Keep the setuid search's time on the caller's clock

SetuidCache compared the caller's now with a time read from the wall clock when the search finished. When the caller's clock ran behind the wall clock, a search was never due again: TestTheSetuidSearchRunsInTheBackgroundAndIsKeptForItsInterval, whose clock is a fixed date, has failed on main and every branch since 2026-10-09 18:00 UTC. A search's time is now the now it was started at; a result reads at most one search older than it is, which errs towards searching again.

internal/accounts/ways.go

@@ -390,7 +390,7 @@ type SetuidCache struct {
 
 	mu        sync.Mutex
 	wg        sync.WaitGroup
-	at        time.Time // when the last search that finished finished
+	at        time.Time // when the last search that finished was started, on the caller's clock
 	found     []string
 	running   bool
 	startedAt time.Time
@@ -431,8 +431,14 @@ const (
 	ReasonIncomplete = rootsearch.ReasonIncomplete
 )
 
-// Look answers the last result and when its search finished, starting a search in the background when one is
+// Look answers the last result and when its search was started, starting a search in the background when one is
 // due; it never waits for one. Nil c searches in place, for a caller that wants to.
+//
+// **One clock: the caller's.** A search's time is the `now` it was started at, never the wall clock read when it
+// finished: due-ness compares a later `now` with it, and mixing the two clocks made a search never due again
+// whenever the caller's clock ran behind the wall clock — the test with a fixed date failed on every branch from
+// 2026-10-09 18:00 UTC (found while checking issue 356). Measured from the start, a result reads at most one
+// search older than it is, which errs towards searching again.
 func (c *SetuidCache) Look(now time.Time, search func(ctx context.Context) ([]string, error)) ([]string, time.Time, error) {
 	if c == nil {
 		ctx, cancel := context.WithTimeout(context.Background(), SearchBound)
@@ -446,6 +452,7 @@ func (c *SetuidCache) Look(now time.Time, search func(ctx context.Context) ([]st
 		(c.failed == nil || now.Sub(c.failedAt) >= c.backoff)
 	if due {
 		c.running, c.startedAt = true, now
+		started := now
 		c.wg.Add(1)
 		go func() {
 			defer c.wg.Done()
@@ -455,13 +462,12 @@ func (c *SetuidCache) Look(now time.Time, search func(ctx context.Context) ([]st
 			c.mu.Lock()
 			defer c.mu.Unlock()
 			c.running = false
-			done := time.Now()
 			if err != nil {
-				c.failed, c.failedAt = err, done
+				c.failed, c.failedAt = err, started
 				c.backoff = min(max(2*c.backoff, c.Every), MaxBackoff)
 				return
 			}
-			c.found, c.at, c.failed, c.backoff = found, done, nil, 0
+			c.found, c.at, c.failed, c.backoff = found, started, nil, 0
 		}()
 	}
 	switch {