SimpleCache: comment a tricky bit of code.

net/disk_cache/simple/simple_synchronous_entry.cc

 // Copyright (c) 2013 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 "net/disk_cache/simple/simple_synchronous_entry.h"
 
 #include <algorithm>
 #include <cstring>
 #include <functional>
 #include <limits>
@@ skipping 1040 matching lines @@
     uint32_t* out_stream_0_crc32) {
   DCHECK(!initialized_);
   if (!OpenFiles(out_entry_stat)) {
     DLOG(WARNING) << "Could not open platform files for entry.";
     return net::ERR_FAILED;
   }
   for (int i = 0; i < kSimpleEntryFileCount; ++i) {
     if (empty_file_omitted_[i])
       continue;
 
-    if (!key_.empty()) {
-      header_and_key_check_needed_[i] = true;
-    } else {
+    if (key_.empty()) {
+      // If |key_| is empty, we were opened via the iterator interface, without
+      // knowing what our key is. We must therefore read the header immediately
+      // to discover it, so SimpleEntryImpl can make it available to
+      // disk_cache::Entry::GetKey().
       if (!CheckHeaderAndKey(i))
         return net::ERR_FAILED;
+    } else {
+      // If we do know which key were are looking for, we still need to
+      // check that the file actually has it (rather than just being a hash
+      // collision or some sort of file system accident), but that can be put
+      // off until opportune time: either the read of the footer, or when we
+      // start reading in the data, depending on stream # and format revision.
+      header_and_key_check_needed_[i] = true;
     }
 
     if (i == 0) {
       // File size for stream 0 has been stored temporarily in data_size[1].
       int ret_value_stream_0 =
           ReadAndValidateStream0(out_entry_stat->data_size(1), out_entry_stat,
                                  stream_0_data, out_stream_0_crc32);
       if (ret_value_stream_0 != net::OK)
         return ret_value_stream_0;
     } else {
@@ skipping 518 matching lines @@
   range.offset = offset;
   range.length = len;
   range.data_crc32 = data_crc32;
   range.file_offset = data_file_offset;
   sparse_ranges_.insert(std::make_pair(offset, range));
 
   return true;
 }
 
 }  // namespace disk_cache