Protect against multiple calls to JLINKARM_Close (#65)

* Protect against multiple calls to JLINKARM_Close

With the latest JLink DLLs (tested with 6.56b), calling jlink.close()
twice can crash. This is a problem because there is an uncoditional call
to close() inside jlink.open(). Any code that attempts to cycle a jlink
connection by calling .close() followed by .open() therefore hits a
double close and the corresponding crash.

Crash from inside JLINKARM_Close looks like:
`double free or corruption (out)`

* Indentation

* Reference counting .open() calls

* Explicitly set the refcount in test

* Remove blank lines to placate pycodestyle

* Test setup ensures .open() called in context manager

Author: rjshade

pylink/jlink.py

@@ -275,6 +275,10 @@ def __init__(self, lib=None, log=None, detailed_log=None, error=None, warn=None,
         self._lock = None
         self._device = None
 
+        # Track the number of .open() calls to avoid multiple calls to
+        # JLINKARM_Close, which can cause a crash.
+        self._open_refcount = 0
+
         # Bind Types for function calls.
         self._dll.JLINKARM_OpenEx.restype = ctypes.POINTER(ctypes.c_char)
         self._dll.JLINKARM_GetCompileDateTime.restype = ctypes.POINTER(ctypes.c_char)
@@ -639,6 +643,9 @@ def open(self, serial_no=None, ip_addr=None):
           TypeError: if ``serial_no`` is present, but not ``int`` coercible.
           AttributeError: if ``serial_no`` and ``ip_addr`` are both ``None``.
         """
+        if self._open_refcount > 0:
+            self._open_refcount += 1
+            return None
 
         # For some reason, the J-Link driver complains if this isn't called
         # first (may have something to do with it trying to establish a
@@ -692,6 +699,7 @@ def open(self, serial_no=None, ip_addr=None):
             func = enums.JLinkFunctions.UNSECURE_HOOK_PROTOTYPE(unsecure_hook)
             self._dll.JLINK_SetHookUnsecureDialog(func)
 
+        self._open_refcount = 1
         return None
 
     def open_tunnel(self, serial_no, port=19020):
@@ -719,6 +727,14 @@ def close(self):
         Raises:
           JLinkException: if there is no connected JLink.
         """
+        if self._open_refcount == 0:
+            # Do nothing if .open() has not been called.
+            return None
+
+        self._open_refcount -= 1
+        if self._open_refcount > 0:
+            return None
+
         self._dll.JLINKARM_Close()
 
         if self._lock is not None:

tests/unit/test_jlink.py

@@ -818,8 +818,7 @@ def test_jlink_open_serial_number_context_manager_manual(self):
             jl.open(serial_no=123456789)
             self.dll.JLINKARM_OpenEx.assert_called()
         self.assertEqual(1, self.dll.JLINKARM_OpenEx.call_count)
-        # 2 instead of 1 because jlink.close() is also called from jlink.open().
-        self.assertEqual(2, self.dll.JLINKARM_Close.call_count)
+        self.assertEqual(1, self.dll.JLINKARM_Close.call_count)
 
     @mock.patch('pylink.jlock.JLock', new=mock.Mock())
     def test_jlink_open_dll_failed(self):
@@ -922,8 +921,21 @@ def test_jlink_close(self):
         Returns:
           ``None``
         """
+        # close() does nothing if there has been no open() call first.
+        self.jlink.close()
+        self.assertEqual(0, self.dll.JLINKARM_Close.call_count)
+
+        # close() decrements the refcount if open() has been called multiple times.
+        self.jlink._open_refcount = 5
+        self.jlink.close()
+        self.assertEqual(0, self.dll.JLINKARM_Close.call_count)
+        self.assertEqual(4, self.jlink._open_refcount)
+
+        # close() calls the DLL close method when refcount is exhausted.
+        self.jlink._open_refcount = 1
         self.jlink.close()
         self.assertEqual(1, self.dll.JLINKARM_Close.call_count)
+        self.assertEqual(0, self.jlink._open_refcount)
 
     def test_jlink_close_context_manager(self):
         """Tests the J-Link ``close()`` method using context manager.
@@ -934,10 +946,11 @@ def test_jlink_close_context_manager(self):
         Returns:
           ``None``
         """
-        # Use open_tunnel=None to avoid implicitly opening the connection.
-        with jlink.JLink(self.lib, open_tunnel=None):
-            self.dll.JLINKARM_OpenEx.assert_not_called()
+        self.dll.JLINKARM_OpenEx.return_value = 0
+        with jlink.JLink(self.lib, ip_addr='127.0.0.1:80') as jl:
+            self.assertTrue(jl.opened())
             self.dll.JLINKARM_Close.assert_not_called()
+
         # .close() is first called when exiting the context manager
         # Depending on the system - GC operation, it can also already be
         # called from __del__ when the object is garbage collected.