[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

services/web/frontend/js/infrastructure/project-snapshot.ts

@@ -1,5 +1,11 @@
 import pLimit from 'p-limit'
-import { Change, Chunk, Snapshot, File } from 'overleaf-editor-core'
+import {
+  BlobStoreBase,
+  Change,
+  Chunk,
+  Snapshot,
+  File,
+} from 'overleaf-editor-core'
 import type { RawChange, RawChunk } from 'overleaf-editor-core/lib/types'
 import { FetchError, getJSON, postJSON } from '@/infrastructure/fetch-json'
 import path from 'path-browserify'
@@ -151,11 +157,13 @@ export class ProjectSnapshot {
     if (byteLength == null) {
       return null
     }
-    let blobStoreOptions
     if (options?.maxSize != null && byteLength > options?.maxSize) {
-      blobStoreOptions = { maxSize: options.maxSize }
+      const blobStoreOptions = { maxSize: options.maxSize }
+      // Straight to fetchString: a binary file large enough to be read with a size
+      // limit is never the empty content the base class answers for.
+      return await this.blobStore.fetchString(hash, blobStoreOptions)
     }
-    return await this.blobStore.getString(hash, blobStoreOptions)
+    return await this.blobStore.getString(hash)
   }
 
   getDocs(): Map<string, File> {
@@ -256,24 +264,20 @@ export class ProjectSnapshot {
 /**
  * Blob store that fetches blobs from the history service
  */
-export class SimpleBlobStore {
+export class SimpleBlobStore extends BlobStoreBase {
   private projectId: string
 
   constructor(projectId: string) {
+    super()
     this.projectId = projectId
   }
 
-  async getString(
+  async fetchString(
     hash: string,
     options?: { maxSize?: number }
   ): Promise<string> {
     return await fetchBlob(this.projectId, hash, options)
   }
-
-  async getObject(hash: string) {
-    const blob = await this.getString(hash)
-    return JSON.parse(blob)
-  }
 }
 
 async function flushHistory(projectId: string) {