Test that a read of an older .meta file keeps the stored content type (closes #70)

A read that finds no .meta file keeps nothing in memory, so the
existing test no longer reaches the step that keeps a type StoreVariant
put there. This one writes a .meta file with a different content type
after the store, as one not yet rewritten by it would hold, and checks
that the read leaves the stored type in memory.

Model: opus-5-5

internal/imgcache/metacache_internal_test.go

@@ -2,6 +2,7 @@ package imgcache
 
 import (
 	"bytes"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"io"
@@ -155,6 +156,54 @@ func TestReadDuringStoreKeepsStoredContentType(t *testing.T) {
 	assertVariantServed(t, cache, testVariantKeyOne, content)
 }
 
+// TestReadOfOlderMetaFileKeepsStoredContentType checks that a read
+// which got its content type from a .meta file that StoreVariant had
+// not yet rewritten cannot replace the type the store kept in memory.
+// The test writes such a .meta file, with a different content type,
+// after the store, then runs the part of GetVariant that comes after
+// its check of memory.
+func TestReadOfOlderMetaFileKeepsStoredContentType(t *testing.T) {
+	t.Parallel()
+
+	cache, _ := newEvictionTestCache(t, 1<<20)
+	content := []byte("webp variant bytes")
+
+	storeEvictionTestVariant(t, cache, testVariantKeyOne, content)
+
+	olderMeta, err := json.Marshal(VariantMeta{
+		ContentType: testContentTypeJPEG,
+		Size:        int64(len(content)),
+	})
+	if err != nil {
+		t.Fatalf("encoding .meta file: %v", err)
+	}
+
+	metaPath := cache.variants.keyToPath(testVariantKeyOne) + ".meta"
+
+	err = os.WriteFile(metaPath, olderMeta, StorageFilePerm)
+	if err != nil {
+		t.Fatalf("writing .meta file: %v", err)
+	}
+
+	reader, _, contentType, err := cache.loadVariantWithMeta(testVariantKeyOne)
+	if err != nil {
+		t.Fatalf("loadVariantWithMeta(%s) error = %v", testVariantKeyOne, err)
+	}
+
+	_ = reader.Close()
+
+	if contentType != testContentTypeJPEG {
+		t.Fatalf("loadVariantWithMeta(%s) content type = %q, want %q from the .meta file",
+			testVariantKeyOne, contentType, testContentTypeJPEG)
+	}
+
+	kept, _ := cache.metaCache.Get(testVariantKeyOne)
+	if kept != testContentTypeWebP {
+		t.Errorf("content type in memory = %q, want the stored %q",
+			kept, testContentTypeWebP)
+	}
+}
+
 // TestFailedReadDuringStoreKeepsStoredContentType checks that a read
 // which found no .meta file cannot leave application/octet-stream in
 // memory, even when another read has removed the content type