fix: FusedCodecPipeline falls back to async path for sharded arrays with async-only inner codecs

docs/release-notes.md

@@ -30,6 +30,7 @@
   emits), instead of falling back to the system's native byte order. ([#3417](https://github.com/zarr-developers/zarr-python/issues/3417))
 - Fixed `save_array`, `Group.__setitem__`, and `load` for 0-dimensional arrays. ([#3469](https://github.com/zarr-developers/zarr-python/issues/3469))
 - Fixed inner-codec spec evolution for sharded arrays. The sharding codec now threads the array spec through its inner codec chain when evolving codecs, so a codec that changes the dtype upstream of `BytesCodec` no longer leaves the inner chain evolved against the wrong spec (which previously failed at decode time). This runs on the default `BatchedCodecPipeline` as well. Standard inner chains (`[BytesCodec]`, `[BytesCodec, ZstdCodec]`, transpose + bytes) are byte-identical to before. Restores the behavior of #2179. ([#3885](https://github.com/zarr-developers/zarr-python/issues/3885))
+- Fixed the opt-in `FusedCodecPipeline` for sharded arrays whose inner or index codec chain contains a codec implementing only the async codec interface (no `SupportsSyncCodec`). Such arrays previously raised `TypeError: All codecs must implement SupportsSyncCodec` on both read and write; the pipeline now declines its synchronous fast path for them and falls back to the async path, matching the behavior of the default `BatchedCodecPipeline`. Fully sync-capable codec chains keep the fast path unchanged. ([#4179](https://github.com/zarr-developers/zarr-python/issues/4179))
 - Make chunk normalization properly handle `-1` as a compact representation of the
   length of an entire axis. Reject several previously-accepted but ill-defined
   chunk specifications: `chunks=True` (previously silently produced size-1 chunks),

src/zarr/abc/codec.py

@@ -82,6 +82,20 @@ def _decode_sync(self, chunk_data: CO, chunk_spec: ArraySpec) -> CI: ...
     def _encode_sync(self, chunk_data: CI, chunk_spec: ArraySpec) -> CO | None: ...
 
 
+def _codec_supports_sync(codec: object) -> bool:
+    """Whether `codec` can actually run on a synchronous (no event loop) path.
+
+    Structural membership in `SupportsSyncCodec` is necessary but not always
+    sufficient: a codec can provide `_decode_sync`/`_encode_sync` whose ability
+    to run depends on runtime configuration the type system cannot see.
+    `ShardingCodec` is the canonical case — its sync methods delegate to its
+    configured inner and index codec chains, so they only work when every codec
+    in those chains is itself sync-capable. Such codecs opt out dynamically via
+    a `_sync_capable` attribute/property (absent means capable).
+    """
+    return isinstance(codec, SupportsSyncCodec) and getattr(codec, "_sync_capable", True)
+
+
 class BaseCodec[CI: CodecInput, CO: CodecOutput](Metadata):
     """Generic base class for codecs.
 

src/zarr/codecs/sharding.py

@@ -14,7 +14,7 @@
     ArrayBytesCodecPartialEncodeMixin,
     Codec,
     CodecPipeline,
-    SupportsSyncCodec,
+    _codec_supports_sync,
 )
 from zarr.abc.store import (
     ByteGetter,
@@ -1406,8 +1406,30 @@ def _is_complete_shard_write(
             is_complete_chunk for *_, is_complete_chunk in indexed_chunks
         )
 
+    @property
+    def _sync_capable(self) -> bool:
+        """Dynamic opt-out consulted by `_codec_supports_sync` / `ChunkTransform`.
+
+        This codec structurally satisfies `SupportsSyncCodec`, but every sync
+        method (`_decode_sync`, `_encode_sync`, `_decode_partial_sync`,
+        `_encode_partial_sync`) delegates to the inner and index codec chains
+        through `ChunkTransform`, so it can only run synchronously when every
+        codec in BOTH chains is itself sync-capable. Reporting False here makes
+        `ChunkTransform` construction raise, which in turn makes
+        `FusedCodecPipeline.evolve_from_array_spec` set `sync_transform=None` —
+        the whole pipeline then declines the sync fast path and routes through
+        the async paths (partial shard decode / async fallback write), exactly
+        as it does for an async-only TOP-level codec or a non-sync store.
+        """
+        return self._inner_codecs_sync_capable() and self._index_codecs_sync_capable()
+
+    def _inner_codecs_sync_capable(self) -> bool:
+        # _codec_supports_sync (not bare isinstance) so a nested sharding codec
+        # with an async-only inner chain propagates its opt-out outward.
+        return all(_codec_supports_sync(c) for c in self.codecs)
+
     def _index_codecs_sync_capable(self) -> bool:
-        return all(isinstance(c, SupportsSyncCodec) for c in self.index_codecs)
+        return all(_codec_supports_sync(c) for c in self.index_codecs)
 
     async def _decode_shard_index(
         self, index_bytes: Buffer, chunks_per_shard: tuple[int, ...]

src/zarr/core/chunk_utils.py

@@ -3,7 +3,7 @@
 from dataclasses import dataclass, field
 from typing import TYPE_CHECKING, Any, cast
 
-from zarr.abc.codec import GetResult, SupportsSyncCodec
+from zarr.abc.codec import GetResult, SupportsSyncCodec, _codec_supports_sync
 from zarr.core.indexing import is_scalar
 
 if TYPE_CHECKING:
@@ -240,7 +240,13 @@ class ChunkTransform:
     def __post_init__(self) -> None:
         from zarr.core.codec_pipeline import codecs_from_list
 
-        non_sync = [c for c in self.codecs if not isinstance(c, SupportsSyncCodec)]
+        # _codec_supports_sync, not a bare isinstance check: a codec can satisfy
+        # the SupportsSyncCodec protocol structurally yet be unable to run
+        # synchronously (ShardingCodec whose inner/index chain contains an
+        # async-only codec). Such codecs opt out via `_sync_capable`, and the
+        # TypeError here is what makes FusedCodecPipeline.evolve_from_array_spec
+        # decline the sync fast path and fall back to the async pipeline.
+        non_sync = [c for c in self.codecs if not _codec_supports_sync(c)]
         if non_sync:
             names = ", ".join(type(c).__name__ for c in non_sync)
             raise TypeError(

tests/test_fused_pipeline.py

@@ -8,6 +8,7 @@
 import pytest
 
 import zarr
+from zarr.abc.codec import BytesBytesCodec
 from zarr.codecs.bytes import BytesCodec
 from zarr.codecs.gzip import GzipCodec
 from zarr.codecs.transpose import TransposeCodec
@@ -582,6 +583,93 @@ def spy_write_sync(self: Any, *args: Any, **kwargs: Any) -> Any:
     )
 
 
+# ---------------------------------------------------------------------------
+# Async-only codecs inside a shard's inner codec chain
+# ---------------------------------------------------------------------------
+
+
+class _AsyncOnlyNoopCodec(BytesBytesCodec):  # type: ignore[misc,unused-ignore]
+    """A no-op BB codec implementing ONLY the async codec interface.
+
+    Deliberately does NOT satisfy `SupportsSyncCodec` (no `_decode_sync` /
+    `_encode_sync`), modelling a third-party codec that predates the sync
+    protocol. Class-level counters prove the codec actually ran.
+    """
+
+    is_fixed_size = True
+    encode_calls = 0
+    decode_calls = 0
+
+    def to_dict(self) -> dict[str, Any]:
+        return {"name": "test-async-only-noop", "configuration": {}}
+
+    @classmethod
+    def from_dict(cls, data: dict[str, Any]) -> _AsyncOnlyNoopCodec:
+        return cls()
+
+    def compute_encoded_size(self, input_byte_length: int, _spec: Any) -> int:
+        return input_byte_length
+
+    async def _encode_single(self, chunk_bytes: Any, chunk_spec: Any) -> Any:
+        type(self).encode_calls += 1
+        return chunk_bytes
+
+    async def _decode_single(self, chunk_bytes: Any, chunk_spec: Any) -> Any:
+        type(self).decode_calls += 1
+        return chunk_bytes
+
+
+def test_sharded_roundtrip_with_async_only_inner_codec() -> None:
+    """A sharded array whose INNER codec chain contains an async-only codec
+    round-trips under FusedCodecPipeline (full write, partial write, full read,
+    partial read).
+
+    Regression: the pipeline's top-level guard (evolve_from_array_spec ->
+    sync_transform=None) only inspected the top-level chain. ShardingCodec
+    structurally satisfies SupportsSyncCodec, so a sync transform was built and
+    the sync fast path dove into ShardingCodec's sync shard paths, which raised
+    TypeError from the inner ChunkTransform. The pipeline must instead decline
+    the sync fast path and fall back to the async inner pipeline, like
+    BatchedCodecPipeline.
+    """
+    _AsyncOnlyNoopCodec.encode_calls = 0
+    _AsyncOnlyNoopCodec.decode_calls = 0
+
+    with zarr_config.set({"codec_pipeline.path": "zarr.core.codec_pipeline.FusedCodecPipeline"}):
+        store = MemoryStore()
+        arr = zarr.create_array(
+            store=store,
+            shape=(16, 16),
+            shards=(8, 8),
+            chunks=(4, 4),
+            dtype="int32",
+            compressors=[_AsyncOnlyNoopCodec()],
+            fill_value=-1,
+        )
+        assert isinstance(arr._async_array.codec_pipeline, FusedCodecPipeline)
+
+        data = np.arange(256, dtype="int32").reshape(16, 16)
+        arr[:] = data  # full write
+        np.testing.assert_array_equal(arr[:], data)  # full read
+        np.testing.assert_array_equal(arr[2:11, 3:14], data[2:11, 3:14])  # partial read
+
+        arr[5:7, 5:13] = 0  # partial write (read-merge-write of existing shards)
+        data[5:7, 5:13] = 0
+        np.testing.assert_array_equal(arr[:], data)
+
+    assert _AsyncOnlyNoopCodec.encode_calls > 0, "async-only inner codec never encoded"
+    assert _AsyncOnlyNoopCodec.decode_calls > 0, "async-only inner codec never decoded"
+
+    # The stored bytes are valid for the default pipeline too: read them back
+    # under BatchedCodecPipeline (default codec_pipeline.path). Opening from
+    # metadata needs the codec name in the registry.
+    from zarr.registry import register_codec
+
+    register_codec("test-async-only-noop", _AsyncOnlyNoopCodec)
+    reread = zarr.open_array(store=store, mode="r")
+    np.testing.assert_array_equal(reread[:], data)
+
+
 # ---------------------------------------------------------------------------
 # AsyncChunkTransform: the async per-chunk codec chain used on the async
 # fallback path. It is the async mirror of ChunkTransform, so it must produce