Start no eviction pass after a stop, and end a pass quietly

Both pass runners do nothing once the loop's context is cancelled, so
a stop no longer starts an eviction pass after a cancelled
reconciliation or on a pending write-pressure wakeup or tick. In
evictBatch, a candidate that fails after cancellation ends the batch
with the context's error instead of logging its own warning; this
replaces the check before each candidate, since every candidate
started after cancellation fails at its first database call. A stop
now logs at most one warning.

Model: opus-5-5

internal/imgcache/eviction.go

@@ -117,8 +117,10 @@ func (c *Cache) EvictToLimit(ctx context.Context) error {
 
 // evictBatch fetches one batch of LRU candidates across variants and
 // source blobs and evicts them oldest-first until excessBytes are
-// freed or the batch is exhausted. It returns the bytes freed. Once ctx
-// is cancelled, it stops at the next candidate and returns ctx's error.
+// freed or the batch is exhausted. It returns the bytes freed. A
+// candidate that fails once ctx is cancelled (every one started after
+// that fails at its first database call) ends the batch with ctx's
+// error, without a warning.
 func (c *Cache) evictBatch(ctx context.Context, excessBytes int64) (int64, error) {
 	candidates, err := c.evictionCandidates(ctx)
 	if err != nil {
@@ -132,12 +134,12 @@ func (c *Cache) evictBatch(ctx context.Context, excessBytes int64) (int64, error
 			break
 		}
 
-		if ctx.Err() != nil {
-			return freed, ctx.Err()
-		}
-
 		err := c.evictCandidate(ctx, candidate)
 		if err != nil {
+			if ctx.Err() != nil {
+				return freed, ctx.Err()
+			}
+
 			c.log.Warn("failed to evict cache entry",
 				"cache_key", candidate.cacheKey,
 				"content_hash", candidate.contentHash,
@@ -463,7 +465,7 @@ func (c *Cache) StopEviction(ctx context.Context) error {
 }
 
 // evictionLoop is the body of the background eviction goroutine. It
-// returns when ctx is cancelled.
+// returns when ctx is cancelled, and starts no pass after that.
 func (c *Cache) evictionLoop(ctx context.Context, interval time.Duration) {
 	defer close(c.evictionDone)
 
@@ -496,8 +498,13 @@ func (c *Cache) evictionLoop(ctx context.Context, interval time.Duration) {
 }
 
 // runEvictionPass runs one eviction pass, logging failures instead of
-// propagating them (the loop must keep running).
+// propagating them (the loop must keep running). It does nothing once
+// ctx is cancelled.
 func (c *Cache) runEvictionPass(ctx context.Context) {
+	if ctx.Err() != nil {
+		return
+	}
+
 	err := c.EvictToLimit(ctx)
 	if err != nil {
 		c.log.Warn("cache eviction pass failed", "error", err)
@@ -505,8 +512,13 @@ func (c *Cache) runEvictionPass(ctx context.Context) {
 }
 
 // runReconciliationPass runs one reconciliation pass, logging failures
-// instead of propagating them (the loop must keep running).
+// instead of propagating them (the loop must keep running). It does
+// nothing once ctx is cancelled.
 func (c *Cache) runReconciliationPass(ctx context.Context) {
+	if ctx.Err() != nil {
+		return
+	}
+
 	err := c.reconcileAccounting(ctx)
 	if err != nil {
 		c.log.Warn("cache accounting reconciliation failed", "error", err)