Initial CL for Downloads resumption.

content/browser/download/base_file.cc

Index: content/browser/download/base_file.cc
diff --git a/content/browser/download/base_file.cc b/content/browser/download/base_file.cc
index 9d55828f5ecbe0e73b9138239899601c7a0fed07..94664c0ad05b06dd16d8754d64b53a317ac131e3 100644
--- a/content/browser/download/base_file.cc
+++ b/content/browser/download/base_file.cc
@@ -296,6 +296,19 @@ DownloadInterruptReason BaseFile::Open() {
     file_stream_->SetBoundNetLogSource(bound_net_log_);
   }
 
+  int64 file_size = file_stream_->SeekSync(net::FROM_END, 0);

[asanka, 2012/12/28 22:01:42]

Wouldn't we need to slurp in the contents of the file up to bytes_so_far_ and
verify the hash?

[Randy Smith (Not in Mondays), 2012/12/29 17:16:16]

Depending on the nature of the filesystem and the amount of stuff we've
downloaded, that could take a while.  I'd like to have some protection against
the file having been changed on us, but I'm inclined to think that it should be
either a last_modified check or asking for some extra bytes from the network and
comparing those bytes with what's in the file.  And either way, I'm inclined to
leave it for another CL (this is one of the items in my list of things we need
to do).  That ok by you, or do you think it should go into this CL?

[asanka, 2013/01/04 22:54:16]

Ok by me. Can you add a TODO here?

+  if (file_size > bytes_so_far_) {
+    // The file is larger than we expected.
+    // This is OK, as long as we don't use the extra.
+    // Truncate the file.
+    int64 truncate_result = file_stream_->Truncate(bytes_so_far_);
+    DCHECK_EQ(bytes_so_far_, truncate_result);

[asanka, 2012/12/28 22:01:42]

This could mean an error. We should handle it instead of a DCHECK.

[Randy Smith (Not in Mondays), 2012/12/29 17:16:16]

Done (though the DCHECK's still there if it's not an error).

+  } else if (file_size < bytes_so_far_) {
+    // The file is shorter than we expected.  Our hashes won't be valid.
+    return LogInterruptReason("Unable to seek to last written point", 0,
+                              DOWNLOAD_INTERRUPT_REASON_FILE_TOO_SHORT);
+  }
+
   return DOWNLOAD_INTERRUPT_REASON_NONE;
 }