Only support seeking file streams from the beginning of the file.

net/base/file_stream_context_win.cc

Index: net/base/file_stream_context_win.cc
diff --git a/net/base/file_stream_context_win.cc b/net/base/file_stream_context_win.cc
index 1a39384c5ee66d3ab843e977aa5ccae36788ec0a..11e5f2d384e59d768629bbc5a2c6a8ddb7111e01 100644
--- a/net/base/file_stream_context_win.cc
+++ b/net/base/file_stream_context_win.cc
@@ -114,17 +114,11 @@ int FileStream::Context::Write(IOBuffer* buf,
   return ERR_IO_PENDING;
 }
 
-FileStream::Context::IOResult FileStream::Context::SeekFileImpl(
-    base::File::Whence whence,
-    int64 offset) {
+FileStream::Context::IOResult FileStream::Context::SeekFileImpl(int64 offset) {

[davidben, 2015/05/21 17:22:12]

If I'm understanding this correctly, not calling seek on the underlying
base::File only works because we always use overlapped IO, correct? This seems
to be correct, but it's probably worth a comment to this effect and why we're
not seeking the file.

There is a file() accessor to FileStream::Context which would be sensitive to
this. If we're relying on it, probably best to not leak that method. It's only
used to implement FileStream::IsOpen and FileStream::GetFileForTesting, which is
only used to assert on things redundant with asserting on IsOpen. Perhaps
replace both of those with FileStream::Context::IsOpen. That way we can be sure
that the base::File doesn't leak.

Actually, does this work if the file doesn't support overlapped IO? Is that
meaningful? This seems to suggest it matters whether the file was opened for
overlapped IO? Though if it wasn't, presumably we'd have problems with doing
disk on the IO thread. I see line 65 has a TODO to check it... perhaps checking
that file_.async() is true is sufficient?

https://msdn.microsoft.com/en-us/library/windows/desktop/aa365747%28v=vs.85%2...

   LARGE_INTEGER result;
-  result.QuadPart = file_.Seek(whence, offset);
-  if (result.QuadPart >= 0) {
-    SetOffset(&io_context_.overlapped, result);
-    return IOResult(result.QuadPart, 0);
-  }
-
-  return IOResult::FromOSError(GetLastError());
+  result.QuadPart = offset;
+  SetOffset(&io_context_.overlapped, result);
+  return IOResult(result.QuadPart, 0);
 }
 
 void FileStream::Context::OnFileOpened() {