Allow this.fileSize to be an async function as well

Instantiating a new client blocks on retrieving filesize.  But there are cases when we really don't need the filesize, for example when we have the metadata cached already.

lib/reader.js

@@ -236,7 +236,7 @@ class ParquetEnvelopeReader {
   }
 
   static async openS3(client, params) {
-    let fileStat = await client.headObject(params).promise();
+    let fileStat = async () => client.headObject(params).promise().then(d => d.ContentLength);
 
     let readFn = async (offset, length) => {
       let Range = `bytes=${offset}-${offset+length-1}`;
@@ -246,7 +246,7 @@ class ParquetEnvelopeReader {
 
     let closeFn = () => ({});
 
-    return new ParquetEnvelopeReader(readFn, closeFn, fileStat.ContentLength);
+    return new ParquetEnvelopeReader(readFn, closeFn, fileStat);
   }
 
   static async openUrl(request, params) {
@@ -259,7 +259,7 @@ class ParquetEnvelopeReader {
 
     let defaultHeaders = params.headers || {};
 
-    let filesize = await new Promise( (resolve, reject) => {
+    let filesize = async () => new Promise( (resolve, reject) => {
       let req = request(params);
       req.on('response', res => {
         req.abort();
@@ -350,6 +350,9 @@ class ParquetEnvelopeReader {
   }
 
   async readFooter() {
+    if (typeof this.fileSize === 'function') {
+      this.fileSize = await this.fileSize();
+    }
     let trailerLen = PARQUET_MAGIC.length + 4;
     let trailerBuf = await this.read(this.fileSize - trailerLen, trailerLen);