fix: make benchmark page-cache clearing opt-in, never prompt for sudo

.github/workflows/codspeed.yml

@@ -33,6 +33,8 @@ jobs:
         version: '1.16.5'
     - name: Run the benchmarks
       uses: CodSpeedHQ/action@f99becdce5e5d51fd556489ebef684f4ecfd6286 # v4.18.5
+      env:
+        ZARR_BENCHMARK_CLEAR_CACHE: '1'
       with:
           mode: walltime
           run: hatch run test.py3.12-minimal:pytest tests/benchmarks --codspeed

.github/workflows/test.yml

@@ -178,6 +178,8 @@ jobs:
     - name: Install Hatch
       run: python -m pip install hatch==1.16.5
     - name: Run Benchmarks
+      env:
+        ZARR_BENCHMARK_CLEAR_CACHE: '1'
       run: |
         hatch env run --env "test.py3.13-minimal" run-benchmark
 

changes/4199.bugfix.md

@@ -0,0 +1 @@
+The end-to-end benchmarks no longer invoke `sudo` to drop the OS page cache during a regular `pytest` run. Cache clearing is now opt-in via the `ZARR_BENCHMARK_CLEAR_CACHE` environment variable, which the benchmark CI jobs set.

tests/benchmarks/test_e2e.py

@@ -4,8 +4,10 @@
 
 from __future__ import annotations
 
+import os
 import platform
 import subprocess
+import warnings
 from functools import lru_cache
 from operator import getitem, setitem
 from typing import TYPE_CHECKING, Any, Literal
@@ -21,12 +23,27 @@
 
 
 def clear_cache() -> None:
+    """Drop the OS page cache between benchmark rounds.
+
+    Requires passwordless sudo, so it is opt-in: set `ZARR_BENCHMARK_CLEAR_CACHE=1`
+    to enable it (as the benchmark CI jobs do). By default this is a no-op, so a
+    plain `pytest` run never prompts for a sudo password (see issue #4199).
+    `sudo -n` guarantees we fail instead of blocking on a password prompt even
+    when the variable is set.
+    """
+    if os.environ.get("ZARR_BENCHMARK_CLEAR_CACHE", "") not in ("1", "true"):
+        return
     if platform.system() == "Darwin":
-        subprocess.call(["sync", "&&", "sudo", "purge"])
+        subprocess.call(["sync"])
+        subprocess.call(["sudo", "-n", "purge"])
     elif platform.system() == "Linux":
-        subprocess.call(["sudo", "sh", "-c", "sync; echo 3 > /proc/sys/vm/drop_caches"])
+        subprocess.call(["sudo", "-n", "sh", "-c", "sync; echo 3 > /proc/sys/vm/drop_caches"])
     else:
-        raise Exception("Unsupported platform")  # noqa: TRY002
+        warnings.warn(
+            f"ZARR_BENCHMARK_CLEAR_CACHE is set but cache clearing is not supported on "
+            f"{platform.system()}; skipping.",
+            stacklevel=2,
+        )
 
 
 if TYPE_CHECKING: