fix(dbpurge): use dbauthz.AsSystemRestricted and add jitter

coadler · coadler · commit b8f215c1c004 · 2023-04-05T12:20:24.000-05:00
Fixes: #7016
diff --git a/coderd/database/dbpurge/dbpurge.go b/coderd/database/dbpurge/dbpurge.go
@@ -10,24 +10,48 @@ import (
 
 	"cdr.dev/slog"
 	"github.com/coder/coder/coderd/database"
+	"github.com/coder/coder/coderd/database/dbauthz"
+	"github.com/coder/coder/cryptorand"
 )
 
+const (
+	jitter = 2 * time.Hour
+	delay  = 23 * time.Hour
+)
+
+func nextTick() (time.Duration, error) {
+	f, err := cryptorand.Float64()
+	if err != nil {
+		return 0, err
+	}
+
+	return delay + time.Duration(float64(jitter)*f), nil
+}
+
 // New creates a new periodically purging database instance.
 // It is the caller's responsibility to call Close on the returned instance.
 //
 // This is for cleaning up old, unused resources from the database that take up space.
 func New(ctx context.Context, logger slog.Logger, db database.Store) io.Closer {
 	closed := make(chan struct{})
 	ctx, cancelFunc := context.WithCancel(ctx)
+	//nolint:gocritic // The system purges old db records without user input.
+	ctx = dbauthz.AsSystemRestricted(ctx)
 	go func() {
 		defer close(closed)
-		ticker := time.NewTicker(24 * time.Hour)
-		defer ticker.Stop()
+		wait, err := nextTick()
+		if err != nil {
+			logger.Error(ctx, "failed to generate random delay; exiting", slog.Error(err))
+			return
+		}
+
+		timer := time.NewTimer(wait)
+		defer timer.Stop()
 		for {
 			select {
 			case <-ctx.Done():
 				return
-			case <-ticker.C:
+			case <-timer.C:
 			}
 
 			var eg errgroup.Group
@@ -44,6 +68,13 @@ func New(ctx context.Context, logger slog.Logger, db database.Store) io.Closer {
 				}
 				logger.Error(ctx, "failed to purge old database entries", slog.Error(err))
 			}
+
+			wait, err := nextTick()
+			if err != nil {
+				logger.Error(ctx, "failed to generate random delay; exiting", slog.Error(err))
+				return
+			}
+			timer.Reset(wait)
 		}
 	}()
 	return &instance{
diff --git a/coderd/database/dbpurge/dbpurge_internal_test.go b/coderd/database/dbpurge/dbpurge_internal_test.go
@@ -0,0 +1,20 @@
+package dbpurge
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func Test_nextTick(t *testing.T) {
+	t.Parallel()
+
+	tick, err := nextTick()
+	require.NoError(t, err)
+
+	t.Log("tick:", tick.String())
+
+	assert.GreaterOrEqual(t, tick, delay)
+	assert.LessOrEqual(t, tick, delay+jitter)
+}
