expose API to fetch a cache entry without promoting uses

Summary: In fblite, we want the ability to query the validity of currently stored PSK tickets, but fetching entries from the cache promotes the uses towards eviction, so I've added the ability to get the stored PSK without promoting its usage count.

Reviewed By: cgrushko, knekritz

Differential Revision: D69534167

fbshipit-source-id: dcb6de72bb0d1addcbc85778daaf2a583d188603

proxygen/lib/transport/PersistentFizzPskCache.h

@@ -44,6 +44,26 @@ class PersistentFizzPskCache : public fizz::client::PskCache {
     return folly::none;
   }
 
+  /**
+   * Returns a PSK, without increasing its usage count. Keep in mind that it
+   * would still update the internal LRU cache
+   */
+  folly::Optional<fizz::client::CachedPsk> peekPsk(
+      const std::string& identity) {
+    auto serialized = cache_.get(identity);
+    if (serialized) {
+      try {
+        return fizz::client::deserializePsk(
+            fizz::openssl::certificateSerializer(),
+            folly::ByteRange(serialized->serialized));
+      } catch (const std::exception& ex) {
+        LOG(ERROR) << "Error deserializing PSK: " << ex.what();
+        cache_.remove(identity);
+      }
+    }
+    return folly::none;
+  }
+
   folly::Optional<fizz::client::CachedPsk> getPsk(
       const std::string& identity) override {
     auto serialized = cache_.get(identity);

proxygen/lib/transport/test/PersistentFizzPskCacheTest.cpp

@@ -292,4 +292,23 @@ TEST_F(PersistentFizzPskCacheTest, TestGetPskUses) {
   cache_->getPsk("facebook.com");
   EXPECT_EQ(folly::none, cache_->getPskUses("facebook.com"));
 }
+
+TEST_F(PersistentFizzPskCacheTest, TestPeekPsk) {
+  cache_->setMaxPskUses(5);
+
+  EXPECT_EQ(folly::none, cache_->getPskUses("facebook.com"));
+  cache_->putPsk("facebook.com", psk1_);
+  EXPECT_EQ(0u, cache_->getPskUses("facebook.com"));
+
+  cache_->getPsk("facebook.com");
+  EXPECT_EQ(1u, cache_->getPskUses("facebook.com"));
+
+  for (size_t i = 0; i < 10; i++) {
+    expectMatch(*cache_->peekPsk("facebook.com"), psk1_);
+    EXPECT_EQ(1u, cache_->getPskUses("facebook.com"));
+  }
+
+  cache_->getPsk("facebook.com");
+  EXPECT_EQ(2u, cache_->getPskUses("facebook.com"));
+}
 }} // namespace proxygen::test