Dynamically load libudev.so.1 on Linux if udev=yes

This makes it possibly to run Linux binaries compiled with udev support on
Linux systems which do not provide udev (typically systemd-less distros).

If udev is missing, we fall back to parsing `/dev/input` like when compiled
without udev support (`udev=no`).

Also adding some verbose debug statements to know which method we're using
when debugging Linux joypad issues.

The libudev so wrappers were generated on Mageia 8 with libudev 246.9 using
https://github.com/hpvb/dynload-wrapper:
```
./generate-wrapper.py --include /usr/include/libudev.h --sys-include '<libudev.h>' \
  --soname libudev.so.1 --init-name libudev --omit-prefix gnu_ \
  --output-header libudev-so_wrap.h --output-implementation libudev-so_wrap.c
```

misc/hooks/pre-commit-clang-format

@@ -125,6 +125,9 @@ do
     if grep -q "platform/android/java/lib/src/com" <<< $file; then
         continue;
     fi
+    if grep -q "\-so_wrap." <<< $file; then
+        continue;
+    fi
 
     # ignore file if we do check for file extensions and the file
     # does not match any of the extensions specified in $FILE_EXTS

misc/scripts/file_format.sh

@@ -30,6 +30,8 @@ while IFS= read -rd '' f; do
         continue
     elif [[ "$f" == "platform/android/java/lib/src/com/google"* ]]; then
         continue
+    elif [[ "$f" == *"-so_wrap."* ]]; then
+        continue
     fi
     # Ensure that files are UTF-8 formatted.
     recode UTF-8 "$f" 2> /dev/null

platform/linuxbsd/SCsub

@@ -16,6 +16,9 @@ common_x11 = [
     "key_mapping_x11.cpp",
 ]
 
+if "udev" in env and env["udev"]:
+    common_x11.append("libudev-so_wrap.c")
+
 prog = env.add_program("#bin/godot", ["godot_linuxbsd.cpp"] + common_x11)
 
 if env["debug_symbols"] and env["separate_debug_symbols"]:

platform/linuxbsd/detect.py

@@ -325,14 +325,14 @@ def configure(env):
 
     if platform.system() == "Linux":
         env.Append(CPPDEFINES=["JOYDEV_ENABLED"])
-
         if env["udev"]:
             if os.system("pkg-config --exists libudev") == 0:  # 0 means found
                 print("Enabling udev support")
                 env.Append(CPPDEFINES=["UDEV_ENABLED"])
-                env.ParseConfig("pkg-config libudev --cflags --libs")
             else:
                 print("libudev development libraries not found, disabling udev support")
+    else:
+        env["udev"] = False  # Linux specific
 
     # Linkflags below this line should typically stay the last ones
     if not env["builtin_zlib"]:

platform/linuxbsd/joypad_linux.cpp

@@ -39,7 +39,7 @@
 #include <unistd.h>
 
 #ifdef UDEV_ENABLED
-#include <libudev.h>
+#include "libudev-so_wrap.h"
 #endif
 
 #define LONG_BITS (sizeof(long) * 8)
@@ -72,13 +72,22 @@ void JoypadLinux::Joypad::reset() {
 }
 
 JoypadLinux::JoypadLinux(Input *in) {
-	exit_udev = false;
+#ifdef UDEV_ENABLED
+	use_udev = initialize_libudev() == 0;
+	if (use_udev) {
+		print_verbose("JoypadLinux: udev enabled and loaded successfully.");
+	} else {
+		print_verbose("JoypadLinux: udev enabled, but couldn't be loaded. Falling back to /dev/input to detect joypads.");
+	}
+#else
+	print_verbose("JoypadLinux: udev disabled, parsing /dev/input to detect joypads.");
+#endif
 	input = in;
 	joy_thread.start(joy_thread_func, this);
 }
 
 JoypadLinux::~JoypadLinux() {
-	exit_udev = true;
+	exit_monitor = true;
 	joy_thread.wait_to_finish();
 	close_joypad();
 }
@@ -92,11 +101,18 @@ void JoypadLinux::joy_thread_func(void *p_user) {
 
 void JoypadLinux::run_joypad_thread() {
 #ifdef UDEV_ENABLED
-	udev *_udev = udev_new();
-	ERR_FAIL_COND(!_udev);
-	enumerate_joypads(_udev);
-	monitor_joypads(_udev);
-	udev_unref(_udev);
+	if (use_udev) {
+		udev *_udev = udev_new();
+		if (!_udev) {
+			use_udev = false;
+			ERR_FAIL_MSG("Failed getting an udev context, falling back to parsing /dev/input.");
+		}
+		enumerate_joypads(_udev);
+		monitor_joypads(_udev);
+		udev_unref(_udev);
+	} else {
+		monitor_joypads();
+	}
 #else
 	monitor_joypads();
 #endif
@@ -137,7 +153,7 @@ void JoypadLinux::monitor_joypads(udev *p_udev) {
 	udev_monitor_enable_receiving(mon);
 	int fd = udev_monitor_get_fd(mon);
 
-	while (!exit_udev) {
+	while (!exit_monitor) {
 		fd_set fds;
 		struct timeval tv;
 		int ret;
@@ -179,7 +195,7 @@ void JoypadLinux::monitor_joypads(udev *p_udev) {
 #endif
 
 void JoypadLinux::monitor_joypads() {
-	while (!exit_udev) {
+	while (!exit_monitor) {
 		{
 			MutexLock lock(joy_mutex);
 

platform/linuxbsd/joypad_linux.h

@@ -70,10 +70,13 @@ class JoypadLinux {
 		void reset();
 	};
 
-	bool exit_udev;
+#ifdef UDEV_ENABLED
+	bool use_udev = false;
+#endif
+	bool exit_monitor = false;
 	Mutex joy_mutex;
 	Thread joy_thread;
-	Input *input;
+	Input *input = nullptr;
 	Joypad joypads[JOYPADS_MAX];
 	Vector<String> attached_devices;