Remove prefetching loop (#1984)

Author: TLCFEM

fsspec/caching.py

@@ -214,7 +214,7 @@ def _fetch(self, start: int | None, end: int | None) -> bytes:
         if self.multi_fetcher:
             logger.debug(f"MMap get blocks {ranges}")
             for idx, r in enumerate(self.multi_fetcher(ranges)):
-                (sstart, send) = ranges[idx]
+                sstart, send = ranges[idx]
                 logger.debug(f"MMap copy block ({sstart}-{send}")
                 self.cache[sstart:send] = r
         else:
@@ -391,19 +391,8 @@ def _fetch(self, start: int | None, end: int | None) -> bytes:
         if start >= self.size or start >= end:
             return b""
 
-        # byte position -> block numbers
-        start_block_number = start // self.blocksize
-        end_block_number = end // self.blocksize
-
-        # these are cached, so safe to do multiple calls for the same start and end.
-        for block_number in range(start_block_number, end_block_number + 1):
-            self._fetch_block_cached(block_number)
-
         return self._read_cache(
-            start,
-            end,
-            start_block_number=start_block_number,
-            end_block_number=end_block_number,
+            start, end, start // self.blocksize, (end - 1) // self.blocksize
         )
 
     def _fetch_block(self, block_number: int) -> bytes:
@@ -439,6 +428,8 @@ def _read_cache(
         """
         start_pos = start % self.blocksize
         end_pos = end % self.blocksize
+        if end_pos == 0:
+            end_pos = self.blocksize
 
         self.hit_count += 1
         if start_block_number == end_block_number:

fsspec/tests/test_caches.py

@@ -65,6 +65,16 @@ def test_block_cache_lru():
     assert cache.total_requested_bytes == block_size * cache.miss_count
 
 
+def test_block_cache_lru_no_redundant_reads():
+    block_size = 4
+    maxblocks = 2
+    cache = BlockCache(
+        block_size, letters_fetcher, len(string.ascii_letters), maxblocks=maxblocks
+    )
+    cache._fetch(0, block_size * (maxblocks + 1))
+    assert cache.cache_info().misses == 3
+
+
 def test_first_cache():
     """
     FirstChunkCache is a cache that only caches the first chunk of data