fix: append bucket name to V4 policy URL in emulator mode

src/file.ts

@@ -2937,7 +2937,11 @@ class File extends ServiceObject<File, FileMetadata> {
 
         let url: string;
 
-        if (this.storage.customEndpoint) {
+        const EMULATOR_HOST = process.env.STORAGE_EMULATOR_HOST;
+
+        if (this.storage.customEndpoint && typeof EMULATOR_HOST === 'string') {
+          url = `${this.storage.apiEndpoint}/${this.bucket.name}`;
+        } else if (this.storage.customEndpoint) {
           url = this.storage.apiEndpoint;
         } else if (options.virtualHostedStyle) {
           url = `https://${this.bucket.name}.storage.${universe}/`;

test/file.ts

@@ -3578,6 +3578,34 @@ describe('File', () => {
       );
     });
 
+    it('should append bucket name to the URL when using the emulator', done => {
+      const emulatorHost = 'http://127.0.0.1:9199';
+      const originalApiEndpoint = STORAGE.apiEndpoint;
+      const originalCustomEndpoint = STORAGE.customEndpoint;
+      const originalEnvHost = process.env.STORAGE_EMULATOR_HOST;
+
+      process.env.STORAGE_EMULATOR_HOST = emulatorHost;
+      STORAGE.apiEndpoint = emulatorHost;
+      STORAGE.customEndpoint = true;
+
+      file.generateSignedPostPolicyV4(
+        CONFIG,
+        (err: Error, res: SignedPostPolicyV4Output) => {
+          STORAGE.apiEndpoint = originalApiEndpoint;
+          STORAGE.customEndpoint = originalCustomEndpoint;
+          if (originalEnvHost) {
+            process.env.STORAGE_EMULATOR_HOST = originalEnvHost;
+          } else {
+            delete process.env.STORAGE_EMULATOR_HOST;
+          }
+
+          assert.ifError(err);
+          assert.strictEqual(res.url, `${emulatorHost}/${BUCKET.name}`);
+          done();
+        }
+      );
+    });
+
     describe('expires', () => {
       it('should accept Date objects', done => {
         const expires = new Date(Date.now() + 1000 * 60);