[magneto] implement execution lock, avoid multiple magneto applets popping up

magneto/src/magneto.py

@@ -11,11 +11,48 @@
 
 """
 import os
+import errno
 import sys
 import time
+import fcntl
 
-if __name__ == "__main__":
+_LOCK_HANDLES = {}
 
+def _acquire_lock(lock_file):
+    lock_f = open(lock_file, "a+")
+    try:
+        fcntl.flock(lock_f.fileno(), fcntl.LOCK_EX | fcntl.LOCK_NB)
+    except IOError as err:
+        if err.errno not in (errno.EACCES, errno.EAGAIN,):
+            # ouch, wtf?
+            raise
+        lock_f.close()
+        return False # lock already acquired
+
+    lock_f.truncate()
+    lock_f.write(str(os.getpid()))
+    lock_f.flush()
+    _LOCK_HANDLES[lock_file] = lock_f
+    return True
+
+def _release_lock(lock_file):
+    try:
+        lock_f = _LOCK_HANDLES.pop(lock_file)
+    except KeyError:
+        lock_f = None
+
+    if lock_f is not None:
+        fcntl.flock(lock_f.fileno(), fcntl.LOCK_UN)
+        lock_f.close()
+
+    try:
+        os.remove(lock_file)
+    except OSError as err:
+        # cope with possible race conditions
+        if err.errno != errno.ENOENT:
+            raise
+
+def _startup():
     sys.path.insert(0, '/usr/lib/entropy/client')
     sys.path.insert(0, '/usr/lib/entropy/libraries')
     sys.path.insert(0, '/usr/lib/entropy/sulfur')
@@ -23,8 +60,8 @@ if __name__ == "__main__":
     sys.path.insert(0, '../../libraries')
     sys.path.insert(0, '../../sulfur/src')
     sys.path.insert(0, '../')
-    sys.argv.append('--no-pid-handling')
 
+    sys.argv.append('--no-pid-handling')
     startup_delay = None
     for arg in sys.argv[1:]:
         if arg.startswith("--startup-delay="):
@@ -69,3 +106,23 @@ if __name__ == "__main__":
         raise
     raise SystemExit(0)
 
+if __name__ == "__main__":
+    # acquire lock
+    magneto_lock_dir = "/tmp"
+    magneto_lock_file = ".magneto.lock"
+    user_home = os.getenv("HOME")
+    if user_home is not None:
+        if os.path.isdir(user_home):
+            magneto_lock_dir = user_home
+
+    magneto_lock = os.path.join(magneto_lock_dir, magneto_lock_file)
+    acquired = _acquire_lock(magneto_lock)
+    try:
+        if acquired:
+            _startup()
+        else:
+            sys.stderr.write("Warning: another Magneto instance is already running.\n")
+            raise SystemExit(1)
+    finally:
+        if acquired:
+            _release_lock(magneto_lock)