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WinRT: bug and data-integrity fixes for SDL_GetPrefPath()

This change does a few things, all with regards to the WinRT implementation of
SDL_GetPrefPath():

1. it fixes a bug whereby SDL_GetPrefPath() did not create the directory it
returned.  On other SDL platforms, SDL_GetPrefPath() will create separate
directories for its 'org' and 'app' folders.  Without this, attempts to create
files in the pref-path would fail, unless those directories were first created
by the app, or by some other library the app used.  This change makes sure
that these directories get created, before SDL_GetPrefPath() returns to its
caller(s).


2. it defaults to having SDL_GetPrefPath() return a WinRT 'Local' folder
on all platforms.  Previously, for Windows Store apps, it would have used a
different, 'Roaming' folder.  Files in Roaming folders can be automatically,
and synchronized across multiple devices by Windows.  This synchronization can
happen while the app runs, with new files being copied into a running app's
pref-path.  Unless an app is specifically designed to handle this scenario,
there is a chance that save-data could be overwritten in unwanted or
unexpected ways.

The default is now to use a Local folder, which does not get synchronized, and
which is arguably a bit safer to use.  Apps that wish to use Roaming folders
can do so by setting SDL_HINT_WINRT_PREF_PATH_ROOT to "roaming", however it
is recommended that one first read Microsoft's documentation for Roaming
files, a link to which is provided in README-winrt.md.

To preserve older pref-path selection behavior (found in SDL 2.0.3, as well as
many pre-2.0.4 versions of SDL from hg.libsdl.org), which uses a Roaming path
in Windows Store apps, and a Local path in Windows Phone, set
SDL_HINT_WINRT_PREF_PATH_ROOT to "old".

Please note that Roaming paths are not supported on Windows Phone 8.0, due to
limitations in the OS itself.  Attempts to use this will fail.
(Windows Phone 8.1 does not have this limitation, however.)


3. It makes SDL_GetPrefPath(), when on Windows Phone 8.0, and when
SDL_HINT_WINRT_PREF_PATH_ROOT is set to "roaming", return NULL, rather than
silently defaulting to a Local path (then switching to a Roaming path if and
when the user upgraded to Windows Phone 8.1).
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DavidLudwig committed Nov 29, 2014
1 parent 4dab32a commit ce64b4ad3a17e455b5d09bcde0efcd912894b84d

File 1 of 3 in ce64b4a

@@ -119,27 +119,64 @@ Here is a rough list of what works, and what doens't:
Caveats
-------

#### SDL_GetPrefPath() usage

SDL_GetPrefPath() is available for use in WinRT apps, however the following
should be noted:

1. It will return different path types, by default, depending on the WinRT
platform. Windows Phone apps will default to using the app's "local" path,
whereas Windows Store (i.e. non-Phone) apps will default to using the app's
"roaming" path. This behavior can be changed by calling SDL_SetHint() with
the key, SDL_HINT_WINRT_PREF_PATH_ROOT, and a value of either "local" or
"roaming".

2. Windows Phone 8.0 does not provide apps access to a "roaming" folder.
Attempts to make SDL_GetPrefPath() return a roaming folder on Windows
Phone 8.0 will be ignored (and a path inside the "local" folder will be
used instead).

Further details on this can be found in the documentation for
SDL_HINT_WINRT_PREF_PATH_ROOT, in SDL_hints.h, as well as the docs for
SDL_WinRT_Path, SDL_WinRTGetFSPathUNICODE, and SDL_WinRTGetFSPathUTF8,
in SDL_system.h.
#### SDL_GetPrefPath() usage when upgrading existing WinRT apps to SDL 2.0.4

SDL 2.0.4 fixes two bugs found in SDL_GetPrefPath() which can affect
an app's save data. These bugs only apply to WinRT apps (and not
Windows Desktop / Win32 apps, or to apps on any other SDL platform).
In particular, for older versions of SDL (anything before 2.0.4):

1. SDL_GetPrefPath() would return an invalid path, one in which attempts
to write files to would fail, in many cases. Some of the path elements
returned by SDL_GetPrefPath() would not get created (as done on other
SDL platforms). Files could be written to this path, however apps would
need to explicitly create the missing directories first.

2. SDL_GetPrefPath() would return a path inside a WinRT 'Roaming' folder,
the contents of which could get automatically synchronized across multiple
devices, by Windows. This process could occur while an app was running.
Apps which were not explicitly built to handle this scenario could
have their SDL_GetPrefPath-backed save data swapped out by Windows at
unexpected times, which raised potential for data-loss (if apps weren't
designed to support live file-synchronization.)


SDL_GetPrefPath(), starting with SDL 2.0.4, addresses these by:

1. making sure that SDL_GetPrefPath() returns a directory in which data
can be written to immediately, without first needing to create directories.

2. basing SDL_GetPrefPath() off of a non-Roaming / 'Local' folder, the
contents of which do not get automatically synchronized across devices,
and which may be safer in terms of data-integrity.

Apps can, at their discretion, choose to utilize WinRT's Roaming
functionality by calling the following before calling SDL_GetPrefPath():

SDL_SetHint(SDL_HINT_WINRT_PREF_PATH_ROOT, "roaming");

Alternatively, to restore SDL_GetPrefPath()'s old behavior (found in
SDL 2.0.3, and in many pre-2.0.4 versions of SDL found on hg.libsdl.org),
whereby a Roaming path is returned for Windows Store apps, and a Local
folder is returned for Windows Phone apps, use the following code:

SDL_SetHint(SDL_HINT_WINRT_PREF_PATH_ROOT, "old");

Before using Roaming data in any capacity, it is highly recommended that
one read the following:

1. Microsoft's documentation on the Roaming data. Details on this can be
found on MSDN, at:
[Guidelines for roaming app data](http://msdn.microsoft.com/en-us/library/windows/apps/hh465094.aspx).

2. the SDL documentation for SDL_HINT_WINRT_PREF_PATH_ROOT, which is
listed inside SDL_hints.h.

Please note that Roaming support is not available on Windows Phone 8.0,
due to limitations in the OS itself. Attempts to use it will fail, with
SDL_GetPrefPath() returning NULL (if SDL_HINT_WINRT_PREF_PATH_ROOT is
set to "roaming" on that platform). Windows Phone 8.1 does not have this
limitation, and does support Roaming data.



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