refactor: move wallpaper.cache to 0:/state

programs/src/login/login_wallpaper.cpp

@@ -12,9 +12,15 @@ namespace {
 constexpr const char* kDefaultWallpaperPath = "0:/os/wallpapers/default.jpg";
 
 // Screen-sized, already tinted pixels, so a re-login skips the JPEG decode and
-// the rescale entirely. Kernel-protected (CAP_USER_ADMIN) because it is drawn
-// on a screen that is about to take a password.
-constexpr const char* kCachePath = "0:/config/wallpaper.cache";
+// the rescale entirely. Kernel-protected (CAP_USER_ADMIN), along with its
+// directory, because it is drawn on a screen that is about to take a password.
+constexpr const char* kStateDir  = "0:/state";
+constexpr const char* kCacheDir  = "0:/state/cache";
+constexpr const char* kCachePath = "0:/state/cache/wallpaper.cache";
+
+// Where the cache lived before it moved out of 0:/config. Deleted when the new
+// one is first written so an upgraded install does not keep the stale copy.
+constexpr const char* kLegacyCachePath = "0:/config/wallpaper.cache";
 
 constexpr uint32_t kCacheMagic   = 0x4350574DU;  // "MWPC"
 constexpr uint32_t kCacheVersion = 1;
@@ -137,6 +143,10 @@ void save_to_cache(const LoginState* ls, const char* srcPath,
     header->overlayAlpha = kLoginOverlayAlpha;
     montauk::strncpy(header->sourcePath, srcPath, sizeof(header->sourcePath));
 
+    montauk::fdelete(kLegacyCachePath);
+    montauk::fmkdir(kStateDir);
+    montauk::fmkdir(kCacheDir);
+
     int fd = montauk::fcreate(kCachePath);
     if (fd < 0) return;  // read-only volume or no authority: not fatal
     // One write, so the file is never briefly visible half-written and the