[monorepo] read empty content from its hash, in a shared blob store base (#36831)

The hash of empty content is a property of the content rather than of anything
stored: every project's empty file has it, whether or not a blob was ever written
under it, and a file may reference it before anything has. Eight blob stores were
each paying a lookup -- a database query, an HTTP request, a read of a cache that
offline mode may be the only thing standing in for -- to be told what the hash
already says, and clsi had already reinvented the answer on its own.

overleaf-editor-core gains BlobStoreBase, which answers getString for that hash
and delegates the rest to the fetchString its subclasses provide. getObject comes
with it: four stores spelled out the same JSON.parse of getString, and a fifth
wrapped it in a log line.

Adopted by history-v1's BlobStore, BackupBlobStore, HashCheckBlobStore and
recover_zip's RecoveryBlobStore, by project-history's BlobStore, by clsi's
BlobStore, and by web's HistoryBlobStore, SimpleBlobStore and FrontendBlobStore.
history-v1 keeps its own answers in getStream, getBlob and getBlobs, which no
other store implements. Writing and copying a blob are untouched, so a project
that wrote empty content still has a blob for it and a read answers the same
either way.

Two paths could not handle a project holding a tracked change or a comment at
all. Loading such a file needs the ranges blob its rangesHash points at, and
hashing one again needs to write it back, so a store missing getObject or
putObject fails outright:

- recover_zip died on `blobStore.getObject is not a function`, making every such
  project unrecoverable. The test covering it asserts a tracked delete is absent
  from the zip, which the script already asked for and never reached.
- GET /projects/:project_id/latest/hashed_content returned 500 from the same
  cause, then would have hit a missing putObject in snapshot.store.
  HashCheckBlobStore becomes a class over the base and gains putObject.

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>
GitOrigin-RevId: c84f62465beff6cf77ffac26aa772984e1b554cb

libraries/overleaf-editor-core/lib/blob_store_base.js

@@ -0,0 +1,68 @@
+// @ts-check
+'use strict'
+
+const Blob = require('./blob')
+
+/**
+ * The parts of reading a blob that every blob store answers the same way.
+ *
+ * The hash of empty content is a property of the content rather than of anything
+ * stored: every project's empty file has it, whether or not anything ever wrote a
+ * blob under it, and a file may reference it before anything has. So no store has
+ * to look it up, and every store that did was paying a lookup -- a database query,
+ * an HTTP request, a read of a cache that offline mode may be the only thing
+ * standing in for -- to be told what the hash already says. Deserializing a blob
+ * that holds JSON is likewise the same wherever it is read from.
+ *
+ * Implementations extend this and provide `fetchString`, which is only ever called
+ * for a hash that needs fetching. Spelling either of these out in each store
+ * instead is a line per store, and the store written next is the one that forgets.
+ *
+ * `getObject` inherits the empty-hash answer through `getString`, so it parses `''`
+ * and fails. That is right: a ranges blob is written from `JSON.stringify`, which
+ * never yields empty content, so a hash reaching `getObject` is one something
+ * stored. A store that reads or repairs its JSON differently overrides it.
+ */
+class BlobStoreBase {
+  /**
+   * Fetch a blob's content as a string.
+   *
+   * @param {string} hash hexadecimal SHA-1 hash
+   * @return {Promise<string>}
+   */
+  async getString(hash) {
+    if (hash === Blob.EMPTY_HASH) {
+      return ''
+    }
+    return await this.fetchString(hash)
+  }
+
+  /**
+   * Fetch a blob holding JSON and deserialize it.
+   *
+   * @template [T=unknown]
+   * @param {string} hash hexadecimal SHA-1 hash
+   * @return {Promise<T>}
+   */
+  async getObject(hash) {
+    return /** @type {T} */ (JSON.parse(await this.getString(hash)))
+  }
+
+  /**
+   * Fetch a blob's content as a string from wherever this store keeps it.
+   *
+   * Called only for a hash that something needs to have stored, so an
+   * implementation may treat a miss as an error.
+   *
+   * @abstract
+   * @param {string} hash hexadecimal SHA-1 hash
+   * @return {Promise<string>}
+   */
+  async fetchString(hash) {
+    throw new Error(
+      `${this.constructor.name} does not implement fetchString(hash)`
+    )
+  }
+}
+
+module.exports = BlobStoreBase