Check cached sources with upstream after the cache TTL (closes #69)

Cached images were served until eviction, so upstream changes were
missed.

After 24 hours a request checks the source with upstream, sending its
stored ETag and Last-Modified. A 304 renews it with no download or
conversion. New bytes replace it; older variants are converted again.
A failed check serves the cached image and puts the URL in the
negative cache. Images not checked for 30 days are evicted.

Variant rows keep the source's ETag and Last-Modified for checking a
variant of an evicted source.

A variant file's modification time is its source's check time when it
was made or renewed. Hits read it without a database call. The ETag
includes it, so new bytes get a new ETag.

Model: opus-5-5

internal/imgcache/evict_not_checked_recently_internal_test.go

@@ -0,0 +1,88 @@
+package imgcache
+
+import (
+	"bytes"
+	"os"
+	"slices"
+	"testing"
+	"time"
+
+	"sneak.berlin/go/pixa/internal/imagerequest"
+)
+
+// TestEvictNotCheckedRecently stores a source and a variant last checked with
+// upstream 31 days ago and a source and a variant checked 29 days ago.
+// EvictNotCheckedRecently must remove the rows and files of the first two and
+// keep the other two.
+func TestEvictNotCheckedRecently(t *testing.T) {
+	t.Parallel()
+
+	cache, _ := newEvictionTestCache(t, 1<<30)
+	old := time.Now().Add(-31 * 24 * time.Hour)
+	recent := time.Now().Add(-29 * 24 * time.Hour)
+
+	oldSource := imagerequest.Source{Host: testHostCDN, Path: "/old.jpg"}
+	recentSource := imagerequest.Source{Host: testHostCDN, Path: "/recent.jpg"}
+
+	for src, checkedAt := range map[imagerequest.Source]time.Time{
+		oldSource: old, recentSource: recent,
+	} {
+		_, err := cache.PutSource(t.Context(), src, bytes.NewReader([]byte("source")),
+			SourceInfo{FetchedAt: checkedAt, CheckedAt: checkedAt})
+		if err != nil {
+			t.Fatalf("PutSource(%s) error = %v", src.Path, err)
+		}
+	}
+
+	oldVariant := imagerequest.VariantKey("aabbccdd0001")
+	recentVariant := imagerequest.VariantKey("aabbccdd0002")
+
+	for key, checkedAt := range map[imagerequest.VariantKey]time.Time{
+		oldVariant: old, recentVariant: recent,
+	} {
+		err := cache.PutVariant(t.Context(), key, SourceInfo{CheckedAt: checkedAt},
+			bytes.NewReader([]byte("variant")))
+		if err != nil {
+			t.Fatalf("PutVariant(%s) error = %v", key, err)
+		}
+	}
+
+	err := cache.EvictNotCheckedRecently(t.Context())
+	if err != nil {
+		t.Fatalf("EvictNotCheckedRecently() error = %v", err)
+	}
+
+	sourceRows, err := queryStringColumn[imagerequest.SourceKey](t.Context(),
+		cache.db, `SELECT source_key FROM source_content`, "sources", "source")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	variantRows, err := queryStringColumn[imagerequest.VariantKey](t.Context(),
+		cache.db, `SELECT cache_key FROM variant_content`, "variants", "variant")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	t.Logf("source rows left = %v, variant rows left = %v", sourceRows, variantRows)
+
+	if !slices.Equal(sourceRows, []imagerequest.SourceKey{recentSource.Key()}) {
+		t.Errorf("source rows left = %v, want only %s", sourceRows, recentSource.Key())
+	}
+
+	if !slices.Equal(variantRows, []imagerequest.VariantKey{recentVariant}) {
+		t.Errorf("variant rows left = %v, want only %s", variantRows, recentVariant)
+	}
+
+	for path, wantKept := range map[string]bool{
+		cache.srcContent.keyToPath(string(oldSource.Key())):    false,
+		cache.srcContent.keyToPath(string(recentSource.Key())): true,
+		cache.variants.keyToPath(string(oldVariant)):           false,
+		cache.variants.keyToPath(string(recentVariant)):        true,
+	} {
+		_, statErr := os.Stat(path)
+		if kept := statErr == nil; kept != wantKept {
+			t.Errorf("file %s kept = %t, want %t (%v)", path, kept, wantKept, statErr)
+		}
+	}
+}