server: prevent multiple instances using file locks

On Linux, this was already prevented when binding socket, but it wasn't
working on Windows. Regardless, this solution is a lot cleaner overall.

Author: johncf

.gitignore

@@ -1,2 +1,3 @@
 __pycache__
 sessions
+instance.lock

server.py

@@ -4,6 +4,7 @@
 import sys
 
 import compiler
+import singleton
 
 frozen = getattr(sys, 'frozen', False)
 
@@ -17,8 +18,7 @@
 app = Flask('javaplay', static_url_path='', static_folder=os.path.join(root_dir, 'static'))
 
 import logging
-log = logging.getLogger('werkzeug')
-log.setLevel(logging.WARNING)
+logging.getLogger('werkzeug').setLevel(logging.WARNING)
 
 socketio = SocketIO(app, async_mode='threading')
 
@@ -98,6 +98,8 @@ def disconnect():
         del sid_program_map[sid]
 
 if frozen or __name__ == "__main__":
-    if not os.path.isdir(sess_dir):
-        os.makedirs(sess_dir)
-    socketio.run(app, port=8040)
+    lockpath = os.path.join(root_dir, 'instance.lock')
+    with singleton.InstanceFileLock(lockpath):
+        if not os.path.isdir(sess_dir):
+            os.makedirs(sess_dir)
+        socketio.run(app, port=8040)

singleton.py

@@ -0,0 +1,39 @@
+import logging
+import os, sys
+
+logger = logging.getLogger("singleton")
+
+# code adapted from https://github.com/pycontribs/tendo/blob/master/tendo/singleton.py
+class InstanceFileLock:
+    def __init__(self, lockpath):
+        self._lock = lockpath
+
+    def __enter__(self):
+        if sys.platform == 'win32':
+            try:
+                if os.path.exists(self._lock):
+                    os.unlink(self._lock)
+                self.fd = os.open(self._lock, os.O_CREAT | os.O_EXCL | os.O_RDWR)
+            except OSError:
+                logger.warning("Another instance is running.")
+                sys.exit(-1)
+        else:
+            import fcntl
+            self.fp = open(self._lock, 'w')
+            try:
+                fcntl.lockf(self.fp, fcntl.LOCK_EX | fcntl.LOCK_NB)
+            except IOError:
+                logger.warning("Another instance is running.")
+                sys.exit(-1)
+
+    def __exit__(self, type, value, traceback):
+        try:
+            if sys.platform == 'win32':
+                os.close(self.fd)
+            else:
+                import fcntl
+                fcntl.lockf(self.fp, fcntl.LOCK_UN)
+                self.fp.close()
+            os.unlink(self._lock)
+        except Exception as e:
+            logger.error(e)