lock on the spectrum thread to avoid double start

owrx/source/__init__.py

@@ -54,6 +54,7 @@ class SdrSource(ABC):
         self.clients = []
         self.spectrumClients = []
         self.spectrumThread = None
+        self.spectrumLock = threading.Lock()
         self.process = None
         self.modificationLock = threading.Lock()
         self.failed = False
@@ -263,11 +264,12 @@ class SdrSource(ABC):
             self.stop()
 
     def addSpectrumClient(self, c):
-        self.spectrumClients.append(c)
-        if self.spectrumThread is None:
         # local import due to circular depencency
         from owrx.fft import SpectrumThread
 
+        self.spectrumClients.append(c)
+        with self.spectrumLock:
+            if self.spectrumThread is None:
                 self.spectrumThread = SpectrumThread(self)
                 self.spectrumThread.start()
 
@@ -276,6 +278,7 @@ class SdrSource(ABC):
             self.spectrumClients.remove(c)
         except ValueError:
             pass
+        with self.spectrumLock:
             if not self.spectrumClients and self.spectrumThread is not None:
                 self.spectrumThread.stop()
                 self.spectrumThread = None