Use stat() to minimize input device opens when not using udev

Calling open() on input devices can generate device I/O which blocks
the main thread and causes dropped frames. Using stat() we can avoid
opening anything unless /dev/input has changed since we last polled.

We could have used something fancy like inotify, but it didn't seem
worth the added complexity for this uncommon non-udev case.

src/joystick/linux/SDL_sysjoystick.c

@@ -80,7 +80,8 @@ static SDL_joylist_item *SDL_joylist_tail = NULL;
 static int numjoysticks = 0;
 
 #if !SDL_USE_LIBUDEV
-static Uint32 last_joy_detect_time = 0;
+static Uint32 last_joy_detect_time;
+static time_t last_input_dir_mtime;
 #endif
 
 #define test_bit(nr, addr) \
@@ -421,6 +422,10 @@ LINUX_JoystickDetect(void)
     Uint32 now = SDL_GetTicks();
 
     if (!last_joy_detect_time || SDL_TICKS_PASSED(now, last_joy_detect_time + SDL_JOY_DETECT_INTERVAL_MS)) {
+        struct stat sb;
+
+        /* Opening input devices can generate synchronous device I/O, so avoid it if we can */
+        if (stat("/dev/input", &sb) == 0 && sb.st_mtime != last_input_dir_mtime) {
             DIR *folder;
             struct dirent *dent;
 
@@ -438,6 +443,9 @@ LINUX_JoystickDetect(void)
                 closedir(folder);
             }
 
+            last_input_dir_mtime = sb.st_mtime;
+        }
+
         last_joy_detect_time = now;
     }
 #endif
@@ -483,6 +491,10 @@ LINUX_JoystickInit(void)
     /* Force a scan to build the initial device list */
     SDL_UDEV_Scan();
 #else
+    /* Force immediate joystick detection */
+    last_joy_detect_time = 0;
+    last_input_dir_mtime = 0;
+
     /* Report all devices currently present */
     LINUX_JoystickDetect();
 #endif