gopls/filecache: prevent premature CAS file eviction

If the CAS file exists with the correct content when setting
a cache value, the timestamp on the CAS file was not being
updated. This causes the garbage collector to evict the CAS
file potentially long before the index file. This can result
in a large cache having many orphaned index files.

This change ensures that the CAS file timestamp is updated
even if it already exists during a Set call.

Change-Id: I1f16253d7a8a5d0f9d2b6d7e1e766f34c5661303
Reviewed-on: https://go-review.googlesource.com/c/tools/+/740900
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Alan Donovan <adonovan@google.com>
Reviewed-by: Madeline Kalil <mkalil@google.com>

Author: leitzler

gopls/internal/filecache/filecache.go

@@ -125,15 +125,7 @@ func Get(kind string, key [32]byte) ([]byte, error) {
 	// automatically, but for efficiency most POSIX systems have
 	// for many years set the noatime mount option to avoid every
 	// open or read operation entailing a metadata write.)
-	now := time.Now()
-	touch := func(filename string) {
-		st, err := os.Stat(filename)
-		if err == nil && now.Sub(st.ModTime()) > time.Hour {
-			os.Chtimes(filename, now, now) // ignore error
-		}
-	}
-	touch(indexName)
-	touch(casName)
+	touch(indexName, casName)
 
 	memCache.Set(memKey{kind, key}, value, len(value))
 
@@ -198,9 +190,22 @@ func Set(kind string, key [32]byte, value []byte) error {
 		return err           // e.g. disk full
 	}
 
+	// Update the CAS file's timestamp to prevent premature LRU eviction.
+	touch(casName)
+
 	return nil
 }
 
+func touch(filenames ...string) {
+	now := time.Now()
+	for _, filename := range filenames {
+		st, err := os.Stat(filename)
+		if err == nil && now.Sub(st.ModTime()) > time.Hour {
+			os.Chtimes(filename, now, now) // ignore error
+		}
+	}
+}
+
 // The active 1-channel is a selectable resettable event
 // indicating recent cache activity.
 var active = make(chan struct{}, 1)