Test the warn lines of failed cache writes and stats queries (closes #72)

A directory where a variant .meta file goes, and dropped tables in the
test database, make the variant .meta write, the two Stats count queries
and the stats counter updates fail. Each test hands the cache or
NewVariantStorage a logger writing to a buffer, checks the warn line, and
checks the store and Stats calls still succeed.

Model: opus-5-5

internal/imgcache/stats_internal_test.go

@@ -1,9 +1,12 @@
 package imgcache
 
 import (
+	"bytes"
 	"context"
 	"database/sql"
+	"log/slog"
 	"math"
+	"strings"
 	"testing"
 	"time"
 
@@ -101,3 +104,82 @@ func TestStats_ZeroCounts(t *testing.T) {
 		t.Errorf("HitRate = %f, want 0.0 for zero counts", stats.HitRate)
 	}
 }
+
+// TestStats_LogsFailedCountQueries verifies that a failed item count query
+// and a failed size query are each logged at warn and Stats still succeeds.
+func TestStats_LogsFailedCountQueries(t *testing.T) {
+	t.Parallel()
+
+	db := setupStatsTestDB(t)
+
+	var logBuf bytes.Buffer
+
+	cache, err := NewCache(db, CacheConfig{
+		StateDir:    t.TempDir(),
+		CacheTTL:    time.Hour,
+		NegativeTTL: 5 * time.Minute,
+		Logger:      slog.New(slog.NewJSONHandler(&logBuf, nil)),
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	_, err = db.ExecContext(t.Context(),
+		`DROP TABLE request_cache; DROP TABLE output_content`)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	_, err = cache.Stats(t.Context())
+	if err != nil {
+		t.Fatalf("Stats() error = %v, want nil", err)
+	}
+
+	for _, msg := range []string{
+		"failed to count cache items for stats",
+		"failed to sum cache size for stats",
+	} {
+		want := `"level":"WARN","msg":"` + msg + `"`
+		if !strings.Contains(logBuf.String(), want) {
+			t.Errorf("log missing %s; got %q", want, logBuf.String())
+		}
+	}
+}
+
+// TestIncrementStats_LogsFailedUpdates verifies that a failed hit or miss
+// count update and a failed upstream fetch count update are each logged at
+// warn.
+func TestIncrementStats_LogsFailedUpdates(t *testing.T) {
+	t.Parallel()
+
+	db := setupStatsTestDB(t)
+
+	var logBuf bytes.Buffer
+
+	cache, err := NewCache(db, CacheConfig{
+		StateDir:    t.TempDir(),
+		CacheTTL:    time.Hour,
+		NegativeTTL: 5 * time.Minute,
+		Logger:      slog.New(slog.NewJSONHandler(&logBuf, nil)),
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	_, err = db.ExecContext(t.Context(), `DROP TABLE cache_stats`)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	cache.IncrementStats(t.Context(), false, 1024)
+
+	for _, msg := range []string{
+		"failed to count cache hit or miss",
+		"failed to count upstream fetch",
+	} {
+		want := `"level":"WARN","msg":"` + msg + `"`
+		if !strings.Contains(logBuf.String(), want) {
+			t.Errorf("log missing %s; got %q", want, logBuf.String())
+		}
+	}
+}