Redo UF2 discovery for Windows compatibility (#2853)

Windows Python doesn't seem to kill the worker Thread properly when the IDE
is exited, leading to a) multiple Python3 instances on a PC after many uses
and b) errors updating the core when it tries to re-install Python3 while
still having the older version's EXE loaded and in use.

When an exception happens on the input() call under Windows, it seems like it
can still leave a thread running.  Add one more flag, caught in a global
exception handler.

Works around https://github.com/arduino/arduino-cli/issues/2867

tools/pluggable_discovery.py

@@ -5,25 +5,35 @@ import sys
 import time
 import threading
 
-
 toolspath = os.path.dirname(os.path.realpath(__file__))
 try:
-    sys.path.insert(0, os.path.join(toolspath, ".")) # Add pyserial dir to search path
+    sys.path.insert(0, os.path.join(toolspath, ".")) # Add uf2conv dir to search path
     import uf2conv # If this fails, we can't continue and will bomb below
 except ImportError:
     sys.stderr.write("uf2conv not found next to this tool.\n")
     sys.exit(1)
 
+scannerStop = threading.Event()
+dropDead = False
 
-scannerGo = False
+class ScannerDarkly(threading.Thread):
 
-def scanner():
-    global scannerGo
-    scannerGo = True
-    boards = False
-    while scannerGo:
+    loopTime = 0.0 # Set to 0 for 1st pass to get immediate response for arduino-cli, then bumped to 2.0 for ongoing checks
+
+    # https://stackoverflow.com/questions/12435211/threading-timer-repeat-function-every-n-seconds
+    def __init__(self, event):
+        threading.Thread.__init__(self)
+        self.stopped = event
+
+    def run(self):
+        global dropDead
+        boards = False;
+        while not self.stopped.wait(self.loopTime):
+            if self.stopped.is_set() or dropDead:
+                return
+            self.loopTime = 2.0
             l = uf2conv.get_drives()
-        if (len(l) > 0) and scannerGo and not boards:
+            if (len(l) > 0) and not boards:
                 boards = True
                 print ("""{
           "eventType": "add",
@@ -39,7 +49,7 @@ def scanner():
             }
           }
         }""", flush=True)
-        elif (len(l) == 0) and scannerGo and boards:
+            elif (len(l) == 0) and boards:
                 boards = False
                 print("""{
           "eventType": "remove",
@@ -48,13 +58,12 @@ def scanner():
             "protocol": "uf2conv"
           }
         }""", flush = True)
-        n = time.time() + 2
-        while scannerGo and (time.time() < n):
-            time.sleep(.1)
-    scannerGo = True
+
 
 def main():
-    global scannerGo
+    global scannerStop
+    global dropDead
+    try:
         while True:
             cmdline = input()
             cmd = cmdline.split()[0]
@@ -70,15 +79,13 @@ def main():
   "message": "OK"
 }""", flush = True);
             elif cmd == "STOP":
-            scannerGo = False
-            while not scannerGo:
-                time.sleep(.1)
+                scannerStop.set()
                 print("""{
   "eventType": "stop",
   "message": "OK"
 }""", flush = True)
             elif cmd == "QUIT":
-            scannerGo = False
+                scannerStop.set()
                 print("""{
   "eventType": "quit",
   "message": "OK"
@@ -113,8 +120,9 @@ def main():
   "eventType": "start_sync",
   "message": "OK"
 }""", flush = True)
-            scannerGo = True
-            threading.Thread(target = scanner).start()
-    time.sleep(.5)
+                thread = ScannerDarkly(scannerStop)
+                thread.start()
+    except:
+          dropDead = True
 
 main()