(core) make sure forks with no changes are persisted

Summary:
This fixes a problem where a fork could be created, have no changes
made, and then (e.g. if worker rolled over) fail to open with a
`cannot create fork` error. Adds a test that fails priot to this diff.

Test Plan: added test

Reviewers: georgegevoian

Reviewed By: georgegevoian

Differential Revision: https://phab.getgrist.com/D3162

app/server/lib/ExternalStorage.ts

@@ -240,9 +240,12 @@ export class ChecksummedExternalStorage implements ExternalStorage {
         // exist in this case, and this should fail if it does.
         await fse.move(tmpPath, fname, {overwrite: false});
         if (fromKey === toKey) {
+          // Save last S3 snapshot id observed for this key.
           await this._options.latestVersion.save(toKey, downloadedSnapshotId);
+          // Save last S3 hash observed for this key (so if we have a version with that hash
+          // locally we can skip pushing it back needlessly later).
+          await this._options.localHash.save(toKey, checksum);
         }
-        await this._options.localHash.save(toKey, checksum);
 
         log.info("ext %s download: %s%s%s with checksum %s and version %s", this.label, fromKey,
                  snapshotId ? ` [VersionId ${snapshotId}]` : '',

app/server/lib/HostedStorageManager.ts

@@ -252,6 +252,9 @@ export class HostedStorageManager implements IDocStorageManager {
    */
   public async prepareFork(srcDocName: string, destDocName: string): Promise<string> {
     await this.prepareLocalDoc(destDocName, srcDocName);
+    this.markAsChanged(destDocName);  // Make sure fork is actually stored in S3, even
+                                      // if no changes are made, since we'd refuse to
+                                      // create it later.
     return this.getPath(destDocName);
   }