Add UMA to track why a download is non-parallel when enabled parallel

content/browser/download/download_stats.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 //
 // Holds helpers for gathering UMA stats about downloads.
 
 #ifndef CONTENT_BROWSER_DOWNLOAD_DOWNLOAD_STATS_H_
 #define CONTENT_BROWSER_DOWNLOAD_DOWNLOAD_STATS_H_
 
 #include <stddef.h>
@@ skipping 127 matching lines @@
 };
 
 enum DownloadDiscardReason {
   // The download is being discarded due to a user action.
   DOWNLOAD_DISCARD_DUE_TO_USER_ACTION,
 
   // The download is being discarded due to the browser being shut down.
   DOWNLOAD_DISCARD_DUE_TO_SHUTDOWN
 };
 
+// When parallel download is enabled, the download may fall back to a normal
+// download for various reasons. This enum counts the number of parallel
+// download and fallbacks. Also records the reasons why the download falls back
+// to a normal download. The reasons are not mutual exclusive.
+enum class ParallelDownloadCreationEvent {

[Ilya Sherman, 2017/04/08 01:22:46]

nit: Please document that this enum is used to back an UMA histogram and should
therefore be treated as append-only.

[xingliu, 2017/04/10 18:56:30]

Done.

+  // The total number of downloads created as parallel download.
+  PARALLEL_DOWNLOAD_COUNT = 0,

[Ilya Sherman, 2017/04/08 01:22:46]

Optional nit: I still think it's weird to use "COUNT" in the name of some
buckets but not others.  I'd name this something like
"STARTED_PARALLEL_DOWNLOAD" and "FELL_BACK_TO_NORMAL_DOWNLOAD" below.

[xingliu, 2017/04/10 18:56:30]

Done.

+
+  // The total number of downloads fall back to normal download when parallel
+  // download is enabled.
+  FALLBACK_TO_NORMAL_DOWNLOAD_COUNT,
+
+  // No ETag or Last-Modified response header.
+  FALLBACK_REASON_STRONG_VALIDATORS,
+
+  // No Accept-Range response header.
+  FALLBACK_REASON_ACCEPT_RANGE_HEADER,
+
+  // No Content-Length response header.
+  FALLBACK_REASON_CONTENT_LENGTH_HEADER,
+
+  // File size is not complied to finch configuration.
+  FALLBACK_REASON_FILE_SIZE,
+
+  // The HTTP connection type does not meet the requirement.
+  FALLBACK_REASON_CONNECTION_TYPE,
+
+  // Last entry of the enum.
+  MAX,

[Ilya Sherman, 2017/04/08 01:22:46]

Optional nit: IMO "max" makes sense to use as an alias for the last item in an
enum, and "count" makes sense to count the number of items in an enum.  So, this
is currently named "max" but is behaving more like "count".  I'd rename it to
"count".

[xingliu, 2017/04/10 18:56:30]

Done.

+};
+
 // Increment one of the above counts.
 void RecordDownloadCount(DownloadCountTypes type);
 
 // Record initiation of a download from a specific source.
 void RecordDownloadSource(DownloadSource source);
 
 // Record COMPLETED_COUNT and how long the download took.
 void RecordDownloadCompleted(const base::TimeTicks& start,
                              int64_t download_len);
 
@@ skipping 71 matching lines @@
 void RecordParallelDownloadAddStreamSuccess(bool success);
 
 // Records the bandwidth for parallel download and estimates the saved time at
 // the file end. Does not count in any hash computation or file open/close time.
 void RecordParallelDownloadStats(
     size_t bytes_downloaded_with_parallel_streams,
     base::TimeDelta time_with_parallel_streams,
     size_t bytes_downloaded_without_parallel_streams,
     base::TimeDelta time_without_parallel_streams);
 
+// Records the parallel download creation counts and the reasons why the
+// download falls back to non-parallel download.
+void RecordParallelDownloadCreationEvent(ParallelDownloadCreationEvent event);
+
 // Record the result of a download file rename.
 void RecordDownloadFileRenameResultAfterRetry(
     base::TimeDelta time_since_first_failure,
     DownloadInterruptReason interrupt_reason);
 
 enum SavePackageEvent {
   // The user has started to save a page as a package.
   SAVE_PACKAGE_STARTED,
 
   // The save package operation was cancelled.
@@ skipping 28 matching lines @@
 
 void RecordDownloadConnectionSecurity(const GURL& download_url,
                                       const std::vector<GURL>& url_chain);
 
 void RecordDownloadSourcePageTransitionType(
     const base::Optional<ui::PageTransition>& transition);
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_DOWNLOAD_DOWNLOAD_STATS_H_