible:

1. Null-fill on dump. _validate pads sparse keys with None so every sample shares one schema:

   # main_ppo_sync.py (~L939-945)
   for key in reward_extra_infos_dict:
       if key != "reward" and key not in reward_extra_info:
           reward_extra_infos_dict[key].append(None)
   for key, value in reward_extra_info.items():
       if key not in reward_extra_infos_dict:
           reward_extra_infos_dict[key] = [None] * n_prior
       reward_extra_infos_dict[key].append(value)
   

2. Aggregation can't tolerate that None. _val_metrics_update -> process_validation_metrics groups values per uid and means each group, but its skip-filter only catches empty lists and strings:

   # metric_utils.py (~L638-646)
   for var_name, var_vals in var2vals.items():
       if not var_vals or isinstance(var_vals[0], str):   # does NOT skip [None]
           continue
       n_resps = len(var_vals)
       metric = {f"mean@{n_resps}": float(np_mean(var_vals))}   # np.mean([None]) -> TypeError
   

A uid whose only rollout omitted the key has var_vals == [None], so:

TypeError: unsupported operand type(s) for /: 'NoneType' and 'int'

Reproduction

A reward function that emits an extra-info key for only some samples:

reward_extra_info = {"reward": r, "parse_success": ok}
if ok:
    reward_extra_info["cp"] = engine_eval   # absent when ok is False

Run validation with at least one sample where ok is False. The dump ({step}.jsonl) is written fine (the missing cells show null), then _val_metrics_update raises.

Minimal trigger for the aggregation bug itself:

import numpy as np
np.mean([None])          # TypeError
np.mean([35.0, None])    # TypeError

Why current workarounds are unsatisfying

• A numeric sentinel for the missing case skews the mean; nan propagates and turns the whole aggregate into nan (verl uses np.mean, not np.nanmean).
• Forcing every sample to emit every key defeats the purpose of optional/diagnostic metrics, and the codebase clearly intends sparse keys, since _validate deliberately null-fills them for the dump.

Reproduction Code

from verl.trainer.ppo.metric_utils import process_validation_metrics

# Three validation prompts, one rollout each (n=1). Two parse-success samples that
# emitted a `cp` metric, and one parse-fail sample that did NOT emit `cp`.
data_sources = ["chess", "chess", "chess"]
sample_uids = ["uid_a", "uid_b", "uid_c"]

# This is exactly what `_validate` hands to `process_validation_metrics` AFTER its
# null-fill (main_ppo_sync.py ~L939-945): every key spans all samples, and the
# sample that never emitted `cp` is padded with None.
infos_dict = {
    "reward": [0.9, 0.1, 0.0],   # always present
    "cp":     [120, -30, None],  # sparse: None for the parse-fail sample (verl-inserted)
}

print("infos_dict =", infos_dict)
print("Calling process_validation_metrics ...\n")

result = process_validation_metrics(data_sources, sample_uids, infos_dict)

# If verl is fixed, we reach here. The `cp` mean should be computed over the
# samples that actually emitted it (120 and -30 -> mean 45), ignoring None.
print("No crash. Result:")
for ds, var2metric in result.items():
    for var, metrics in var2metric.items():
        print(f"  {ds}/{var}: {metrics}")

Expected behavior

Suggested Fix

Make process_validation_metrics treat None as "absent" instead of a value, e.g. drop None entries per group before the mean (and skip groups that become empty):

var_vals = [v for v in var_vals if v is not None]
if not var_vals or isinstance(var_vals[0], str):
    continue

This keeps the dump behavior (sparse keys still appear with null cells) while computing each key's mean only over the samples that actually emitted it. Alternatively, gate the null-fill so sparse keys are excluded from metric aggregation entirely.