derp: start adding flow tracking statspatrickod/bradtfitz-flow-rebased

This starts adding flow tracking stats, without exposing them anywhere
yet. Flow structs are created as needed and metrics are bumped, and
benchmarks show no change in performance.

Updates #3560

Change-Id: I376187a8452ec92d49effcbf48a6fb4f4d787b8a
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

1 files changed, 9 insertions, 0 deletions

diff --git a/util/lru/lru.go b/util/lru/lru.go
index 8e4dd417b..d4e836cf1 100644
--- a/util/lru/lru.go
+++ b/util/lru/lru.go
@@ -133,6 +133,15 @@ func (c *Cache[K, V]) DeleteOldest() {
 	}
 }
 
+// OldestKey returns the oldest key, without bumping it to the head.
+// If the cache is empty, it returns ok false.
+func (c *Cache[K, V]) OldestKey() (key K, ok bool) {
+	if c.head == nil {
+		return key, false
+	}
+	return c.head.prev.key, true
+}
+
 // Len returns the number of items in the cache.
 func (c *Cache[K, V]) Len() int { return len(c.lookup) }