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

net/disk_cache/mapped_file_posix.cc

Index: net/disk_cache/mapped_file_posix.cc
===================================================================
--- net/disk_cache/mapped_file_posix.cc	(revision 139948)
+++ net/disk_cache/mapped_file_posix.cc	(working copy)
@@ -24,11 +24,16 @@
   buffer_ = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED,
                  platform_file(), 0);
   init_ = true;
+  view_size_ = size;
   DCHECK(reinterpret_cast<intptr_t>(buffer_) != -1);
   if (reinterpret_cast<intptr_t>(buffer_) == -1)
     buffer_ = 0;
 
-  view_size_ = size;
+  // Make sure we detect hardware failures reading the headers.
+  scoped_array<char> temp(new char[size]);

[gavinp, 2012/06/01 12:00:34]

Now there's another difference with windows.  When size is passed as 0 to the
MappedFile::Init on windows, we just read the 8192 byte header.  But for Posix,
we read the whole file.  Will that slow us down on Posix?

+  if (!Read(temp.get(), size, 0))
+    return NULL;
+
   return buffer_;
 }