(core) start reconciling forking with granular access

Summary:
This allows a fork to be made by a user if:
 * That user is an owner of the document being forked, or
 * That user has full read access to the document being forked.

The bulk of the diff is reorganization of how forking is done.  ActiveDoc.fork is now responsible for creating a fork, not just a docId/urlId for the fork. Since fork creation should not be limited to the doc worker hosting the trunk, a helper endpoint is added for placing the fork.

The change required sanitizing worker allocation a bit, and allowed session knowledge to be removed from HostedStorageManager.

Test Plan: Added test; existing tests pass.

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2700

app/common/KeyedOps.ts

@@ -99,6 +99,7 @@ export class KeyedOps {
       await status.promise;
       return;
     }
+    if (!this._changed.has(key)) { return; }
     const callback = new Promise((resolve) => {
       status.callbacks.push(resolve);
     });

app/gen-server/lib/DocApiForwarder.ts

@@ -42,6 +42,8 @@ export class DocApiForwarder {
     app.use('/api/docs/:docId/remove', withDoc);
     app.delete('/api/docs/:docId', withDoc);
     app.use('/api/docs/:docId/download', withDoc);
+    app.use('/api/docs/:docId/fork', withDoc);
+    app.use('/api/docs/:docId/create-fork', withDoc);
     app.use('/api/docs/:docId/apply', withDoc);
     app.use('/api/docs/:docId/attachments', withDoc);
     app.use('/api/docs/:docId/snapshots', withDoc);

app/gen-server/lib/DocWorkerMap.ts

@@ -328,8 +328,29 @@ export class DocWorkerMap implements IDocWorkerMap {
    * A preferred doc worker can be specified, which will be assigned
    * if no assignment is already made.
    *
+   * For the special docId "import", return a potential assignment.
+   * It will be up to the doc worker to assign the eventually
+   * created document, if desired.
+   *
    */
   public async assignDocWorker(docId: string, workerId?: string): Promise<DocStatus> {
+    if (docId === 'import') {
+      const lock = await this._redlock.lock(`workers-lock`, LOCK_TIMEOUT);
+      try {
+        const workerId = await this._client.srandmemberAsync(`workers-available-default`);
+        if (!workerId) { throw new Error('no doc worker available'); }
+        const docWorker = await this._client.hgetallAsync(`worker-${workerId}`) as DocWorkerInfo|null;
+        if (!docWorker) { throw new Error('no doc worker contact info available'); }
+        return {
+          docMD5: null,
+          docWorker,
+          isActive: false
+        };
+      } finally {
+        await lock.unlock();
+      }
+    }
+
     // Check if a DocWorker is already assigned; if so return result immediately
     // without locking.
     let docStatus = await this.getDocWorker(docId);

app/server/lib/ActiveDoc.ts

@@ -52,13 +52,14 @@ import {ActionHistoryImpl} from './ActionHistoryImpl';
 import {ActiveDocImport} from './ActiveDocImport';
 import {DocClients} from './DocClients';
 import {DocPluginManager} from './DocPluginManager';
-import {DocSession, getDocSessionAccess, getDocSessionUserId, makeExceptionalDocSession,
-        OptDocSession} from './DocSession';
+import {DocSession, getDocSessionAccess, getDocSessionUser, getDocSessionUserId,
+        makeExceptionalDocSession, OptDocSession} from './DocSession';
 import {DocStorage} from './DocStorage';
 import {expandQuery} from './ExpandedQuery';
 import {GranularAccess} from './GranularAccess';
 import {OnDemandActions} from './OnDemandActions';
 import {findOrAddAllEnvelope, Sharing} from './Sharing';
+import fetch from 'node-fetch';
 
 bluebird.promisifyAll(tmp);
 
@@ -328,8 +329,7 @@ export class ActiveDoc extends EventEmitter {
     const startTime = Date.now();
     this.logDebug(docSession, "loadDoc");
     try {
-      const isNew: boolean = await this._docManager.storageManager.prepareLocalDoc(this.docName,
-                                                                                   docSession);
+      const isNew: boolean = await this._docManager.storageManager.prepareLocalDoc(this.docName);
       if (isNew) {
         await this.createDoc(docSession);
         await this.addInitialTable(docSession);
@@ -571,8 +571,7 @@ export class ActiveDoc extends EventEmitter {
 
   // Check if user has rights to download this doc.
   public async canDownload(docSession: OptDocSession) {
-    return this._granularAccess.hasViewAccess(docSession) &&
-      await this._granularAccess.canReadEverything(docSession);
+    return this._granularAccess.canCopyEverything(docSession);
   }
 
   /**
@@ -885,19 +884,47 @@ export class ActiveDoc extends EventEmitter {
    * Fork the current document.  In fact, all that requires is calculating a good
    * ID for the fork.  TODO: reconcile the two ways there are now of preparing a fork.
    */
-  public async fork(docSession: DocSession): Promise<ForkResult> {
-    if (!await this._granularAccess.canReadEverything(docSession)) {
-      throw new Error('cannot confirm authority to copy document');
+  public async fork(docSession: OptDocSession): Promise<ForkResult> {
+    const user = getDocSessionUser(docSession);
+    // For now, fork only if user can read everything (or is owner).
+    // TODO: allow forks with partial content.
+    if (!user || !await this._granularAccess.canCopyEverything(docSession)) {
+      throw new ApiError('Insufficient access to document to copy it entirely', 403);
     }
-    const userId = docSession.client.getCachedUserId();
-    const isAnonymous = docSession.client.isAnonymous();
+    const userId = user.id;
+    const isAnonymous = this._docManager.isAnonymous(userId);
     // Get fresh document metadata (the cached metadata doesn't include the urlId).
-    const doc = await docSession.authorizer.getDoc();
+    const doc = await docSession.authorizer?.getDoc();
     if (!doc) { throw new Error('document id not known'); }
     const trunkDocId = doc.id;
     const trunkUrlId = doc.urlId || doc.id;
     await this.flushDoc();  // Make sure fork won't be too out of date.
-    return makeForkIds({userId, isAnonymous, trunkDocId, trunkUrlId});
+    const forkIds = makeForkIds({userId, isAnonymous, trunkDocId, trunkUrlId});
+
+    // To actually create the fork, we call an endpoint.  This is so the fork
+    // can be associated with an arbitrary doc worker, rather than tied to the
+    // same worker as the trunk.  We use a Permit for authorization.
+    const permitStore = this._docManager.gristServer.getPermitStore();
+    const permitKey = await permitStore.setPermit({docId: forkIds.docId,
+                                                   otherDocId: this.docName});
+    try {
+      const url = await this._docManager.gristServer.getHomeUrlByDocId(forkIds.docId, `/api/docs/${forkIds.docId}/create-fork`);
+      const resp = await fetch(url, {
+        method: 'POST',
+        body: JSON.stringify({ srcDocId: this.docName }),
+        headers: {
+          Permit: permitKey,
+          'Content-Type': 'application/json',
+        },
+      });
+      if (resp.status !== 200) {
+        throw new ApiError(resp.statusText, resp.status);
+      }
+    } finally {
+      await permitStore.removePermit(permitKey);
+    }
+
+    return forkIds;
   }
 
   /**

app/server/lib/Client.ts

@@ -3,6 +3,7 @@ import {BrowserSettings} from 'app/common/BrowserSettings';
 import {ErrorWithCode} from 'app/common/ErrorWithCode';
 import {UserProfile} from 'app/common/LoginSessionAPI';
 import {getLoginState, LoginState} from 'app/common/LoginState';
+import {ANONYMOUS_USER_EMAIL} from 'app/common/UserAPI';
 import {User} from 'app/gen-server/entity/User';
 import {HomeDBManager} from 'app/gen-server/lib/HomeDBManager';
 import {ActiveDoc} from 'app/server/lib/ActiveDoc';
@@ -297,10 +298,17 @@ export class Client {
     // practice via a change to profile, but let's not make any assumptions here.)
     this._userId = null;
     this._firstLoginAt = null;
-    this._isAnonymous = false;
+    this._isAnonymous = !profile;
   }
 
   public getProfile(): UserProfile|null {
+    if (this._isAnonymous) {
+      return {
+        name: 'Anonymous',
+        email: ANONYMOUS_USER_EMAIL,
+        anonymous: true,
+      };
+    }
     return this._profile;
   }
 
@@ -315,10 +323,16 @@ export class Client {
   // Returns the userId for profile.email, or null when profile is not set; with caching.
   public async getUserId(dbManager: HomeDBManager): Promise<number|null> {
     if (!this._userId) {
-      const user = await this._fetchUser(dbManager);
-      this._userId = (user && user.id) || null;
-      this._isAnonymous = this._userId && dbManager.getAnonymousUserId() === this._userId || false;
-      this._firstLoginAt = (user && user.firstLoginAt) || null;
+      if (this._profile) {
+        const user = await this._fetchUser(dbManager);
+        this._userId = (user && user.id) || null;
+        this._isAnonymous = this._userId && dbManager.getAnonymousUserId() === this._userId || false;
+        this._firstLoginAt = (user && user.firstLoginAt) || null;
+      } else {
+        this._userId = dbManager.getAnonymousUserId();
+        this._isAnonymous = true;
+        this._firstLoginAt = null;
+      }
     }
     return this._userId;
   }

app/server/lib/DocApi.ts

@@ -207,6 +207,22 @@ export class DocWorkerApi {
       }
     }));
 
+    // Fork the specified document.
+    this._app.post('/api/docs/:docId/fork', canView, withDoc(async (activeDoc, req, res) => {
+      const result = await activeDoc.fork(docSessionFromRequest(req));
+      res.json(result);
+    }));
+
+    // Initiate a fork.  Used internally to implement ActiveDoc.fork.  Only usable via a Permit.
+    this._app.post('/api/docs/:docId/create-fork', canEdit, throttled(async (req, res) => {
+      const mreq = req as RequestWithLogin;
+      const docId = stringParam(req.params.docId);
+      const srcDocId = stringParam(req.body.srcDocId);
+      if (srcDocId !== mreq.specialPermit?.otherDocId) { throw new Error('access denied'); }
+      await this._docManager.storageManager.prepareFork(srcDocId, docId);
+      res.json({srcDocId, docId});
+    }));
+
     // Update records. The records to update are identified by their id column. Any invalid id fails
     // the request and returns a 400 error code.
     this._app.patch('/api/docs/:docId/tables/:tableId/data', canEdit, withDoc(async (activeDoc, req, res) => {
@@ -458,7 +474,7 @@ export class DocWorkerApi {
       const isAnonymous = isAnonymousUser(req);
       const {docId} = makeForkIds({userId, isAnonymous, trunkDocId: NEW_DOCUMENT_CODE,
                                    trunkUrlId: NEW_DOCUMENT_CODE});
-      await this._docManager.fetchDoc(makeExceptionalDocSession('nascent', {
+      await this._docManager.createNamedDoc(makeExceptionalDocSession('nascent', {
         req: req as RequestWithLogin,
         browserSettings
       }), docId);

app/server/lib/DocManager.ts

@@ -109,7 +109,11 @@ export class DocManager extends EventEmitter {
   public async createNewDoc(client: Client): Promise<string> {
     log.debug('DocManager.createNewDoc');
     const docSession = makeExceptionalDocSession('nascent', {client});
-    const activeDoc: ActiveDoc = await this.createNewEmptyDoc(docSession, 'Untitled');
+    return this.createNamedDoc(docSession, 'Untitled');
+  }
+
+  public async createNamedDoc(docSession: OptDocSession, docId: string): Promise<string> {
+    const activeDoc: ActiveDoc = await this.createNewEmptyDoc(docSession, docId);
     await activeDoc.addInitialTable(docSession);
     return activeDoc.docName;
   }
@@ -188,9 +192,10 @@ export class DocManager extends EventEmitter {
       }
     }
 
-    // Ship the import to S3, since it isn't associated with any particular worker at this time.
-    // We could associate it with the current worker, but that is not necessarily desirable.
-    await this.storageManager.addToStorage(result.id);
+    // The imported document is associated with the worker that did the import.
+    // We could break that association (see /api/docs/:docId/assign for how) if
+    // we start using dedicated import workers.
+
     return result;
   }
 
@@ -397,6 +402,11 @@ export class DocManager extends EventEmitter {
     return makeAccessId(this.gristServer, userId);
   }
 
+  public isAnonymous(userId: number): boolean {
+    if (!this._homeDbManager) { throw new Error("HomeDbManager not available"); }
+    return userId === this._homeDbManager.getAnonymousUserId();
+  }
+
   /**
    * Helper function for creating a new shared document given the doc snapshot bundles received
    * from the sharing hub.
@@ -458,6 +468,7 @@ export class DocManager extends EventEmitter {
         const docName = await this._createNewDoc(id);
         const docPath = await this.storageManager.getPath(docName);
         await docUtils.copyFile(uploadInfo.files[0].absPath, docPath);
+        await this.storageManager.addToStorage(docName);
         return {title: basename, id: docName};
       } else {
         const doc = await this.createNewEmptyDoc(docSession, id);

app/server/lib/DocSnapshots.ts

@@ -121,6 +121,13 @@ export class DocSnapshotInventory implements IInventory {
     });
   }
 
+  /**
+   * Return true if document inventory does not need to be saved and is not in flux.
+   */
+  public isSaved(key: string) {
+    return !this._needFlush.has(key) && !this._mutex.isLocked(key);
+  }
+
   /**
    * Add a new snapshot of a document to the existing inventory.  A prevSnapshotId may
    * be supplied as a cross-check.  It will be matched against the most recent

app/server/lib/DocStorageManager.ts

@@ -8,7 +8,6 @@ import {DocEntry, DocEntryTag} from 'app/common/DocListAPI';
 import {DocSnapshots} from 'app/common/DocSnapshot';
 import * as gutil from 'app/common/gutil';
 import * as Comm from 'app/server/lib/Comm';
-import {OptDocSession} from 'app/server/lib/DocSession';
 import * as docUtils from 'app/server/lib/docUtils';
 import {GristServer} from 'app/server/lib/GristServer';
 import {IDocStorageManager} from 'app/server/lib/IDocStorageManager';
@@ -85,12 +84,16 @@ export class DocStorageManager implements IDocStorageManager {
    * Prepares a document for use locally. Returns whether the document is new (needs to be
    * created). This is a no-op in the local DocStorageManager case.
    */
-  public async prepareLocalDoc(docName: string, docSession: OptDocSession): Promise<boolean> { return false; }
+  public async prepareLocalDoc(docName: string): Promise<boolean> { return false; }
 
   public async prepareToCreateDoc(docName: string): Promise<void> {
     // nothing to do
   }
 
+  public async prepareFork(srcDocName: string, destDocName: string): Promise<void> {
+    // nothing to do
+  }
+
   /**
    * Returns a promise for the list of docNames to show in the doc list. For the file-based
    * storage, this will include all .grist files under the docsRoot.

app/server/lib/FlexServer.ts

@@ -35,6 +35,7 @@ import {IDocStorageManager} from 'app/server/lib/IDocStorageManager';
 import {INotifier} from 'app/server/lib/INotifier';
 import * as log from 'app/server/lib/log';
 import {getLoginMiddleware} from 'app/server/lib/logins';
+import {IPermitStore} from 'app/server/lib/Permit';
 import {getAppPathTo, getAppRoot, getUnpackedAppRoot} from 'app/server/lib/places';
 import {addPluginEndpoints, limitToPlugins} from 'app/server/lib/PluginEndpoint';
 import {PluginManager} from 'app/server/lib/PluginManager';
@@ -221,6 +222,11 @@ export class FlexServer implements GristServer {
     return this.server ? (this.server.address() as AddressInfo).port : this.port;
   }
 
+  public getPermitStore(): IPermitStore {
+    if (!this._docWorkerMap) { throw new Error('no permit store available'); }
+    return this._docWorkerMap;
+  }
+
   public addLogging() {
     if (this._check('logging')) { return; }
     if (process.env.GRIST_LOG_SKIP_HTTP) { return; }

app/server/lib/GranularAccess.ts

@@ -324,13 +324,24 @@ export class GranularAccess {
   }
 
   /**
-   * Check whether user can read everything in document.
+   * Check whether user can read everything in document.  Checks both home-level and doc-level
+   * permissions.
    */
   public async canReadEverything(docSession: OptDocSession): Promise<boolean> {
+    const access = getDocSessionAccess(docSession);
+    if (!canView(access)) { return false; }
     const permInfo = await this._getAccess(docSession);
     return permInfo.getFullAccess().read === 'allow';
   }
 
+  /**
+   * Check whether user can copy everything in document.  Owners can always copy
+   * everything, even if there are rules that specify they cannot.
+   */
+  public async canCopyEverything(docSession: OptDocSession): Promise<boolean> {
+    return this.isOwner(docSession) || this.canReadEverything(docSession);
+  }
+
   /**
    * Check whether user has full access to the document.  Currently that is interpreted
    * as equivalent owner-level access to the document.
@@ -349,16 +360,6 @@ export class GranularAccess {
     return access === 'owners';
   }
 
-  /**
-   * Check for view access to the document.  For most code paths, a request or message
-   * won't even be considered if there isn't view access, but there's no harm in double
-   * checking.
-   */
-  public hasViewAccess(docSession: OptDocSession): boolean {
-    const access = getDocSessionAccess(docSession);
-    return canView(access);
-  }
-
   /**
    *
    * If the user does not have access to the full document, we need to filter out

app/server/lib/GristServer.ts

@@ -2,6 +2,7 @@ import {SessionUserObj} from 'app/server/lib/BrowserSession';
 import * as Comm from 'app/server/lib/Comm';
 import {Hosts} from 'app/server/lib/extractOrg';
 import {ICreate} from 'app/server/lib/ICreate';
+import {IPermitStore} from 'app/server/lib/Permit';
 import {Sessions} from 'app/server/lib/Sessions';
 import * as express from 'express';
 
@@ -14,6 +15,7 @@ export interface GristServer {
   getHost(): string;
   getHomeUrl(req: express.Request, relPath?: string): string;
   getHomeUrlByDocId(docId: string, relPath?: string): Promise<string>;
+  getPermitStore(): IPermitStore;
 }
 
 export interface GristLoginMiddleware {

app/server/lib/HostedStorageManager.ts

@@ -7,9 +7,7 @@ import {buildUrlId, parseUrlId} from 'app/common/gristUrls';
 import {KeyedOps} from 'app/common/KeyedOps';
 import {DocReplacementOptions, NEW_DOCUMENT_CODE} from 'app/common/UserAPI';
 import {HomeDBManager} from 'app/gen-server/lib/HomeDBManager';
-import {getUserId} from 'app/server/lib/Authorizer';
 import {checksumFile} from 'app/server/lib/checksumFile';
-import {OptDocSession} from 'app/server/lib/DocSession';
 import {DocSnapshotInventory, DocSnapshotPruner} from 'app/server/lib/DocSnapshots';
 import {IDocWorkerMap} from 'app/server/lib/DocWorkerMap';
 import {ChecksummedExternalStorage, DELETED_TOKEN, ExternalStorage} from 'app/server/lib/ExternalStorage';
@@ -216,8 +214,10 @@ export class HostedStorageManager implements IDocStorageManager {
    * Returns whether the document is new (needs to be created).
    * Calling this method multiple times in parallel for the same document is treated as a sign
    * of a bug.
+   *
+   * The optional srcDocName parameter is set when preparing a fork.
    */
-  public async prepareLocalDoc(docName: string, docSession: OptDocSession): Promise<boolean> {
+  public async prepareLocalDoc(docName: string, srcDocName?: string): Promise<boolean> {
     // We could be reopening a document that is still closing down.
     // Wait for that to happen.  TODO: we could also try to interrupt the closing-down process.
     await this.closeDocument(docName);
@@ -228,7 +228,7 @@ export class HostedStorageManager implements IDocStorageManager {
 
     try {
       this._prepareFiles.add(docName);
-      const isNew = !(await this._ensureDocumentIsPresent(docName, docSession));
+      const isNew = !(await this._claimDocument(docName, srcDocName));
       return isNew;
     } finally {
       this._prepareFiles.delete(docName);
@@ -236,17 +236,27 @@ export class HostedStorageManager implements IDocStorageManager {
   }
 
   public async prepareToCreateDoc(docName: string): Promise<void> {
+    await this.prepareLocalDoc(docName, 'new');
     if (this._inventory) {
       await this._inventory.create(docName);
       this._onInventoryChange(docName);
     }
+    this.markAsChanged(docName);
+  }
+
+  /**
+   * Initialize one document from another, associating the result with the current
+   * worker.
+   */
+  public async prepareFork(srcDocName: string, destDocName: string): Promise<void> {
+    await this.prepareLocalDoc(destDocName, srcDocName);
   }
 
   // Gets a copy of the document, eg. for downloading.  Returns full file path.
   // Copy won't change if edits are made to the document.  It is caller's responsibility
   // to delete the result.
   public async getCopy(docName: string): Promise<string> {
-    const present = await this._ensureDocumentIsPresent(docName, {client: null});
+    const present = await this._claimDocument(docName);
     if (!present) {
       throw new Error('cannot copy document that does not exist yet');
     }
@@ -395,9 +405,10 @@ export class HostedStorageManager implements IDocStorageManager {
     return this._baseStore;
   }
 
-  // return true if document is backed up to s3.
-  public isSaved(docName: string): boolean {
-    return !this._uploads.hasPendingOperation(docName);
+  // return true if document and inventory is backed up to external store (if attached).
+  public isAllSaved(docName: string): boolean {
+    return !this._uploads.hasPendingOperation(docName) &&
+      (this._inventory ? this._inventory.isSaved(docName) : true);
   }
 
   // pick up the pace of pushing to s3, from leisurely to urgent.
@@ -423,10 +434,11 @@ export class HostedStorageManager implements IDocStorageManager {
    * Make sure document is backed up to s3.
    */
   public async flushDoc(docName: string): Promise<void> {
-    while (!this.isSaved(docName)) {
+    while (!this.isAllSaved(docName)) {
       log.info('HostedStorageManager: waiting for document to finish: %s', docName);
       await this._uploads.expediteOperationAndWait(docName);
-      if (!this.isSaved(docName)) {
+      await this._inventory?.flush(docName);
+      if (!this.isAllSaved(docName)) {
         // Throttle slightly in case this operation ends up looping excessively.
         await delay(1000);
       }
@@ -502,25 +514,28 @@ export class HostedStorageManager implements IDocStorageManager {
   }
 
   /**
-   * Makes sure a document is present locally, fetching it from S3 if necessary.
-   * Returns true on success, false if document not found.  It is safe to call
-   * this method multiple times in parallel.
+   * Makes sure a document is assigned to this worker, adding an
+   * assignment if it has none.  If the document is present in
+   * external storage, fetch it.  Return true if the document was
+   * fetched.
+   *
+   * The document can optionally be copied from an alternative
+   * source (srcDocName).  This is useful for forking.
+   *
+   * If srcDocName is 'new', checks for the document in external storage
+   * are skipped.
    */
-  private async _ensureDocumentIsPresent(docName: string,
-                                         docSession: OptDocSession): Promise<boolean> {
+  private async _claimDocument(docName: string,
+                               srcDocName?: string): Promise<boolean> {
     // AsyncCreate.mapGetOrSet ensures we don't start multiple promises to talk to S3/Redis
     // and that we clean up the failed key in case of failure.
     return mapGetOrSet(this._localFiles, docName, async () => {
       if (this._closed) { throw new Error("HostedStorageManager._ensureDocumentIsPresent called after closing"); }
       checkValidDocId(docName);
 
-      const {trunkId, forkId, forkUserId, snapshotId} = parseUrlId(docName);
+      const {trunkId, forkId, snapshotId} = parseUrlId(docName);
 
-      // If forkUserId is set to a valid user id, we can only create a fork if we know the
-      // requesting user and their id matches the forkUserId.
-      const userId = (docSession.client && docSession.client.getCachedUserId()) ||
-        (docSession.req && getUserId(docSession.req));
-      const canCreateFork = forkUserId ? (forkUserId === userId) : true;
+      const canCreateFork = Boolean(srcDocName);
 
       const docStatus = await this._docWorkerMap.getDocWorkerOrAssign(docName, this._docWorkerId);
       if (!docStatus.isActive) { throw new Error(`Doc is not active on a DocWorker: ${docName}`); }
@@ -528,10 +543,12 @@ export class HostedStorageManager implements IDocStorageManager {
         throw new Error(`Doc belongs to a different DocWorker (${docStatus.docWorker.id}): ${docName}`);
       }
 
+      if (srcDocName === 'new') { return false; }
+
       if (this._disableS3) {
         // skip S3, just use file system
         let present: boolean = await fse.pathExists(this.getPath(docName));
-        if (forkId && !present) {
+        if ((forkId || snapshotId) && !present) {
           if (!canCreateFork) { throw new Error(`Cannot create fork`); }
           if (snapshotId && snapshotId !== 'current') {
             throw new Error(`cannot find snapshot ${snapshotId} of ${docName}`);
@@ -569,6 +586,7 @@ export class HostedStorageManager implements IDocStorageManager {
         }
       }
       return this._fetchFromS3(docName, {
+        sourceDocId: srcDocName,
         trunkId: forkId ? trunkId : undefined, snapshotId, canCreateFork
       });
     });

app/server/lib/IDocStorageManager.ts

@@ -1,7 +1,6 @@
 import {DocEntry} from 'app/common/DocListAPI';
 import {DocSnapshots} from 'app/common/DocSnapshot';
 import {DocReplacementOptions} from 'app/common/UserAPI';
-import {OptDocSession} from 'app/server/lib/DocSession';
 
 export interface IDocStorageManager {
   getPath(docName: string): string;
@@ -11,8 +10,9 @@ export interface IDocStorageManager {
   // This method must not be called for the same docName twice in parallel.
   // In the current implementation, it is called in the context of an
   // AsyncCreate[docName].
-  prepareLocalDoc(docName: string, docSession: OptDocSession): Promise<boolean>;
+  prepareLocalDoc(docName: string): Promise<boolean>;
   prepareToCreateDoc(docName: string): Promise<void>;
+  prepareFork(srcDocName: string, destDocName: string): Promise<void>;
 
   listDocs(): Promise<DocEntry[]>;
   deleteDoc(docName: string, deletePermanently?: boolean): Promise<void>;

app/server/lib/Permit.ts

@@ -28,6 +28,7 @@ export interface Permit {
   docId?: string;
   workspaceId?: number;
   org?: string|number;
+  otherDocId?: string;  // For operations involving two documents.
 }
 
 /* A store of permits */

app/server/lib/idUtils.ts

@@ -35,14 +35,8 @@ export function makeForkIds(options: { userId: number|null, isAnonymous: boolean
   };
 }
 
-// For importing, we can assign any worker to the job.  As a hack, we reuse the document
-// assignment mechanism.  To spread the work around a bit if we have several doc workers,
-// we use a fake document id between import0 and import9.
-// This method takes a DocWorkerMap to allow for something smarter in future.
+// This used to do a hack for importing, but now does nothing.
+// Instead, the server will interpret the special docId "import".
 export function getAssignmentId(docWorkerMap: IDocWorkerMap, docId: string): string {
-  let assignmentId = docId;
-  if (assignmentId === 'import') {
-    assignmentId = `import${Math.round(Math.random() * 10)}`;
-  }
-  return assignmentId;
+  return docId;
 }