Skip reads when completely overwriting boundary chunks

Uses `slice(..., None)` to indicate that a `chunk_selection`
ends at the boundary of the current chunk. Also does so for a last
chunk that is shorter than the chunk size.

`is_total_slice` now understands this convention, and correctly
detects boundary chunks as total slices.

Closes zarr-developers#757

Author: dcherian

src/zarr/core/indexing.py

@@ -403,16 +403,24 @@ def __iter__(self) -> Iterator[ChunkDimProjection]:
                 dim_chunk_sel_start = self.start - dim_offset
                 dim_out_offset = 0
 
+            # Use None to indicate this selection ends at the chunk edge
+            # This is useful for is_total_slice at boundary chunks,
             if self.stop > dim_limit:
                 # selection ends after current chunk
-                dim_chunk_sel_stop = dim_chunk_len
+                dim_chunk_sel_stop = None  # dim_chunk_len
 
             else:
                 # selection ends within current chunk
-                dim_chunk_sel_stop = self.stop - dim_offset
+                if dim_chunk_ix == (self.nchunks - 1):
+                    # all of the last chunk is included
+                    dim_chunk_sel_stop = None
+                else:
+                    dim_chunk_sel_stop = self.stop - dim_offset
 
             dim_chunk_sel = slice(dim_chunk_sel_start, dim_chunk_sel_stop, self.step)
-            dim_chunk_nitems = ceildiv((dim_chunk_sel_stop - dim_chunk_sel_start), self.step)
+            dim_chunk_nitems = ceildiv(
+                ((dim_chunk_sel_stop or dim_chunk_len) - dim_chunk_sel_start), self.step
+            )
 
             # If there are no elements on the selection within this chunk, then skip
             if dim_chunk_nitems == 0:
@@ -1376,7 +1384,17 @@ def is_total_slice(item: Selection, shape: ChunkCoords) -> bool:
             isinstance(dim_sel, slice)
             and (
                 (dim_sel == slice(None))
-                or ((dim_sel.stop - dim_sel.start == dim_len) and (dim_sel.step in [1, None]))
+                or (
+                    dim_sel.stop is not None
+                    and (dim_sel.stop - dim_sel.start == dim_len)
+                    and (dim_sel.step in [1, None])
+                )
+                # starts exactly at a chunk
+                or (
+                    (dim_sel.start % dim_len == 0)
+                    and dim_sel.stop is None
+                    and (dim_sel.step in [1, None])
+                )
             )
             for dim_sel, dim_len in zip(item, shape, strict=False)
         )

src/zarr/storage/_logging.py

@@ -88,7 +88,7 @@ def log(self, hint: Any = "") -> Generator[None, None, None]:
         op = f"{type(self._store).__name__}.{method}"
         if hint:
             op = f"{op}({hint})"
-        self.logger.info("Calling %s", op)
+        self.logger.info(" Calling %s", op)
         start_time = time.time()
         try:
             self.counter[method] += 1