Add support for loading pak files from arbitrary file regions.

base/files/memory_mapped_file.h

Index: base/files/memory_mapped_file.h
diff --git a/base/files/memory_mapped_file.h b/base/files/memory_mapped_file.h
index b02d8cfbdae09b89159675495313eae521a7576e..dc5680615daf4fb10762ae663f358d0f0f2c7a98 100644
--- a/base/files/memory_mapped_file.h
+++ b/base/files/memory_mapped_file.h
@@ -24,6 +24,26 @@ class BASE_EXPORT MemoryMappedFile {
   MemoryMappedFile();
   ~MemoryMappedFile();
 
+  // Used to hold information about a region [offset + size] of a file.
+  struct BASE_EXPORT Region {
+    static const Region kWholeFile;

[sky, 2014/07/28 22:07:43]

Doesn't this result in a static constructor, which is disallowed?

[Primiano Tucci (use gerrit), 2014/07/29 09:05:49]

We had an offline discussion with Will on this point (I had the same doubt), and
my understanding is:  if the only thing the ctor does is initializing POD fields
(in this specific case, to 0), it would not be a problem. Essentially here
kWholeFile is a struct with two 0-initialized fields. I'd expect kWholeFile to
be just a carve-out of .bss.

I did a quick test by turning kWholeFile into a static method which returns a
new Region every time, and if I diff the program headers, I can see a 16 bytes
delta in bss (for libbase.so):

With kWholeFile being a static const:
[27] .bss              NOBITS          00000000003e3660 3e2660 002714 00  WA  0 
 0 16

With kWholeFile being a method which retuns a new object
[27] .bss              NOBITS          00000000003e3660 3e2660 002704 00  WA  0 
 0 16

(Size column is the 002714, 002704)
Also, I assume we have checks in place for this (I am sure I've seen clang
barfing for at-exit destructors).

Does it sound right?

+
+    Region(int64 offset, int64 size);
+
+    bool operator==(const Region& other) const;
+
+    // Start of the region (measured in bytes from the beginning of the file).
+    int64 offset;
+
+    // Length of the region in bytes.
+    int64 size;
+
+   private:
+    // This ctor is used only by kWholeFile, to construct a zero-sized Region
+    // (which is forbidden by the public ctor) and uniquely identify kWholeFile.
+    Region();
+  };
+
   // Opens an existing file and maps it into memory. Access is restricted to
   // read only. If this object already points to a valid memory mapped file
   // then this method will fail and return false. If it cannot open the file,
@@ -35,6 +55,9 @@ class BASE_EXPORT MemoryMappedFile {
   // ownership of |file| and closes it when done.
   bool Initialize(File file);
 
+  // As above, but works with a region of an already-opened file.
+  bool Initialize(File file, const Region& region);
+
 #if defined(OS_WIN)
   // Opens an existing file and maps it as an image section. Please refer to
   // the Initialize function above for additional information.
@@ -48,9 +71,21 @@ class BASE_EXPORT MemoryMappedFile {
   bool IsValid() const;
 
  private:
+  // Given the arbitrarily aligned memory region [start, size], returns the
+  // boundaries of the region aligned to the granularity specified by the OS,
+  // (a page on Linux, ~32k on Windows) as follows:
+  // - |aligned_start| is page aligned and <= |start|.
+  // - |aligned_size| is a multiple of the VM granularity and >= |size|.
+  // - |offset| is the displacement of |start| w.r.t |aligned_start|.
+  static void CalculateVMAlignedBoundaries(int64 start,
+                                           int64 size,
+                                           int64* aligned_start,
+                                           int64* aligned_size,
+                                           int32* offset);
+
   // Map the file to memory, set data_ to that memory address. Return true on
   // success, false on any kind of failure. This is a helper for Initialize().
-  bool MapFileToMemory();
+  bool MapFileRegionToMemory(const Region& region);
 
   // Closes all open handles.
   void CloseHandles();