[VFS] Use SectorsPerCluster provided by game for cache drive.

This fixes some games (eg. Halo: CEA) not mounting cache.
It seems some games use different SectorsPerClusters values, which then changes the value it expects from NtQueryVolumeInformationFile.
Since we'd always respond with a static value this would make the game think that mounting failed, and it'd bail out of the cache-mounting code.

Now we capture the SectorsPerCluster value when the game writes it to the cache-partition header, and update the NullDevice's sectors_per_allocation_unit with it.

src/xenia/vfs/devices/null_device.h

@@ -34,13 +34,23 @@ class NullDevice : public Device {
   uint32_t total_allocation_units() const override { return 128 * 1024; }
   uint32_t available_allocation_units() const override { return 128 * 1024; }
 
-  // STFC/cache code seems to require the product of these two to equal 0x10000!
+  // STFC/cache code seems to require the product of these two to equal 0x10000
-  uint32_t sectors_per_allocation_unit() const override { return 1; }
+  // or 0x8000! (depending on SectorsPerCluster value written to partition
-  uint32_t bytes_per_sector() const override { return 0x10000; }
+  // header)
+  uint32_t sectors_per_allocation_unit() const override {
+    return sectors_per_allocation_unit_;
+  }
+  uint32_t bytes_per_sector() const override { return 512; }
+
+  void sectors_per_allocation_unit(uint32_t value) {
+    sectors_per_allocation_unit_ = value;
+  }
 
  private:
   std::unique_ptr<Entry> root_entry_;
   std::vector<std::string> null_paths_;
+
+  uint32_t sectors_per_allocation_unit_ = 0x80;
 };
 
 }  // namespace vfs

src/xenia/vfs/devices/null_file.cc

@@ -9,6 +9,7 @@
 
 #include "xenia/vfs/devices/null_file.h"
 
+#include "xenia/vfs/devices/null_device.h"
 #include "xenia/vfs/devices/null_entry.h"
 
 namespace xe {
@@ -37,6 +38,20 @@ X_STATUS NullFile::WriteSync(const void* buffer, size_t buffer_length,
     return X_STATUS_ACCESS_DENIED;
   }
 
+  // Check if game is writing a FATX header...
+  if (byte_offset == 0 && buffer_length >= (4 * 3)) {
+    auto* header = (uint32_t*)buffer;
+    if (xe::load_and_swap<uint32_t>(header) == 0x58544146) {
+      // This is a FATX header - read the SectorsPerCluster value from it
+      // Game will try reading this back through NtQueryVolumeInformationFile
+      // later on, if it doesn't match, cache partition mount won't succeed
+      auto sectors_per_cluster = xe::byte_swap(header[2]);
+      // Update NullDevice with the SectorsPerCluster value
+      auto* null_device = (NullDevice*)entry_->device();
+      null_device->sectors_per_allocation_unit(sectors_per_cluster);
+    }
+  }
+
   return X_STATUS_SUCCESS;
 }