Only encode the streaming content once.

In order to calculate the contentLength, we must write the encode the
data first. The encoded data is written to a buffer using a
ByteArrayOutputStream implementation and we use that to figure out how
many bytes of data is to be sent.

Previously, this data was thrown away and the content was re-encoded
when it was actually time to send the data.

Instead, we now replace the content with a ByteArrayContent which
contains the buffer we wrote to when calculating the size.

We implemented a new CachingByteArrayOutputStream so that we can access
the byte buffer directly rather than copying into a new byte array (for
memory performance)

google-http-client/src/main/java/com/google/api/client/http/HttpRequest.java

@@ -15,7 +15,7 @@
 package com.google.api.client.http;
 
 import com.google.api.client.util.Beta;
-import com.google.api.client.util.IOUtils;
+import com.google.api.client.util.CachingByteArrayOutputStream;
 import com.google.api.client.util.LoggingStreamingContent;
 import com.google.api.client.util.ObjectParser;
 import com.google.api.client.util.Preconditions;
@@ -932,7 +932,14 @@ public HttpResponse execute() throws IOException {
         } else {
           contentEncoding = encoding.getName();
           streamingContent = new HttpEncodingStreamingContent(streamingContent, encoding);
-          contentLength = contentRetrySupported ? IOUtils.computeLength(streamingContent) : -1;
+          if (contentRetrySupported) {
+            CachingByteArrayOutputStream outputStream = new CachingByteArrayOutputStream();
+            streamingContent.writeTo(outputStream);
+            contentLength = outputStream.getContentLength();
+            streamingContent = new ByteArrayContent(contentType, outputStream.getBuffer());
+          } else {
+            contentLength = -1;
+          }
         }
         // append content headers to log buffer
         if (loggable) {

google-http-client/src/main/java/com/google/api/client/util/CachingByteArrayOutputStream.java

@@ -0,0 +1,43 @@
+/*
+ * Copyright 2019 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
+ * in compliance with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software distributed under the License
+ * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
+ * or implied. See the License for the specific language governing permissions and limitations under
+ * the License.
+ */
+
+package com.google.api.client.util;
+
+import java.io.ByteArrayOutputStream;
+
+/**
+ * Output stream that extends the built-in {@link ByteArrayOutputStream} to return the internal
+ * byte buffer rather than creating a copy.
+ */
+public class CachingByteArrayOutputStream extends ByteArrayOutputStream {
+
+  /**
+   * Returns the content length of the buffer.
+   *
+   * @return tthe content length of the buffer.
+   */
+  public int getContentLength() {
+    return count;
+  }
+
+  /**
+   * Returns the buffer where the byte data is stored.
+   *
+   * @return the buffer where the byte data is stored.
+   */
+  public byte[] getBuffer() {
+    return buf;
+  }
+
+}