Store a source the way a variant is stored, one row and one file per source URL (closes #308)

A source is now one source_content row and one file named by its
SourceKey, the SHA-256 of its host, path and query, made as the variant
key is. A store writes the file, then upserts the row, and removes the
file when the row cannot be written; an eviction deletes the row, then
the file, with no lock. source_metadata, the JSON files under
cache/metadata/, MetadataStorage, the content lock, the eviction
transaction and its test hook are gone, as is the reconciliation pass
that dropped a source row whose file is missing. Two source URLs
serving the same bytes store them twice.

Model: opus-5-5

internal/imgcache/service_internal_test.go

@@ -550,21 +550,17 @@ func TestService_Get_Caching(t *testing.T) {
 
 // TestService_Get_ServesSourceAndVariantCachedTwoDaysAgo caches a source and
 // a variant of it and sets back two days every time pixa stored for them.
-// For the source: fetched_at in source_metadata, fetched_at and
-// last_accessed_at in source_content, the fetched_at in its metadata file,
-// and the access and modification times of its file and of its metadata
-// file. For the variant: created_at and last_accessed_at in variant_content,
-// the created_at in its .meta file, and the access and modification times of
-// its file and of its .meta file. It then checks that the variant is still
-// served from the cache and that a new variant is made from the cached source
-// without fetching it again.
+// For the source: fetched_at and last_accessed_at in source_content, and the
+// access and modification times of its file. For the variant: created_at and
+// last_accessed_at in variant_content, the created_at in its .meta file, and
+// the access and modification times of its file and of its .meta file. It
+// then checks that the variant is still served from the cache and that a new
+// variant is made from the cached source without fetching it again.
 //
 // Left at the present: the files' status change and birth times, which a
 // program cannot set, and the times of the directories holding the files,
-// which other entries share. The source's stored response headers are empty,
-// as the mock fetcher sends none. expires_at, etag and last_modified in
-// source_metadata and in the metadata file are never written, nor are the
-// request_cache and output_content tables.
+// which other entries share. The request_cache and output_content tables are
+// never written.
 func TestService_Get_ServesSourceAndVariantCachedTwoDaysAgo(t *testing.T) {
 	t.Parallel()
 
@@ -615,39 +611,19 @@ func backdateSource(
 ) {
 	t.Helper()
 
-	pathHash := HashPath(req.Source.Path + "?" + req.Source.Query)
-
-	meta, err := cache.srcMetadata.Load(req.Source.Host, pathHash)
-	if err != nil {
-		t.Fatalf("reading the source's metadata file: %v", err)
-	}
-
-	meta.FetchedAt = when.Unix()
-
-	err = cache.srcMetadata.Store(req.Source.Host, pathHash, meta)
-	if err != nil {
-		t.Fatalf("writing the source's metadata file: %v", err)
-	}
-
+	key := req.Source.Key()
 	dbTime := when.UTC().Format(sqliteTimestampFormat)
 
-	updateOneRow(t, cache, `
-		UPDATE source_metadata SET fetched_at = ?
-		WHERE source_host = ? AND source_path = ? AND source_query = ?
-	`, dbTime, req.Source.Host, req.Source.Path, req.Source.Query)
 	updateOneRow(t, cache,
-		`UPDATE source_content SET fetched_at = ? WHERE content_hash = ?`,
-		dbTime, meta.ContentHash)
-	setSourceLastAccessed(t, cache, ContentHash(meta.ContentHash), when)
+		`UPDATE source_content SET fetched_at = ? WHERE source_key = ?`,
+		dbTime, string(key))
+	setSourceLastAccessed(t, cache, key, when)
 
-	for _, path := range []string{
-		cache.srcContent.hashToPath(ContentHash(meta.ContentHash)),
-		cache.srcMetadata.metaPath(req.Source.Host, pathHash),
-	} {
-		err = os.Chtimes(path, when, when)
-		if err != nil {
-			t.Fatalf("setting back the times of %s: %v", path, err)
-		}
+	path := cache.srcContent.keyToPath(key)
+
+	err := os.Chtimes(path, when, when)
+	if err != nil {
+		t.Fatalf("setting back the times of %s: %v", path, err)
 	}
 }