Use If-Range instead of If-Match/If-Unmodified-Since for partial requests.

content/browser/download/download_manager_impl.cc

 // 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.
 
 #include "content/browser/download/download_manager_impl.h"
 
 #include <iterator>
 
 #include "base/bind.h"
 #include "base/callback.h"
@@ skipping 71 matching lines @@
   // If we're not at the beginning of the file, retrieve only the remaining
   // portion.
   bool has_last_modified = !params->last_modified().empty();
   bool has_etag = !params->etag().empty();
 
   // If we've asked for a range, we want to make sure that we only
   // get that range if our current copy of the information is good.
   // We shouldn't be asked to continue if we don't have a verifier.
   DCHECK(params->offset() == 0 || has_etag || has_last_modified);
 
-  if (params->offset() > 0) {
+  if (params->offset() > 0 && (has_etag || has_last_modified)) {
     request->SetExtraRequestHeaderByName(
         "Range",
         base::StringPrintf("bytes=%" PRId64 "-", params->offset()),
         true);
 
-    if (has_last_modified) {
-      request->SetExtraRequestHeaderByName("If-Unmodified-Since",
-                                           params->last_modified(),
-                                           true);
-    }
-    if (has_etag) {
-      request->SetExtraRequestHeaderByName("If-Match", params->etag(), true);
-    }
+    // In accordance with RFC 2616 Section 14.27, use If-Range to specify that
+    // the server return the entire entity if the validator doesn't match.
+    // Last-Modified can be used in the absence of ETag as a validator if the
+    // response headers satisfied the HttpUtil::HasStrongValidators() predicate.
+    //
+    // This function assumes that HasStrongValidators() was true and that the
+    // ETag and Last-Modified header values supplied are valid.
+    request->SetExtraRequestHeaderByName(
+        "If-Range", has_etag ? params->etag() : params->last_modified(), true);
   }
 
   for (DownloadUrlParameters::RequestHeadersType::const_iterator iter
            = params->request_headers_begin();
        iter != params->request_headers_end();
        ++iter) {
     request->SetExtraRequestHeaderByName(
         iter->first, iter->second, false /*overwrite*/);
   }
 
@@ skipping 633 matching lines @@
   if (delegate_)
     delegate_->OpenDownload(download);
 }
 
 void DownloadManagerImpl::ShowDownloadInShell(DownloadItemImpl* download) {
   if (delegate_)
     delegate_->ShowDownloadInShell(download);
 }
 
 }  // namespace content