Only write PID after last fork

- should fix systemd integration

src/bitmessagemain.py

@@ -363,7 +363,7 @@ class Main:
             # fork not implemented
             pass
         else:
-            shared.thisapp.lock() # relock
+            shared.thisapp.lock(True) # relock and write pid
         shared.thisapp.lockPid = None # indicate we're the final child
         sys.stdout.flush()
         sys.stderr.flush()

src/singleinstance.py

@@ -36,7 +36,7 @@ class singleinstance:
         self.initialized = True
         atexit.register(self.cleanup)
 
-    def lock(self):
+    def lock(self, writePid = False):
         if self.lockPid is None:
             self.lockPid = os.getpid()
         if sys.platform == 'win32':
@@ -68,9 +68,10 @@ class singleinstance:
                 sys.exit(-1)
             else:
                 pidLine = "%i\n" % self.lockPid
-                self.fp.truncate(0)
-                self.fp.write(pidLine)
-                self.fp.flush()
+                if writePid:
+                    self.fp.truncate(0)
+                    self.fp.write(pidLine)
+                    self.fp.flush()
 
     def cleanup(self):
         if not self.initialized: