bugfix: ZENKO-2866 abort request on backend if S3 client disconnects

Call request.abort() on the backend side when an S3 client
disconnects, to avoid leaking sockets. Also make sure request.abort()
through the stream destroy() call is called asynchronously from the
stream creation.

Author: jonathan-gramain

lib/storage/data/external/AwsClient.js

@@ -178,7 +178,18 @@ class AwsClient {
               { responseHeaders: response.httpResponse.headers,
                 backendType: this.clientType });
         });
-        const stream = request.createReadStream().on('error', err => {
+        const stream = request.createReadStream();
+
+        // modify the stream destroy() behavior to also abort the HTTP request
+        const streamDestroy = stream._destroy.bind(stream);
+        stream._destroy = (err, cb) => {
+            log.debug('aborting GET request in progress', { objectGetInfo });
+            request.abort();
+            if (streamDestroy) {
+                streamDestroy(err, cb);
+            }
+        };
+        stream.on('error', err => {
             let logLevel;
             if (err.code === 'NotFound') {
                 logLevel = 'info';
@@ -189,7 +200,11 @@ class AwsClient {
               `error streaming data from ${this.type}`,
               err, this._dataStoreName, this.clientType);
         });
-        return callback(null, stream);
+        // Always call the callback asynchronously: the caller may
+        // destroy the stream with destroy(), which MUST be
+        // asynchronous wrt. request.createReadStream() to avoid
+        // socket leaks, notably because of the request.abort() call.
+        return process.nextTick(() => callback(null, stream));
     }
     delete(objectGetInfo, reqUids, callback) {
         const { key, dataStoreVersionId, deleteVersion } = objectGetInfo;