[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/index.js

@@ -2,6 +2,7 @@ const AddCommentOperation = require('./lib/operation/add_comment_operation')
 const Author = require('./lib/author')
 const AuthorList = require('./lib/author_list')
 const Blob = require('./lib/blob')
+const BlobStoreBase = require('./lib/blob_store_base')
 const Change = require('./lib/change')
 const ChangeRequest = require('./lib/change_request')
 const ChangeNote = require('./lib/change_note')
@@ -66,6 +67,7 @@ exports.AddCommentOperation = AddCommentOperation
 exports.Author = Author
 exports.AuthorList = AuthorList
 exports.Blob = Blob
+exports.BlobStoreBase = BlobStoreBase
 exports.Change = Change
 exports.ChangeRequest = ChangeRequest
 exports.ChangeNote = ChangeNote

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

libraries/overleaf-editor-core/test/unit/blob_store_base.test.js

@@ -0,0 +1,95 @@
+'use strict'
+
+const { expect } = require('chai')
+
+const { Blob, BlobStoreBase } = require('../..')
+
+class TestBlobStore extends BlobStoreBase {
+  constructor(contentByHash = {}) {
+    super()
+    this.contentByHash = contentByHash
+    this.fetched = []
+  }
+
+  async fetchString(hash) {
+    this.fetched.push(hash)
+    if (!(hash in this.contentByHash)) {
+      throw new Error(`nothing stored under ${hash}`)
+    }
+    return this.contentByHash[hash]
+  }
+}
+
+const HASH = 'a1b2c3d4e5f60718293a4b5c6d7e8f9012345678'
+
+describe('BlobStoreBase', function () {
+  describe('getString', function () {
+    it('fetches the content of a hash something stored', async function () {
+      const blobStore = new TestBlobStore({ [HASH]: 'hello world' })
+
+      expect(await blobStore.getString(HASH)).to.equal('hello world')
+      expect(blobStore.fetched).to.deep.equal([HASH])
+    })
+
+    it('answers the hash of empty content without fetching', async function () {
+      // The store below has nothing stored under it and throws when asked, which is
+      // the point: the hash says what the content is, so no store has to be asked.
+      const blobStore = new TestBlobStore()
+
+      expect(await blobStore.getString(Blob.EMPTY_HASH)).to.equal('')
+      expect(blobStore.fetched).to.have.length(0)
+    })
+
+    it('passes on what the implementation throws', async function () {
+      const blobStore = new TestBlobStore()
+
+      try {
+        await blobStore.getString(HASH)
+        expect.fail('should have thrown')
+      } catch (err) {
+        expect(err.message).to.equal(`nothing stored under ${HASH}`)
+      }
+    })
+  })
+
+  describe('getObject', function () {
+    it('deserializes the content of the hash', async function () {
+      const blobStore = new TestBlobStore({
+        [HASH]: '{"comments":[],"trackedChanges":[]}',
+      })
+
+      expect(await blobStore.getObject(HASH)).to.deep.equal({
+        comments: [],
+        trackedChanges: [],
+      })
+    })
+
+    it('fails on the hash of empty content, which is not JSON', async function () {
+      // Nothing writes an empty ranges blob -- JSON.stringify never yields empty
+      // content -- so a hash reaching here is one something stored.
+      const blobStore = new TestBlobStore()
+
+      try {
+        await blobStore.getObject(Blob.EMPTY_HASH)
+        expect.fail('should have thrown')
+      } catch (err) {
+        expect(err).to.be.an.instanceof(SyntaxError)
+      }
+    })
+  })
+
+  describe('fetchString', function () {
+    it('names the store that has not implemented it', async function () {
+      class IncompleteBlobStore extends BlobStoreBase {}
+
+      try {
+        await new IncompleteBlobStore().getString(HASH)
+        expect.fail('should have thrown')
+      } catch (err) {
+        expect(err.message).to.equal(
+          'IncompleteBlobStore does not implement fetchString(hash)'
+        )
+      }
+    })
+  })
+})

services/clsi/app/js/HistoryResourceWriter.js

@@ -9,9 +9,9 @@ import Errors from './Errors.js'
 import { callbackify, promisify } from 'node:util'
 import {
   AddFileOperation,
+  BlobStoreBase,
   Change,
   EditFileOperation,
-  File,
   MoveFileOperation,
   Snapshot,
 } from 'overleaf-editor-core'
@@ -799,7 +799,7 @@ export async function syncResourcesToDisk(
   }
 }
 
-class BlobStore {
+class BlobStore extends BlobStoreBase {
   /** @type {string} */
   #historyId
   /** @type {string[]} */
@@ -816,6 +816,7 @@ class BlobStore {
    * @param {string[]} globalBlobs
    */
   constructor(historyId, filestoreBlobPrefix, clsiPerfVariant, globalBlobs) {
+    super()
     this.#historyId = historyId
     this.#filestoreBlobPrefix = filestoreBlobPrefix
     this.#clsiPerfVariant = clsiPerfVariant
@@ -845,8 +846,7 @@ class BlobStore {
    * @param {string} hash
    * @return {Promise<string>}
    */
-  async getString(hash) {
-    if (hash === File.EMPTY_FILE_HASH) return ''
+  async fetchString(hash) {
     const u = this.getBlobURL(hash)
     let remainingAttempts = 3
     while (true) {
@@ -866,13 +866,4 @@ class BlobStore {
       }
     }
   }
-
-  /**
-   * @param {string} hash
-   * @return {Promise<any>}
-   */
-  async getObject(hash) {
-    const string = await this.getString(hash)
-    return JSON.parse(string)
-  }
 }

services/history-v1/storage/lib/backupArchiver.mjs

@@ -29,7 +29,7 @@ import { NoKEKMatchedError } from '@overleaf/object-persistor/src/Errors.js'
 
 const globalBlobsBucket = config.get('blobStore.globalBucket')
 
-class BackupBlobStore {
+class BackupBlobStore extends core.BlobStoreBase {
   /**
    *
    * @param {string} historyId
@@ -38,6 +38,7 @@ class BackupBlobStore {
    * @param {boolean} useBackupGlobalBlobs
    */
   constructor(historyId, tmp, persistor, useBackupGlobalBlobs) {
+    super()
     this.historyId = historyId
     this.tmp = tmp
     this.blobs = new Map()
@@ -46,17 +47,13 @@ class BackupBlobStore {
   }
 
   /**
-   * Required for BlobStore interface - not supported.
-   *
    * @template T
    * @param {string} hash
    * @return {Promise<T>}
    */
   async getObject(hash) {
     try {
-      const stream = await this.getStream(hash)
-      const buffer = await streams.readStreamToBuffer(stream)
-      return JSON.parse(buffer.toString())
+      return await super.getObject(hash)
     } catch (err) {
       logger.warn({ err, hash }, 'Failed to fetch chunk blob')
       throw err
@@ -146,7 +143,7 @@ class BackupBlobStore {
    * @param {string} hash
    * @return {Promise<string>}
    */
-  async getString(hash) {
+  async fetchString(hash) {
     const stream = await this.getStream(hash)
     const buffer = await streams.readStreamToBuffer(stream)
     return buffer.toString()

services/history-v1/storage/lib/blob_store/index.js

@@ -28,6 +28,21 @@ const { promiseMapWithLimit } = require('@overleaf/promise-utils')
 /** @type {Map<string, { blob: core.Blob, demoted: boolean}>} */
 const GLOBAL_BLOBS = new Map()
 
+/**
+ * Empty content, which reads need nothing stored to answer.
+ *
+ * Its hash is a property of the content rather than of anything written, and every
+ * project's empty file has it, so a read answers it from here rather than looking for
+ * it: a file may reference it before anything has stored it. Writes are unchanged and
+ * still store it, so a project that wrote empty content also has a blob for it; a read
+ * answers the same either way.
+ *
+ * Left out of `GLOBAL_BLOBS` deliberately, because that is loaded from the database and
+ * can be configured away, and because a global blob is fetched from the global bucket
+ * rather than not fetched at all.
+ */
+const EMPTY_BLOB = new Blob(Blob.EMPTY_HASH, 0, 0)
+
 function makeGlobalKey(hash) {
   return `${hash.slice(0, 2)}/${hash.slice(2, 4)}/${hash.slice(4)}`
 }
@@ -163,12 +178,13 @@ async function getProjectBlobsBatch(projectIds) {
  * blob store manages both content and metadata (byte and UTF-8 length) for
  * blobs.
  */
-class BlobStore {
+class BlobStore extends core.BlobStoreBase {
   /**
    * @constructor
    * @param {string} projectId the project for which we'd like to find blobs
    */
   constructor(projectId) {
+    super()
     assert.projectId(projectId)
     this.projectId = projectId
     this.backend = getBackend(this.projectId)
@@ -279,7 +295,7 @@ class BlobStore {
    * @param {string} hash hexadecimal SHA-1 hash
    * @return {Promise.<string>} promise for the content of the file
    */
-  async getString(hash) {
+  async fetchString(hash) {
     assert.blobHash(hash, 'bad hash')
 
     const projectId = this.projectId
@@ -333,6 +349,10 @@ class BlobStore {
    */
   async getStream(hash, opts = {}) {
     assert.blobHash(hash, 'bad hash')
+    if (hash === Blob.EMPTY_HASH) {
+      // Any range of empty content is empty, so opts needs no honouring.
+      return new ReadableString('')
+    }
 
     const { bucket, key } = getBlobLocation(this.projectId, hash)
     try {
@@ -354,6 +374,9 @@ class BlobStore {
    */
   async getBlob(hash) {
     assert.blobHash(hash, 'bad hash')
+    if (hash === Blob.EMPTY_HASH) {
+      return EMPTY_BLOB
+    }
     const globalBlob = GLOBAL_BLOBS.get(hash)
     if (globalBlob != null) {
       return globalBlob.blob
@@ -372,6 +395,10 @@ class BlobStore {
     const nonGlobalHashes = []
     const blobs = []
     for (const hash of hashes) {
+      if (hash === Blob.EMPTY_HASH) {
+        blobs.push(EMPTY_BLOB)
+        continue
+      }
       const globalBlob = GLOBAL_BLOBS.get(hash)
       if (globalBlob != null) {
         blobs.push(globalBlob.blob)

services/history-v1/storage/lib/hash_check_blob_store.js

@@ -1,30 +1,52 @@
-const Blob = require('overleaf-editor-core').Blob
+const { Blob, BlobStoreBase } = require('overleaf-editor-core')
 const blobHash = require('./blob_hash')
-const BPromise = require('bluebird')
 
 // We want to simulate applying all of the operations so we can return the
 // resulting hashes to the caller for them to check. To do this, we need to be
 // able to take the lazy files in the final snapshot, fetch their content, and
 // compute the new content hashes. We don't, however, need to actually store
 // that content; we just need to get the hash.
-function HashCheckBlobStore(realBlobStore) {
-  this.realBlobStore = realBlobStore
-}
-
-HashCheckBlobStore.prototype.getString = BPromise.method(
-  function hashCheckBlobStoreGetString(hash) {
-    return this.realBlobStore.getString(hash)
+//
+// A file's ranges go through here the same way its content does: reading one
+// needs getObject, which the base class builds on fetchString, and hashing one
+// again needs putObject, which is putString of its JSON. Without both, a project
+// holding a tracked change or a comment could not be hashed at all.
+class HashCheckBlobStore extends BlobStoreBase {
+  /**
+   * @param {{ getString(hash: string): Promise<string> }} realBlobStore
+   */
+  constructor(realBlobStore) {
+    super()
+    this.realBlobStore = realBlobStore
   }
-)
 
-HashCheckBlobStore.prototype.putString = BPromise.method(
-  function hashCheckBlobStorePutString(string) {
+  /**
+   * @param {string} hash
+   * @return {Promise<string>}
+   */
+  async fetchString(hash) {
+    return await this.realBlobStore.getString(hash)
+  }
+
+  /**
+   * @param {string} string
+   * @return {Promise<Blob>}
+   */
+  async putString(string) {
     return new Blob(
       blobHash.fromString(string),
       Buffer.byteLength(string),
       string.length
     )
   }
-)
+
+  /**
+   * @param {object} obj
+   * @return {Promise<Blob>}
+   */
+  async putObject(obj) {
+    return await this.putString(JSON.stringify(obj))
+  }
+}
 
 module.exports = HashCheckBlobStore

services/history-v1/storage/scripts/recover_zip.js

@@ -114,8 +114,9 @@ async function loadChunk(historyPathname, blobStore) {
   return new core.Chunk(history, 0)
 }
 
-class RecoveryBlobStore {
+class RecoveryBlobStore extends core.BlobStoreBase {
   constructor(historyId, tmp) {
+    super()
     this.historyId = historyId
     this.tmp = tmp
     this.blobs = new Map()
@@ -166,7 +167,7 @@ class RecoveryBlobStore {
     return new core.Blob(hash, byteLength, stringLength)
   }
 
-  async getString(hash) {
+  async fetchString(hash) {
     const stream = await this.getStream(hash)
     const buffer = await streams.readStreamToBuffer(stream)
     return buffer.toString()

services/history-v1/test/acceptance/js/api/project_hashed_content.test.js

@@ -0,0 +1,98 @@
+'use strict'
+
+const { expect } = require('chai')
+
+const cleanup = require('../storage/support/cleanup')
+const fixtures = require('../storage/support/fixtures')
+const testServer = require('./support/test_server')
+const testProjects = require('./support/test_projects')
+
+const core = require('overleaf-editor-core')
+const Change = core.Change
+const File = core.File
+const Operation = core.Operation
+
+const blobHash = require('../../../../storage/lib/blob_hash')
+const persistChanges = require('../../../../storage/lib/persist_changes')
+
+describe('project hashed content', function () {
+  beforeEach(cleanup.everything)
+  beforeEach(fixtures.create)
+
+  let basicAuthClient
+  let limitsToPersistImmediately
+
+  before(async function () {
+    basicAuthClient = testServer.basicAuthClient
+
+    const farFuture = new Date()
+    farFuture.setTime(farFuture.getTime() + 7 * 24 * 3600 * 1000)
+    limitsToPersistImmediately = {
+      minChangeTimestamp: farFuture,
+      maxChangeTimestamp: farFuture,
+      maxChanges: 10,
+      maxChunkChanges: 10,
+    }
+  })
+
+  it('hashes a file that carries tracked changes', async function () {
+    // A file with tracked changes keeps its ranges in a second blob, which it needs
+    // alongside its content both to be read back and to be hashed again.
+    const projectId = await testProjects.createEmptyProject()
+    const content = 'the quick brown fox'
+    const change = new Change(
+      [
+        Operation.addFile(
+          'tracked.tex',
+          File.fromRaw({
+            content,
+            trackedChanges: [
+              {
+                range: { pos: 4, length: 'quick '.length },
+                tracking: {
+                  type: 'delete',
+                  ts: '2024-01-01T00:00:00.000Z',
+                  userId: 'user1',
+                },
+              },
+            ],
+          })
+        ),
+      ],
+      new Date(),
+      []
+    )
+    await persistChanges(projectId, [change], limitsToPersistImmediately, 0)
+
+    const response = await basicAuthClient.apis.Project.getLatestHashedContent({
+      project_id: projectId,
+    })
+
+    expect(response.status).to.equal(200)
+    const file = response.obj.files['tracked.tex']
+    // The content is hashed as it stands, tracked delete included, and the ranges
+    // are hashed separately.
+    expect(file.hash).to.equal(blobHash.fromString(content))
+    expect(file.rangesHash).to.match(/^[0-9a-f]{40}$/)
+  })
+
+  it('hashes a file that carries no ranges', async function () {
+    const projectId = await testProjects.createEmptyProject()
+    const content = 'hello world'
+    const change = new Change(
+      [Operation.addFile('main.tex', File.fromString(content))],
+      new Date(),
+      []
+    )
+    await persistChanges(projectId, [change], limitsToPersistImmediately, 0)
+
+    const response = await basicAuthClient.apis.Project.getLatestHashedContent({
+      project_id: projectId,
+    })
+
+    expect(response.status).to.equal(200)
+    const file = response.obj.files['main.tex']
+    expect(file.hash).to.equal(blobHash.fromString(content))
+    expect(file.rangesHash).to.be.undefined
+  })
+})

services/history-v1/test/acceptance/js/storage/batch_blob_store.test.js

@@ -5,9 +5,6 @@ const { expect } = require('chai')
 const cleanup = require('./support/cleanup')
 const testFiles = require('./support/test_files')
 
-const core = require('overleaf-editor-core')
-const File = core.File
-
 const storage = require('../../../../storage')
 const BatchBlobStore = storage.BatchBlobStore
 const BlobStore = storage.BlobStore
@@ -16,6 +13,10 @@ const projectId = '123'
 const blobStore = new BlobStore(projectId)
 const batchBlobStore = new BatchBlobStore(blobStore)
 
+// A well-formed hash that nothing has been stored under. Not the empty file's hash,
+// which the blob store answers from the content it stands for rather than looking it up.
+const MISSING_HASH = '0'.repeat(40)
+
 describe('BatchBlobStore', function () {
   beforeEach(cleanup.everything)
 
@@ -29,7 +30,7 @@ describe('BatchBlobStore', function () {
     // Cache some blobs (one that exists and another that doesn't)
     await batchBlobStore.preload([
       testFiles.GRAPH_PNG_HASH,
-      File.EMPTY_FILE_HASH, // not found
+      MISSING_HASH, // not found
     ])
     expect(batchBlobStore.blobs.size).to.equal(1)
 
@@ -37,7 +38,7 @@ describe('BatchBlobStore', function () {
       await Promise.all([
         batchBlobStore.getBlob(testFiles.GRAPH_PNG_HASH), // cached
         batchBlobStore.getBlob(testFiles.HELLO_TXT_HASH), // not cached; exists
-        batchBlobStore.getBlob(File.EMPTY_FILE_HASH), // not cached; not exists
+        batchBlobStore.getBlob(MISSING_HASH), // not cached; not exists
         batchBlobStore.getBlob(testFiles.GRAPH_PNG_HASH), // duplicate
       ])
 

services/history-v1/test/acceptance/js/storage/blob_store.test.js

@@ -109,6 +109,45 @@ describe('BlobStore', function () {
         expect(content).to.equal(helloWorldString)
       })
 
+      describe('empty content', function () {
+        // A read answers the hash of empty content without looking anything up: the
+        // hash is a property of the content, so every project has it whether or not
+        // anything wrote it, and a file may reference it before anything has. Writing
+        // it is unchanged and still stores a blob.
+        const emptyHash = Blob.EMPTY_HASH
+
+        it('is fetched without anything having stored it', async function () {
+          const blob = await blobStore.getBlob(emptyHash)
+
+          expect(blob.getHash()).to.equal(emptyHash)
+          expect(blob.getByteLength()).to.equal(0)
+          expect(blob.getStringLength()).to.equal(0)
+          expect(await blobStore.getString(emptyHash)).to.equal('')
+          const stream = await blobStore.getStream(emptyHash)
+          expect(await streams.readStreamToBuffer(stream)).to.have.length(0)
+        })
+
+        it('is fetched alongside blobs that were stored', async function () {
+          await blobStore.putString(helloWorldString)
+
+          const blobs = await blobStore.getBlobs([emptyHash, helloWorldHash])
+
+          expect(blobs.map(blob => blob.getHash())).to.have.members([
+            emptyHash,
+            helloWorldHash,
+          ])
+        })
+
+        it('is still stored when it is written', async function () {
+          const blob = await blobStore.putString('')
+
+          expect(blob.getHash()).to.equal(emptyHash)
+          expect(
+            (await blobStore.getProjectBlobs()).map(blob => blob.getHash())
+          ).to.include(emptyHash)
+        })
+      })
+
       it('can store and fetch utf-8 files', async function () {
         const testFile = 'hello.txt'
 

services/history-v1/test/acceptance/js/storage/recover_zip.test.mjs

@@ -15,7 +15,7 @@ import ChunkStore from '../../../../storage/lib/chunk_store/index.js'
 import persistChanges from '../../../../storage/lib/persist_changes.js'
 import testFiles from '../storage/support/test_files.js'
 import cleanup from './support/cleanup.js'
-import { getZipEntries } from './support/unzip.js'
+import { getZipEntries, getZipEntryContents } from './support/unzip.js'
 
 describe('recover_zip script', function () {
   let projectId
@@ -119,6 +119,54 @@ describe('recover_zip script', function () {
     }
   })
 
+  it('recovers a file that carries tracked changes', async function () {
+    this.timeout(30 * 1000)
+
+    // A file with tracked changes has its ranges in a second blob, which the file
+    // needs alongside its content to be loaded at all. The zip holds the content as
+    // accepting the changes would leave it, so a tracked delete is not in it.
+    const content = 'the quick brown fox'
+    const addTracked = Operation.addFile(
+      'tracked.tex',
+      File.fromRaw({
+        content,
+        trackedChanges: [
+          {
+            range: { pos: 4, length: 'quick '.length },
+            tracking: {
+              type: 'delete',
+              ts: '2024-01-01T00:00:00.000Z',
+              userId: 'user1',
+            },
+          },
+        ],
+      })
+    )
+    const change3 = new Change([addTracked], new Date(), [])
+    await persistChanges(projectId, [change3], limitsToPersistImmediately, 2)
+
+    const zipPath = `/tmp/test-recover-zip-tracked-${projectId}.zip`
+    try {
+      const { stdout } = await runRecoverZipScript([projectId])
+
+      const urlMatch = stdout.match(/(https?:\/\/[^\s]+)/)
+      expect(urlMatch).to.not.be.null
+      const res = await fetch(urlMatch[1])
+      expect(res.ok).to.be.true
+      await fs.promises.writeFile(zipPath, Buffer.from(await res.arrayBuffer()))
+
+      const contents = await getZipEntryContents(zipPath)
+      expect([...contents.keys()].sort()).to.deep.equal([
+        'graph.png',
+        'main.tex',
+        'tracked.tex',
+      ])
+      expect(contents.get('tracked.tex').toString()).to.equal('the brown fox')
+    } finally {
+      await fs.promises.unlink(zipPath).catch(() => {})
+    }
+  })
+
   it('supports the --verbose flag', async function () {
     this.timeout(30 * 1000)
 

services/history-v1/test/acceptance/js/storage/support/unzip.js

@@ -19,4 +19,35 @@ function getZipEntries(pathname) {
   return yauzl.openAsync(pathname).then(readEntries)
 }
 
+/**
+ * The contents of every entry in a zip, by name.
+ *
+ * @param {string} pathname
+ * @return {Promise<Map<string, Buffer>>}
+ */
+function getZipEntryContents(pathname) {
+  return yauzl.openAsync(pathname, { lazyEntries: true }).then(
+    zip =>
+      new BPromise((resolve, reject) => {
+        const contents = new Map()
+        zip.on('entry', entry => {
+          zip.openReadStream(entry, (err, stream) => {
+            if (err) return reject(err)
+            const chunks = []
+            stream.on('data', chunk => chunks.push(chunk))
+            stream.on('error', reject)
+            stream.on('end', () => {
+              contents.set(entry.fileName, Buffer.concat(chunks))
+              zip.readEntry()
+            })
+          })
+        })
+        zip.on('error', reject)
+        zip.on('end', () => resolve(contents))
+        zip.readEntry()
+      })
+  )
+}
+
 exports.getZipEntries = getZipEntries
+exports.getZipEntryContents = getZipEntryContents

services/project-history/app/js/HistoryStoreManager.js

@@ -18,6 +18,7 @@ import * as LocalFileWriter from './LocalFileWriter.js'
 import * as HashManager from './HashManager.js'
 import * as HistoryBlobTranslator from './HistoryBlobTranslator.js'
 import { callbackify, promisifyMultiResult } from '@overleaf/promise-utils'
+import { BlobStoreBase } from 'overleaf-editor-core'
 
 const HTTP_REQUEST_TIMEOUT = Settings.overleaf.history.requestTimeout
 
@@ -555,19 +556,15 @@ export function deleteProject(projectId, callback) {
 
 const getProjectBlobAsync = promisify(getProjectBlob)
 
-class BlobStore {
+class BlobStore extends BlobStoreBase {
   constructor(projectId) {
+    super()
     this.projectId = projectId
   }
 
-  async getString(hash) {
+  async fetchString(hash) {
     return await getProjectBlobAsync(this.projectId, hash)
   }
-
-  async getObject(hash) {
-    const string = await this.getString(hash)
-    return JSON.parse(string)
-  }
 }
 
 export function getBlobStore(projectId) {

services/web/app/src/Features/History/HistoryBlobStore.mjs

@@ -1,4 +1,5 @@
 import { text } from 'node:stream/consumers'
+import { BlobStoreBase } from 'overleaf-editor-core'
 import HistoryManager from './HistoryManager.mjs'
 
 /**
@@ -8,11 +9,12 @@ import HistoryManager from './HistoryManager.mjs'
  * eagerly here -- needed for the ones with pending edit operations, whose content
  * is not in a single blob.
  */
-class HistoryBlobStore {
+class HistoryBlobStore extends BlobStoreBase {
   /**
    * @param {string} historyId
    */
   constructor(historyId) {
+    super()
     this.historyId = historyId
   }
 
@@ -20,21 +22,13 @@ class HistoryBlobStore {
    * @param {string} hash
    * @return {Promise<string>}
    */
-  async getString(hash) {
+  async fetchString(hash) {
     const { stream } = await HistoryManager.promises.requestBlob(
       this.historyId,
       hash
     )
     return await text(stream)
   }
-
-  /**
-   * @param {string} hash
-   * @return {Promise<object>}
-   */
-  async getObject(hash) {
-    return JSON.parse(await this.getString(hash))
-  }
 }
 
 export default HistoryBlobStore

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) {

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

@@ -1,5 +1,6 @@
 import { expect } from 'chai'
 import fetchMock from 'fetch-mock'
+import { File } from 'overleaf-editor-core'
 import { ProjectSnapshot } from '@/infrastructure/project-snapshot'
 
 describe('ProjectSnapshot', function () {
@@ -714,4 +715,90 @@ describe('ProjectSnapshot', function () {
       )
     })
   })
+
+  describe('blob with the hash of empty content', function () {
+    // Nothing is stored under that hash, on the server or here, so the content is
+    // answered from the hash rather than fetched. A new empty doc references it before
+    // anything has written it, so a fetch would 404 and take the whole snapshot down.
+
+    afterEach(function () {
+      fetchMock.removeRoutes().clearHistory()
+    })
+
+    it('loads an empty doc without fetching the blob', async function () {
+      const emptyChunk = {
+        history: {
+          snapshot: { files: {} },
+          changes: [
+            {
+              operations: [
+                {
+                  pathname: 'fresh.tex',
+                  file: { hash: File.EMPTY_FILE_HASH, stringLength: 0 },
+                },
+              ],
+              timestamp: '2025-01-01T12:00:00.000Z',
+            },
+          ],
+        },
+        startVersion: 0,
+      }
+
+      fetchMock.post(`/project/${projectId}/flush`, 200)
+      fetchMock.getOnce(`/project/${projectId}/latest/history`, {
+        chunk: emptyChunk,
+      })
+      // Served only if the short circuit does not answer first, in which case the
+      // content below is what comes back.
+      fetchMock.get(
+        `/project/${projectId}/blob/${File.EMPTY_FILE_HASH}`,
+        'fetched from the server'
+      )
+
+      await snapshot.refresh()
+
+      expect(snapshot.getDocPaths()).to.have.members(['fresh.tex'])
+      expect(snapshot.getDocContents('fresh.tex')).to.equal('')
+    })
+
+    it('applies edits made on top of an empty doc', async function () {
+      const insertedText = '\\documentclass{article}\n'
+      const emptyChunk = {
+        history: {
+          snapshot: { files: {} },
+          changes: [
+            {
+              operations: [
+                {
+                  pathname: 'fresh.tex',
+                  file: { hash: File.EMPTY_FILE_HASH, stringLength: 0 },
+                },
+              ],
+              timestamp: '2025-01-01T12:00:00.000Z',
+            },
+            {
+              operations: [
+                { pathname: 'fresh.tex', textOperation: [insertedText] },
+              ],
+              timestamp: '2025-01-01T12:01:00.000Z',
+            },
+          ],
+        },
+        startVersion: 0,
+      }
+
+      fetchMock.post(`/project/${projectId}/flush`, 200)
+      fetchMock.getOnce(`/project/${projectId}/latest/history`, {
+        chunk: emptyChunk,
+      })
+      fetchMock.get(
+        `/project/${projectId}/blob/${File.EMPTY_FILE_HASH}`,
+        'fetched from the server'
+      )
+
+      await snapshot.refresh()
+
+      expect(snapshot.getDocContents('fresh.tex')).to.equal(insertedText)
+    })
+  })
 })

services/web/test/unit/src/History/HistoryBlobStore.test.mjs

@@ -0,0 +1,73 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest'
+import sinon from 'sinon'
+import { Readable } from 'node:stream'
+import { Blob } from 'overleaf-editor-core'
+
+const modulePath = '../../../../app/src/Features/History/HistoryBlobStore.mjs'
+
+const HASH = 'a1b2c3d4e5f60718293a4b5c6d7e8f9012345678'
+
+describe('HistoryBlobStore', function () {
+  beforeEach(async function (ctx) {
+    ctx.HistoryManager = {
+      promises: { requestBlob: sinon.stub() },
+    }
+    vi.doMock(
+      '../../../../app/src/Features/History/HistoryManager.mjs',
+      () => ({
+        default: ctx.HistoryManager,
+      })
+    )
+
+    const HistoryBlobStore = (await import(modulePath)).default
+    ctx.blobStore = new HistoryBlobStore('history-id')
+  })
+
+  describe('getString', function () {
+    it('reads the blob of the given hash', async function (ctx) {
+      ctx.HistoryManager.promises.requestBlob.resolves({
+        stream: Readable.from(['hello ', 'world']),
+      })
+
+      expect(await ctx.blobStore.getString(HASH)).to.equal('hello world')
+      expect(ctx.HistoryManager.promises.requestBlob).to.have.been.calledWith(
+        'history-id',
+        HASH
+      )
+    })
+
+    it('passes on errors from history', async function (ctx) {
+      ctx.HistoryManager.promises.requestBlob.rejects(new Error('boom'))
+
+      await expect(ctx.blobStore.getString(HASH)).to.be.rejectedWith('boom')
+    })
+
+    it('answers the hash of empty content without asking history', async function (ctx) {
+      // Nothing is stored under it, so history-v1 answers it without a lookup of its
+      // own: the round-trip to ask can only be slow or fail.
+      ctx.HistoryManager.promises.requestBlob.resolves({
+        stream: Readable.from(['fetched from history-v1']),
+      })
+
+      expect(await ctx.blobStore.getString(Blob.EMPTY_HASH)).to.equal('')
+    })
+
+    it('answers the hash of empty content when history is down', async function (ctx) {
+      ctx.HistoryManager.promises.requestBlob.rejects(new Error('boom'))
+
+      expect(await ctx.blobStore.getString(Blob.EMPTY_HASH)).to.equal('')
+    })
+  })
+
+  describe('getObject', function () {
+    it('parses the blob of the given hash as JSON', async function (ctx) {
+      ctx.HistoryManager.promises.requestBlob.resolves({
+        stream: Readable.from(['{"comments":[]}']),
+      })
+
+      expect(await ctx.blobStore.getObject(HASH)).to.deep.equal({
+        comments: [],
+      })
+    })
+  })
+})