Add support for downloading binary files using S3Client.getObject

This commit adds an `additionalHeaders` parameter to S3Client.getObject.
The operation additionally receives support for the `Accept` header.
When the header is provided with the value `application/octet-stream`
the operation will, as a result, treat the response as binary content
and return an ArrayBuffer.

We decided to opt for the header approach, as it doesn't leak nor depend
on k6 interfaces, and adopts a standard HTTP behavior, indicating to
the server the type of content we expect in return, through the Accept
header.

src/internal/s3.ts

@@ -168,7 +168,11 @@ export class S3Client extends AWSClient {
      * @throws  {S3ServiceError}
      * @throws  {InvalidSignatureError}
      */
-    async getObject(bucketName: string, objectKey: string): Promise<S3Object> {
+    async getObject(
+        bucketName: string,
+        objectKey: string,
+        additionalHeaders: object = {}
+    ): Promise<S3Object> {
         // Prepare request
         const method = 'GET'
 
@@ -177,13 +181,27 @@ export class S3Client extends AWSClient {
                 method: method,
                 endpoint: this.endpoint,
                 path: encodeURI(`/${bucketName}/${objectKey}`),
-                headers: {},
+                headers: {
+                    ...additionalHeaders,
+                },
             },
             {}
         )
 
+        // If the Accept header is set to 'application/octet-stream', we want to
+        // return the response as binary data.
+        let responseType: ResponseType = 'text'
+        if (
+            'Accept' in additionalHeaders &&
+            additionalHeaders['Accept'] !== undefined &&
+            additionalHeaders['Accept'] === 'application/octet-stream'
+        ) {
+            responseType = 'binary'
+        }
+
         const res = await http.asyncRequest(method, signedRequest.url, null, {
             headers: signedRequest.headers,
+            responseType: responseType as ResponseType,
         })
         this._handle_error('GetObject', res)
 
@@ -537,7 +555,7 @@ export class S3Object {
     etag: string
     size: number
     storageClass: StorageClass
-    data?: string | bytes | null
+    data?: string | ArrayBuffer | null
 
     /**
      * Create an S3 Object
@@ -547,15 +565,15 @@ export class S3Object {
      * @param  {string} etag - S3 object's etag
      * @param  {number} size - S3 object's size
      * @param  {StorageClass} storageClass - S3 object's storage class
-     * @param  {string | bytes | null} data=null - S3 Object's data
+     * @param  {string | ArrayBuffer | null} data=null - S3 Object's data
      */
     constructor(
         key: string,
         lastModified: number,
         etag: string,
         size: number,
         storageClass: StorageClass,
-        data?: string | bytes | null
+        data?: string | ArrayBuffer | null
     ) {
         this.key = key
         this.lastModified = lastModified

tests/internal/s3.js

@@ -73,6 +73,21 @@ export async function s3TestSuite(data) {
         expect(getNonExistingObjectError).to.be.an.instanceOf(S3ServiceError)
     })
 
+    await asyncDescribe('s3.getObject [binary]', async (expect) => {
+        // Act
+        const gotBinaryObject = await s3Client.getObject(
+            data.s3.testBucketName,
+            data.s3.testObjects[0].key,
+            { Accept: 'application/octet-stream' }
+        )
+
+        // Assert
+        expect(gotBinaryObject).to.be.an('object')
+        expect(gotBinaryObject.key).to.equal(data.s3.testObjects[0].key)
+        expect(gotBinaryObject.data).to.be.an('ArrayBuffer')
+        expect(gotBinaryObject.data.byteLength).to.equal(data.s3.testObjects[0].body.length)
+    })
+
     await asyncDescribe('s3.putObject', async (expect) => {
         // Act
         let putObectError