perf(cache): skip redundant store writes and scans on cache requests (#1222)

Every cache hit rewrote the entry's access time, costing a bolt write and fsync per restore. Completed entries accessed within the last minute are no longer rewritten, and the in-use and retention windows grow by that minute so nothing is evicted earlier. Same-second rewrites are skipped, reservations are inserted in one transaction, and prefix lookups use the repository index instead of scanning every entry.

The store is still opened per request. Keeping it open, as this commit first did, is faster under concurrency but stops a second process sharing the cache directory from starting its cache server.

The README caching section is condensed and corrected.

| Benchmark, v2 | main | store kept open | this PR |
| --- | ---: | ---: | ---: |
| Exact hit, 1 client | 16.42 | 0.15 | 0.24 |
| Exact hit, 8 clients | 13.70 | 0.08 | 1.28 |
| Exact hit, 32 clients | 16.01 | 0.07 | 4.26 |
| Prefix hit, 8 clients | 18.34 | 0.12 | 1.28 |
| Miss, 8 clients | 3.21 | 0.11 | 1.28 |
| Save and restore 1 MiB, 8 clients | 74.4 | 22.9 | 34.9 |
| Geomean, all 22 benchmarks | 14.03 | 0.31 | 2.11 |

Mean ms per request, medians of 4 rounds of 300 requests over 1,000 entries on an Apple M3 Max.

---------

Co-authored-by: silverwind <[email protected]>
Co-authored-by: silverwind <[email protected]>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1222
Reviewed-by: silverwind <[email protected]>

act/artifactcache/handler_test.go

@@ -771,7 +771,7 @@ func TestHandler_gcCache(t *testing.T) {
 				Key:       "test_key_3",
 				Version:   "test_version",
 				Complete:  true,
-				UsedAt:    now.Add(-(testRetention + time.Second)).Unix(),
+				UsedAt:    now.Add(-(testRetention + touchStale + time.Second)).Unix(),
 				CreatedAt: now.Add(-(testRetention + time.Hour)).Unix(),
 			},
 			Kept: false,
@@ -1046,6 +1046,26 @@ func TestHandler_gcCacheInterval(t *testing.T) {
 	assert.Empty(t, keptKeys(t, handler, []*Cache{cache}))
 }
 
+func TestHandler_touchCacheSkipsFreshCompletedEntries(t *testing.T) {
+	fresh := &Cache{Repo: testRepo, Key: "fresh", Version: "v", Complete: true, UsedAt: time.Now().Unix()}
+	pending := &Cache{Repo: testRepo, Key: "pending", Version: "v", UsedAt: time.Now().Add(-time.Second).Unix()}
+	handler := newTestHandler(t, Policy{}, fresh, pending)
+	txID := func() int {
+		db, err := handler.openDB()
+		require.NoError(t, err)
+		defer db.Close()
+		var id int
+		require.NoError(t, db.Bolt().View(func(tx *bbolt.Tx) error { id = tx.ID(); return nil }))
+		return id
+	}
+
+	before := txID()
+	require.NoError(t, handler.touchCache(fresh.ID, false))
+	assert.Equal(t, before, txID())
+	require.NoError(t, handler.touchCache(pending.ID, false))
+	assert.Greater(t, txID(), before)
+}
+
 // newTestHandler starts a handler with testToken registered, seeded with entries.
 func newTestHandler(t *testing.T, policy Policy, entries ...*Cache) *Handler {
 	t.Helper()