Redo UF2 discovery for Windows compatibility (earlephilhower#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 arduino/arduino-cli#2867

Author: earlephilhower

tools/pluggable_discovery.py

@@ -5,89 +5,96 @@
 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):
+
+    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 not boards:
+                boards = True
+                print ("""{
+          "eventType": "add",
+          "port": {
+            "address": "UF2_Board",
+            "label": "UF2 Board",
+            "protocol": "uf2conv",
+            "protocolLabel": "UF2 Devices",
+            "properties": {
+                "mac": "ffffffffffff",
+                "pid" : "0x2e8a",
+                "vid" : "0x000a"
+            }
+          }
+        }""", flush=True)
+            elif (len(l) == 0) and boards:
+                boards = False
+                print("""{
+          "eventType": "remove",
+          "port": {
+            "address": "UF2_Board",
+            "protocol": "uf2conv"
+          }
+        }""", flush = True)
 
-def scanner():
-    global scannerGo
-    scannerGo = True
-    boards = False
-    while scannerGo:
-        l = uf2conv.get_drives()
-        if (len(l) > 0) and scannerGo and not boards:
-            boards = True
-            print ("""{
-  "eventType": "add",
-  "port": {
-    "address": "UF2_Board",
-    "label": "UF2 Board",
-    "protocol": "uf2conv",
-    "protocolLabel": "UF2 Devices",
-    "properties": {
-        "mac": "ffffffffffff",
-        "pid" : "0x2e8a",
-        "vid" : "0x000a"
-    }
-  }
-}""", flush=True)
-        elif (len(l) == 0) and scannerGo and boards:
-            boards = False
-            print("""{
-  "eventType": "remove",
-  "port": {
-    "address": "UF2_Board",
-    "protocol": "uf2conv"
-  }
-}""", flush = True)
-        n = time.time() + 2
-        while scannerGo and (time.time() < n):
-            time.sleep(.1)
-    scannerGo = True
 
 def main():
-    global scannerGo
-    while True:
-        cmdline = input()
-        cmd = cmdline.split()[0]
-        if cmd == "HELLO":
-            print(""" {
+    global scannerStop
+    global dropDead
+    try:
+        while True:
+            cmdline = input()
+            cmd = cmdline.split()[0]
+            if cmd == "HELLO":
+                print(""" {
   "eventType": "hello",
   "message": "OK",
   "protocolVersion": 1
 }""", flush = True)
-        elif cmd == "START":
-            print("""{
+            elif cmd == "START":
+                print("""{
   "eventType": "start",
   "message": "OK"
 }""", flush = True);
-        elif cmd == "STOP":
-            scannerGo = False
-            while not scannerGo:
-                time.sleep(.1)
-            print("""{
+            elif cmd == "STOP":
+                scannerStop.set()
+                print("""{
   "eventType": "stop",
   "message": "OK"
 }""", flush = True)
-        elif cmd == "QUIT":
-            scannerGo = False
-            print("""{
+            elif cmd == "QUIT":
+                scannerStop.set()
+                print("""{
   "eventType": "quit",
   "message": "OK"
 }""", flush = True)
-            return
-        elif cmd == "LIST":
-            l = uf2conv.get_drives()
-            if len(l) > 0:
-                        print ("""{
+                return
+            elif cmd == "LIST":
+                l = uf2conv.get_drives()
+                if len(l) > 0:
+                    print ("""{
   "eventType": "list",
   "ports": [
    {
@@ -103,18 +110,19 @@ def main():
    }
  ]
 }""", flush=True)
-            else:
-                print ("""{
+                else:
+                    print ("""{
   "eventType": "list",
   "ports": [ ]
 }""", flush=True)
-        elif cmd == "START_SYNC":
-            print("""{
+            elif cmd == "START_SYNC":
+                print("""{
   "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()