RegexMatchCache::hasItemsToPurge

Summary:
A `const` check for whether `RegexMatchCache::purge`, which is non-`cont`, would have any work to do.

For concurrent access, `hasItemsToPurge` can be invoked with a shared lock held and, only if it returns `true`, would `purge` be invoked with an exclusive lock held.

Reviewed By: Gownta

Differential Revision: D66730362

fbshipit-source-id: 678872260faff868671c76875408f4357511b58b

folly/container/RegexMatchCache.cpp

@@ -497,6 +497,16 @@ std::vector<RegexMatchCache::string_pointer> RegexMatchCache::findMatches(
   return {matches.begin(), matches.end()};
 }
 
+bool RegexMatchCache::hasItemsToPurge(time_point const expiry) const noexcept {
+  for (auto const& [regex, entry] : cacheRegexToMatch_) {
+    auto const accessed_at = entry.accessed_at.load(std::memory_order_relaxed);
+    if (accessed_at <= expiry) {
+      return true;
+    }
+  }
+  return false;
+}
+
 void RegexMatchCache::clear() {
   std::exchange(stringQueueReverse_, {});
   std::exchange(stringQueueForward_, {});

folly/container/RegexMatchCache.h

@@ -691,6 +691,8 @@ class RegexMatchCache {
   std::vector<string_pointer> findMatches(
       regex_key const& regex, time_point now) const;
 
+  bool hasItemsToPurge(time_point expiry) const noexcept;
+
   void clear();
   void purge(time_point expiry);
 };