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

content/browser/download/download_job_factory.cc

Index: content/browser/download/download_job_factory.cc
diff --git a/content/browser/download/download_job_factory.cc b/content/browser/download/download_job_factory.cc
index a619b04e66559165d5bcc0f6d202fe10db66d151..aa077c73061f5acbde6a2a747cf5c029ce78a9a9 100644
--- a/content/browser/download/download_job_factory.cc
+++ b/content/browser/download/download_job_factory.cc
@@ -10,12 +10,73 @@
 #include "content/browser/download/download_item_impl.h"
 #include "content/browser/download/download_job.h"
 #include "content/browser/download/download_job_impl.h"
+#include "content/browser/download/download_stats.h"
 #include "content/browser/download/parallel_download_job.h"
 #include "content/browser/download/parallel_download_utils.h"
 #include "content/public/common/content_features.h"
 
 namespace content {
 
+namespace {
+
+// Returns if the download should be a parallel download.
+bool ShouldUseParallelDownload(const DownloadCreateInfo& create_info) {
+  // To enable parallel download, following conditions need to be satisfied.
+  // 1. Feature |kParallelDownloading| enabled.
+  // 2. Strong validators response headers. i.e. ETag and Last-Modified.
+  // 3. Accept-Ranges header.
+  // 4. Content-Length header.
+  // 5. Content-Length is no less than the minimum slice size configuration.
+  // 6. HTTP/1.1 protocol, not QUIC nor HTTP/1.0.
+  if (!base::FeatureList::IsEnabled(features::kParallelDownloading))
+    return false;
+
+  // Etag and last modified are stored into DownloadCreateInfo in
+  // DownloadRequestCore only if the response header complies to the strong
+  // validator rule.
+  bool has_strong_validator =
+      !create_info.etag.empty() || !create_info.last_modified.empty();
+  bool has_content_length = create_info.total_bytes > 0;
+  bool satisfy_min_file_size =
+      create_info.total_bytes >= GetMinSliceSizeConfig();
+  bool satisfy_connection_type = create_info.connection_info ==
+                                 net::HttpResponseInfo::CONNECTION_INFO_HTTP1_1;
+
+  bool should_use_parallel_download =
+      has_strong_validator && create_info.accept_range && has_content_length &&
+      satisfy_min_file_size && satisfy_connection_type;
+
+  RecordParallelDownloadCreationEvent(
+      should_use_parallel_download
+          ? ParallelDownloadCreationEvent::PARALLEL_DOWNLOAD_COUNT
+          : ParallelDownloadCreationEvent::FALLBACK_TO_NORMAL_DOWNLOAD_COUNT);
+
+  if (!has_strong_validator) {
+    RecordParallelDownloadCreationEvent(
+        ParallelDownloadCreationEvent::FALLBACK_REASON_STRONG_VALIDATORS);
+  }
+  if (!create_info.accept_range) {
+    RecordParallelDownloadCreationEvent(
+        ParallelDownloadCreationEvent::FALLBACK_REASON_ACCEPT_RANGE_HEADER);
+  }
+  if (!has_content_length) {
+    RecordParallelDownloadCreationEvent(
+        ParallelDownloadCreationEvent::FALLBACK_REASON_CONTENT_LENGTH_HEADER);
+  }
+  if (!satisfy_min_file_size) {
+    RecordParallelDownloadCreationEvent(
+        ParallelDownloadCreationEvent::FALLBACK_REASON_FILE_SIZE);
+  }
+  if (!satisfy_connection_type) {
+    RecordParallelDownloadCreationEvent(
+        ParallelDownloadCreationEvent::FALLBACK_REASON_CONNECTION_TYPE);
+  }
+
+  return should_use_parallel_download;
+}
+
+}  // namespace
+
 std::unique_ptr<DownloadJob> DownloadJobFactory::CreateJob(
     DownloadItemImpl* download_item,
     std::unique_ptr<DownloadRequestHandleInterface> req_handle,