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

content/browser/download/download_stats.h

Index: content/browser/download/download_stats.h
diff --git a/content/browser/download/download_stats.h b/content/browser/download/download_stats.h
index 0c4a5be726edf4de5dd19f07118aec46ae9dce35..cd28ce944397f6820f71fd4a0189ad90d76a2786 100644
--- a/content/browser/download/download_stats.h
+++ b/content/browser/download/download_stats.h
@@ -145,6 +145,37 @@ enum DownloadDiscardReason {
   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);
 
@@ -236,6 +267,10 @@ void RecordParallelDownloadStats(
     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,