fix(generate): avoid None entries in merged logits_processors

[BLuchterhand]

Bug

PromptProcessingBatch.extend filled missing per-slot logits_processors with [None] * N. Merging an unconfigured batch with a processor-equipped batch produced a list shaped [None, ..., [fn], ...]. GenerationBatch._step (line 1346) iterates self.logits_processors[e] under the any() guard at line 1337, which raises TypeError: 'NoneType' object is not iterable on the None slots.

Reproduce

a = PromptProcessingBatch.empty(model, fallback)
a.uids = [0]; a.logits_processors = []
b = PromptProcessingBatch.empty(model, fallback)
b.uids = [1]; b.logits_processors = [make_logits_processors({0: 2000.0})]
a.extend(b)
# a.logits_processors == [None, [<fn>]]   ← later crashes _step

Fix

-            self.logits_processors = [None] * len(self.uids)
+            self.logits_processors = [[]] * len(self.uids)
...
-            else [None] * len(batch.uids)
+            else [[]] * len(batch.uids)

Per-slot type is List[Callable], so the absent-value sentinel should be [], not None. Matches the existing [[]] * len(keep) at line 1120 (PromptProcessingBatch.filter).

Test

test_prompt_processing_batch_extend_mixes_logits_processors in tests/test_generate.py. Asserts no None entries remain in the merged list. Fails on main (AssertionError: None is not an instance of <class 'list'>), passes with the fix.

Related

#1225 fixes the symmetric stale-length bug in GenerationBatch.filter. This is situated in a different code path so both can land independently.

Traceback (production hit)

File "mlx_lm/generate.py", line 1346, in _step
    for processor in self.logits_processors[e]:
TypeError: 'NoneType' object is not iterable

[mloiterman]

+1 — independent production hit, batched-inference server, mlx-lm 0.31.3.

Our workload mixes grammar-mode requests (RAG hybrid search using response_format=json_schema) with plain chat requests, both arriving concurrently and batched together by BatchGenerator. The mixed batch is exactly the shape your repro produces: at least one request with logits_processors and at least one without. Single-workload runs (all-grammar or all-plain) don't trip it.

Live traceback at batch_size=8:

File "mlx_lm/generate.py", line 1346, in _step
    for processor in self.logits_processors[e]:
TypeError: 'NoneType' object is not iterable

The diagnosis matches yours exactly. The supervisor in our setup catches the crash via BrokenPipeError, respawns the engine subprocess (~60 s), and in-flight requests return 500 — but that's our restart machinery doing its job, not a real recovery. Without the patch the bug is fatal to any heterogeneous batched workload.

We're applying your diff to our local .venv as a stopgap (mirrors how we're running #1225's patch today, since neither has merged yet). Vouching for the fix shape: [] is the right sentinel for List[Callable], matching the existing [[]] * len(keep) in PromptProcessingBatch.filter:1120.

Validated locally: a regression probe that fires a long plain request followed by a grammar request ~300 ms later (so the grammar request extend()s the existing plain batch) reliably FAILS with 500 {"error":"Generation failed"} on the un-patched engine, and PASSES once the engine respawns onto the patched code.

Related: #1225 lands the symmetric fix in GenerationBatch.filter. Different code paths, same sentinel argument — both are safe to land independently.