Avoid emulator data loss when there an error during export

CHANGELOG.md

@@ -0,0 +1 @@
+- Avoid emulator data loss when there an error during export (#3504)

src/emulator/controller.ts

@@ -774,15 +774,6 @@ export async function exportEmulatorData(exportPath: string, options: any) {
     }
   }
 
-  // Remove all existing data (metadata.json will be overwritten automatically)
-  if (existingMetadata) {
-    if (existingMetadata.firestore) {
-      const firestorePath = path.join(exportAbsPath, existingMetadata.firestore.path);
-      utils.logBullet(`Deleting directory ${firestorePath}`);
-      rimraf.sync(firestorePath);
-    }
-  }
-
   utils.logBullet(`Exporting data to: ${exportAbsPath}`);
   try {
     await hubClient.postExport(exportAbsPath);

src/emulator/hubExport.ts

@@ -12,6 +12,7 @@ import { EmulatorHub } from "./hub";
 import { getDownloadDetails } from "./downloadableEmulators";
 import { DatabaseEmulator } from "./databaseEmulator";
 import { StorageEmulator } from "./storage";
+import * as rimraf from "rimraf";
 
 export interface FirestoreExportMetadata {
   version: string;
@@ -45,7 +46,11 @@ export interface ExportMetadata {
 export class HubExport {
   static METADATA_FILE_NAME = "firebase-export-metadata.json";
 
-  constructor(private projectId: string, private exportPath: string) {}
+  private tmpDir: string;
+
+  constructor(private projectId: string, private exportPath: string) {
+    this.tmpDir = fs.mkdtempSync(`firebase-export-${new Date().getTime()}`);
+  }
 
   public static readMetadata(exportPath: string): ExportMetadata | undefined {
     const metadataPath = path.join(exportPath, this.METADATA_FILE_NAME);
@@ -102,8 +107,20 @@ export class HubExport {
       await this.exportStorage(metadata);
     }
 
-    const metadataPath = path.join(this.exportPath, HubExport.METADATA_FILE_NAME);
+    // Make sure the export directory exists
+    if (!fs.existsSync(this.exportPath)) {
+      fs.mkdirSync(this.exportPath);
+    }
+
+    // Write the metadata file after everything else has succeeded
+    const metadataPath = path.join(this.tmpDir, HubExport.METADATA_FILE_NAME);
     fs.writeFileSync(metadataPath, JSON.stringify(metadata, undefined, 2));
+
+    // Remove any existing data in the directory and then swap it with the
+    // temp directory.
+    logger.debug(`hubExport: swapping ${this.tmpDir} with ${this.exportPath}`);
+    rimraf.sync(this.exportPath);
+    fse.moveSync(this.tmpDir, this.exportPath);
   }
 
   private async exportFirestore(metadata: ExportMetadata): Promise<void> {
@@ -112,7 +129,7 @@ export class HubExport {
 
     const firestoreExportBody = {
       database: `projects/${this.projectId}/databases/(default)`,
-      export_directory: this.exportPath,
+      export_directory: this.tmpDir,
       export_name: metadata.firestore!!.path,
     };
 
@@ -155,12 +172,7 @@ export class HubExport {
       }
     }
 
-    // Make sure the export directory exists
-    if (!fs.existsSync(this.exportPath)) {
-      fs.mkdirSync(this.exportPath);
-    }
-
-    const dbExportPath = path.join(this.exportPath, metadata.database!.path);
+    const dbExportPath = path.join(this.tmpDir, metadata.database!.path);
     if (!fs.existsSync(dbExportPath)) {
       fs.mkdirSync(dbExportPath);
     }
@@ -185,7 +197,7 @@ export class HubExport {
   private async exportAuth(metadata: ExportMetadata): Promise<void> {
     const { host, port } = EmulatorRegistry.get(Emulators.AUTH)!.getInfo();
 
-    const authExportPath = path.join(this.exportPath, metadata.auth!.path);
+    const authExportPath = path.join(this.tmpDir, metadata.auth!.path);
     if (!fs.existsSync(authExportPath)) {
       fs.mkdirSync(authExportPath);
     }
@@ -221,7 +233,7 @@ export class HubExport {
     const storageEmulator = EmulatorRegistry.get(Emulators.STORAGE) as StorageEmulator;
 
     // Clear the export
-    const storageExportPath = path.join(this.exportPath, metadata.storage!.path);
+    const storageExportPath = path.join(this.tmpDir, metadata.storage!.path);
     if (fs.existsSync(storageExportPath)) {
       fse.removeSync(storageExportPath);
     }