[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/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)
+    })
+  })
 })