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).

src/filesystem/winrt/SDL_sysfilesystem.cpp

@@ -144,49 +144,121 @@ SDL_GetPrefPath(const char *org, const char *app)
      * without violating Microsoft's app-store requirements.
      */
 
-#if WINAPI_FAMILY == WINAPI_FAMILY_PHONE_APP
-    /* A 'Roaming' folder is not available in Windows Phone 8.0, however a
-     * 'Local' folder is.  Use the 'Local' folder in order to preserve
-     * compatibility with Windows Phone 8.0, and with app-installs that have
-     * been updated from 8.0-based, to 8.1-based apps.
+    /* Default to using a Local/non-Roaming path.  WinRT will often attempt
+     * to synchronize files in Roaming paths, and will do so while an app is
+     * running.  Using a Local path prevents the possibility that an app's
+     * save-data files will get changed from underneath it, without it
+     * being ready.
+     *
+     * This behavior can be changed via use of the
+     * SDL_HINT_WINRT_PREF_PATH_ROOT hint.
      */
     SDL_WinRT_Path pathType = SDL_WINRT_PATH_LOCAL_FOLDER;
-#else
-    /* A 'Roaming' folder is available on Windows 8 and 8.1.  Use that.
-     */
-    SDL_WinRT_Path pathType = SDL_WINRT_PATH_ROAMING_FOLDER;
-#endif
 
     const char * hint = SDL_GetHint(SDL_HINT_WINRT_PREF_PATH_ROOT);
     if (hint) {
         if (SDL_strcasecmp(hint, "local") == 0) {
             pathType = SDL_WINRT_PATH_LOCAL_FOLDER;
-        }
+        } else if (SDL_strcasecmp(hint, "roaming") == 0) {
 #if (WINAPI_FAMILY != WINAPI_FAMILY_PHONE_APP) || (NTDDI_VERSION > NTDDI_WIN8)
-        else if (SDL_strcasecmp(hint, "roaming") == 0) {
             pathType = SDL_WINRT_PATH_ROAMING_FOLDER;
-        }
+#else
+            /* Don't apply a 'Roaming' path on Windows Phone 8.0.  Roaming
+             * data is not supported by that version of the operating system.
+             */
+            SDL_SetError("A Roaming path was specified via SDL_HINT_WINRT_PREF_PATH_ROOT, but Roaming is not supported on Windows Phone 8.0");
+            return NULL;
 #endif
+        } else if (SDL_strcasecmp(hint, "old") == 0) {
+            /* Older versions of SDL/WinRT, including 2.0.3, would return a
+             * pref-path that used a Roaming folder on non-Phone versions of
+             * Windows, such as Windows 8.0 and Windows 8.1.  This has since
+             * been reverted to using a Local folder, in order to prevent
+             * problems arising from WinRT automatically synchronizing files
+             * during an app's lifetime.  In case this functionality is
+             * desired, setting SDL_HINT_WINRT_PREF_PATH_ROOT to "old" will
+             * trigger the older behavior.
+             */
+#if WINAPI_FAMILY == WINAPI_FAMILY_PHONE_APP
+            pathType = SDL_WINRT_PATH_LOCAL_FOLDER;
+#else
+            pathType = SDL_WINRT_PATH_ROAMING_FOLDER;
+#endif
+        }
     }
 
-    const char * srcPath = SDL_WinRTGetFSPathUTF8(pathType);
-    size_t destPathLen;
-    char * destPath = NULL;
+    const WCHAR * srcPath = NULL;
+    WCHAR path[MAX_PATH];
+    char *retval = NULL;
+    WCHAR* worg = NULL;
+    WCHAR* wapp = NULL;
+    size_t new_wpath_len = 0;
+    BOOL api_result = FALSE;
 
-    if (!srcPath) {
-        SDL_SetError("Couldn't locate our basepath: %s", SDL_GetError());
+    srcPath = SDL_WinRTGetFSPathUNICODE(pathType);
+    if ( ! srcPath) {
+        SDL_SetError("Unable to find a source path");
         return NULL;
     }
 
-    destPathLen = SDL_strlen(srcPath) + SDL_strlen(org) + SDL_strlen(app) + 4;
-    destPath = (char *) SDL_malloc(destPathLen);
-    if (!destPath) {
+    if (SDL_wcslen(srcPath) >= MAX_PATH) {
+        SDL_SetError("Path too long.");
+        return NULL;
+    }
+    SDL_wcslcpy(path, srcPath, SDL_arraysize(path));
+
+    worg = WIN_UTF8ToString(org);
+    if (worg == NULL) {
         SDL_OutOfMemory();
         return NULL;
     }
 
-    SDL_snprintf(destPath, destPathLen, "%s\\%s\\%s\\", srcPath, org, app);
-    return destPath;
+    wapp = WIN_UTF8ToString(app);
+    if (wapp == NULL) {
+        SDL_free(worg);
+        SDL_OutOfMemory();
+        return NULL;
+    }
+
+    new_wpath_len = SDL_wcslen(worg) + SDL_wcslen(wapp) + SDL_wcslen(path) + 3;
+
+    if ((new_wpath_len + 1) > MAX_PATH) {
+        SDL_free(worg);
+        SDL_free(wapp);
+        SDL_SetError("Path too long.");
+        return NULL;
+    }
+
+    SDL_wcslcat(path, L"\\", new_wpath_len + 1);
+    SDL_wcslcat(path, worg, new_wpath_len + 1);
+    SDL_free(worg);
+
+    api_result = CreateDirectoryW(path, NULL);
+    if (api_result == FALSE) {
+        if (GetLastError() != ERROR_ALREADY_EXISTS) {
+            SDL_free(wapp);
+            WIN_SetError("Couldn't create a prefpath.");
+            return NULL;
+        }
+    }
+
+    SDL_wcslcat(path, L"\\", new_wpath_len + 1);
+    SDL_wcslcat(path, wapp, new_wpath_len + 1);
+    SDL_free(wapp);
+
+    api_result = CreateDirectoryW(path, NULL);
+    if (api_result == FALSE) {
+        if (GetLastError() != ERROR_ALREADY_EXISTS) {
+            WIN_SetError("Couldn't create a prefpath.");
+            return NULL;
+        }
+    }
+
+    SDL_wcslcat(path, L"\\", new_wpath_len + 1);
+
+    retval = WIN_StringToUTF8(path);
+
+    return retval;
 }
 
 #endif /* __WINRT__ */