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 mutually exclusive.
+// Used in histogram "Download.ParallelDownload.CreationEvent".

[Ilya Sherman, 2017/04/10 21:54:15]

nit: Please also document that "and therefore, should be treated as
append-only".  Metrics data is persisted, so it's important that semantics
remain consistent over time.

[xingliu, 2017/04/10 22:07:12]

Done,

sorry I probably forgot this one.

+enum class ParallelDownloadCreationEvent {
+  // The total number of downloads started as parallel download.
+  STARTED_PARALLEL_DOWNLOAD = 0,
+
+  // The total number of downloads fell back to normal download when parallel
+  // download is enabled.
+  FELL_BACK_TO_NORMAL_DOWNLOAD,
+
+  // 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.
+  COUNT,
+};
+
 // 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_