Disk cache: Pre-read file headers when opening files (posix).

net/disk_cache/mapped_file_posix.cc

 // Copyright (c) 2011 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/mapped_file.h"
 
 #include <errno.h>
 #include <sys/mman.h>
 
 #include "base/file_path.h"
 #include "base/logging.h"
 #include "net/disk_cache/disk_cache.h"
 
 namespace disk_cache {
 
 void* MappedFile::Init(const FilePath& name, size_t size) {
   DCHECK(!init_);
   if (init_ || !File::Init(name))
     return NULL;
 
   if (!size)
     size = GetLength();

[gavinp, 2012/05/31 20:51:32]

Here's where size gets set.  GetLength() returns zero on error or a file of
length zero.  So I think we shouldn't use temp_len below in the posix case.

 
   buffer_ = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED,
                  platform_file(), 0);
   init_ = true;
   DCHECK(reinterpret_cast<intptr_t>(buffer_) != -1);
   if (reinterpret_cast<intptr_t>(buffer_) == -1)
     buffer_ = 0;
 
+  // Make sure we detect hardware failures reading the headers.
+  size_t temp_len = size ? size : 4096 * 2;

[gavinp, 2012/05/31 19:59:50]

Isn't it just an error if size is 0 here?  How would File::GetLength() return 0,
but we have a file that allows an 8k read without error?

[rvargas (doing something else), 2012/05/31 20:07:05]

Well... that's one of the details that I was hoping you picked when I asked you
to look at this function the other day.

For some reason, it seems to work "as unexpected". We pass 0 all the time,
expecting the whole file to be mapped.

If you think that should be changed, we should do that on a separate CL.

[gavinp, 2012/05/31 20:50:34]

If I missed a request to help earlier, I'm very sorry; but I'm here now!

Passing zero should work; I commented the line up above where it gets filled in,
and why I think the test here is not required.  In the windows version, the test
is needed though.

On 2012/05/31 20:07:05, rvargas wrote:

+  scoped_array<char> temp(new char[temp_len]);
+  if (!Read(temp.get(), temp_len, 0))
+    return NULL;
+
   view_size_ = size;
   return buffer_;
 }
 
 bool MappedFile::Load(const FileBlock* block) {
   size_t offset = block->offset() + view_size_;
   return Read(block->buffer(), block->size(), offset);
 }
 
 bool MappedFile::Store(const FileBlock* block) {
   size_t offset = block->offset() + view_size_;
   return Write(block->buffer(), block->size(), offset);
 }
 
 MappedFile::~MappedFile() {
   if (!init_)
     return;
 
   if (buffer_) {
     int ret = munmap(buffer_, view_size_);
     DCHECK_EQ(0, ret);
   }
 }
 
 }  // namespace disk_cache