fix: use deterministic hash for ETag generation

Python's built-in hash() is salted by default (PYTHONHASHSEED),
producing different values across processes. This causes ETags to
change on service restart or differ between backends behind a load
balancer, invalidating the cache unexpectedly.

Replace hash() with hashlib.md5() which produces consistent values.
MD5 is used for speed since this is for caching, not security.

Fixes #400

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: awanawona

fastapi_cache/decorator.py

@@ -1,3 +1,4 @@
+import hashlib
 import logging
 import sys
 from functools import wraps
@@ -66,6 +67,16 @@ def _locate_param(
     return param
 
 
+def _compute_etag(data: bytes) -> str:
+    """Compute a deterministic ETag hash from cache data.
+
+    Uses MD5 for speed since this is for caching, not security.
+    Unlike Python's built-in hash(), this produces consistent values
+    across different processes and service restarts.
+    """
+    return hashlib.md5(data).hexdigest()
+
+
 def _uncacheable(request: Optional[Request]) -> bool:
     """Determine if this request should not be cached
 
@@ -199,14 +210,14 @@ async def ensure_async_func(*args: P.args, **kwargs: P.kwargs) -> R:
                     response.headers.update(
                         {
                             "Cache-Control": f"max-age={expire}",
-                            "ETag": f"W/{hash(to_cache)}",
+                            "ETag": f"W/{_compute_etag(to_cache)}",
                             cache_status_header: "MISS",
                         }
                     )
 
             else:  # cache hit
                 if response:
-                    etag = f"W/{hash(cached)}"
+                    etag = f"W/{_compute_etag(cached)}"
                     response.headers.update(
                         {
                             "Cache-Control": f"max-age={ttl}",