fix: Special-case suffix requests in obstore backend to support Azure (#2994)

* Special-case suffix requests in obstore backend

Co-authored-by: Davis Bennett <davis.v.bennett@gmail.com>

Author: kylebarron

src/zarr/storage/_obstore.py

@@ -106,10 +106,25 @@ async def get(
                 )
                 return prototype.buffer.from_bytes(await resp.bytes_async())  # type: ignore[arg-type]
             elif isinstance(byte_range, SuffixByteRequest):
-                resp = await obs.get_async(
-                    self.store, key, options={"range": {"suffix": byte_range.suffix}}
-                )
-                return prototype.buffer.from_bytes(await resp.bytes_async())  # type: ignore[arg-type]
+                # some object stores (Azure) don't support suffix requests. In this
+                # case, our workaround is to first get the length of the object and then
+                # manually request the byte range at the end.
+                try:
+                    resp = await obs.get_async(
+                        self.store, key, options={"range": {"suffix": byte_range.suffix}}
+                    )
+                    return prototype.buffer.from_bytes(await resp.bytes_async())  # type: ignore[arg-type]
+                except obs.exceptions.NotSupportedError:
+                    head_resp = await obs.head_async(self.store, key)
+                    file_size = head_resp["size"]
+                    suffix_len = byte_range.suffix
+                    buffer = await obs.get_range_async(
+                        self.store,
+                        key,
+                        start=file_size - suffix_len,
+                        length=suffix_len,
+                    )
+                    return prototype.buffer.from_bytes(buffer)  # type: ignore[arg-type]
             else:
                 raise ValueError(f"Unexpected byte_range, got {byte_range}")
         except _ALLOWED_EXCEPTIONS:
@@ -265,10 +280,29 @@ class _OtherRequest(TypedDict):
     path: str
     """The path to request from."""
 
-    range: OffsetRange | SuffixRange | None
+    range: OffsetRange | None
+    # Note: suffix requests are handled separately because some object stores (Azure)
+    # don't support them
     """The range request type."""
 
 
+class _SuffixRequest(TypedDict):
+    """Offset or suffix range requests.
+
+    These requests cannot be concurrent on the Rust side, and each need their own call
+    to `obstore.get_async`, passing in the `range` parameter.
+    """
+
+    original_request_index: int
+    """The positional index in the original key_ranges input"""
+
+    path: str
+    """The path to request from."""
+
+    range: SuffixRange
+    """The suffix range."""
+
+
 class _Response(TypedDict):
     """A response buffer associated with the original index that it should be restored to."""
 
@@ -317,7 +351,7 @@ async def _make_other_request(
     prototype: BufferPrototype,
     semaphore: asyncio.Semaphore,
 ) -> list[_Response]:
-    """Make suffix or offset requests.
+    """Make offset or full-file requests.
 
     We return a `list[_Response]` for symmetry with `_make_bounded_requests` so that all
     futures can be gathered together.
@@ -339,6 +373,46 @@ async def _make_other_request(
     ]
 
 
+async def _make_suffix_request(
+    store: _UpstreamObjectStore,
+    request: _SuffixRequest,
+    prototype: BufferPrototype,
+    semaphore: asyncio.Semaphore,
+) -> list[_Response]:
+    """Make suffix requests.
+
+    This is separated out from `_make_other_request` because some object stores (Azure)
+    don't support suffix requests. In this case, our workaround is to first get the
+    length of the object and then manually request the byte range at the end.
+
+    We return a `list[_Response]` for symmetry with `_make_bounded_requests` so that all
+    futures can be gathered together.
+    """
+    import obstore as obs
+
+    async with semaphore:
+        try:
+            resp = await obs.get_async(store, request["path"], options={"range": request["range"]})
+            buffer = await resp.bytes_async()
+        except obs.exceptions.NotSupportedError:
+            head_resp = await obs.head_async(store, request["path"])
+            file_size = head_resp["size"]
+            suffix_len = request["range"]["suffix"]
+            buffer = await obs.get_range_async(
+                store,
+                request["path"],
+                start=file_size - suffix_len,
+                length=suffix_len,
+            )
+
+    return [
+        {
+            "original_request_index": request["original_request_index"],
+            "buffer": prototype.buffer.from_bytes(buffer),  # type: ignore[arg-type]
+        }
+    ]
+
+
 async def _get_partial_values(
     store: _UpstreamObjectStore,
     prototype: BufferPrototype,
@@ -358,6 +432,7 @@ async def _get_partial_values(
     key_ranges = list(key_ranges)
     per_file_bounded_requests: dict[str, list[_BoundedRequest]] = defaultdict(list)
     other_requests: list[_OtherRequest] = []
+    suffix_requests: list[_SuffixRequest] = []
 
     for idx, (path, byte_range) in enumerate(key_ranges):
         if byte_range is None:
@@ -381,7 +456,7 @@ async def _get_partial_values(
                 }
             )
         elif isinstance(byte_range, SuffixByteRequest):
-            other_requests.append(
+            suffix_requests.append(
                 {
                     "original_request_index": idx,
                     "path": path,
@@ -402,6 +477,9 @@ async def _get_partial_values(
     for request in other_requests:
         futs.append(_make_other_request(store, request, prototype, semaphore=semaphore))  # noqa: PERF401
 
+    for suffix_request in suffix_requests:
+        futs.append(_make_suffix_request(store, suffix_request, prototype, semaphore=semaphore))  # noqa: PERF401
+
     buffers: list[Buffer | None] = [None] * len(key_ranges)
 
     for responses in await asyncio.gather(*futs):