libfetchers: Add CachingFilteringInputAccessor

Co-authored-by: Eelco Dolstra <edolstra@gmail.com>

src/libfetchers/filtering-input-accessor.cc

@@ -80,4 +80,13 @@ ref<AllowListInputAccessor> AllowListInputAccessor::create(
     return make_ref<AllowListInputAccessorImpl>(next, std::move(allowedPaths), std::move(makeNotAllowedError));
 }
 
+bool CachingFilteringInputAccessor::isAllowed(const CanonPath & path)
+{
+    auto i = cache.find(path);
+    if (i != cache.end()) return i->second;
+    auto res = isAllowedUncached(path);
+    cache.emplace(path, res);
+    return res;
+}
+
 }

src/libfetchers/filtering-input-accessor.hh

@@ -71,4 +71,18 @@ struct AllowListInputAccessor : public FilteringInputAccessor
     using FilteringInputAccessor::FilteringInputAccessor;
 };
 
+/**
+ * A wrapping `InputAccessor` mix-in where `isAllowed()` caches the result of virtual `isAllowedUncached()`.
+ */
+struct CachingFilteringInputAccessor : FilteringInputAccessor
+{
+    std::map<CanonPath, bool> cache;
+
+    using FilteringInputAccessor::FilteringInputAccessor;
+
+    bool isAllowed(const CanonPath & path) override;
+
+    virtual bool isAllowedUncached(const CanonPath & path) = 0;
+};
+
 }