[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/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: [],
+      })
+    })
+  })
+})