binary cache: add use-bloom-filter setting (default true)

Lets a client opt out of consulting a cache's advertised Bloom filter.
isDefinitelyMissing bails out early when it's false.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: edolstra

src/libstore/binary-cache-store.cc

@@ -140,7 +140,7 @@ bool BinaryCacheStore::fetchBloomFilter(const std::string & uri)
 
 bool BinaryCacheStore::isDefinitelyMissing(const StorePath & storePath) noexcept
 {
-    if (!diskCache || !bloomFilterUrl)
+    if (!diskCache || !bloomFilterUrl || !config.useBloomFilter)
         return false;
 
     const auto uri = config.getReference().render(/*withParams=*/false);

src/libstore/include/nix/store/binary-cache-store.hh

@@ -68,6 +68,17 @@ struct BinaryCacheStoreConfig : virtual StoreConfig
           The meaning and accepted values depend on the compression method selected.
           `-1` specifies that the default compression level should be used.
         )"};
+
+    Setting<bool> useBloomFilter{
+        this,
+        true,
+        "use-bloom-filter",
+        R"(
+          Whether to use the Bloom filter advertised by this binary cache (if
+          any) to avoid querying `.narinfo` files for store paths that are
+          definitely not in the cache. Set to `false` to disable this
+          optimization.
+        )"};
 };
 
 /**