Use dir command on Windows for checking mount points

If you call `os.path.exists` on a removable drive that is currently in the
"ejected" state (which often happens when flashing multiple devices at the
same time), then an error dialog will be shown that blocks the python
process until the user interacts with it. You can globally disable these
error boxes, but this is undesirable as it may mask other errors that occur.

Windows has APIs that allow it to check if a device is ready before
accessing the file system, however this is hard to access from Python.
Instead, we use the `dir` command, which will error but not cause a blocking
error dialog. For other OSes, we just use `os.path.exists`.

Author: bridadan

mbed_lstools/lstools_base.py

@@ -720,6 +720,19 @@ def scan_html_line_for_target_id(self, line):
                 return result
         return None
 
+    def mount_point_ready(self, path):
+        """! Check if a mount point is ready for file operations
+        @return Returns True if the given path exists, False otherwise
+        """
+        result = os.path.exists(path)
+
+        if result:
+            self.debug(self.mount_point_ready.__name__, "Mount point %s is ready" % path)
+        else:
+            self.debug(self.mount_point_ready.__name__, "Mount point %s does not exist" % (path))
+
+        return result
+
     @staticmethod
     def run_cli_process(cmd, shell=True):
         """! Runs command as a process and return stdout, stderr and ret code

mbed_lstools/lstools_win7.py

@@ -132,7 +132,7 @@ def get_connected_mbeds(self):
         @return Returns [(<mbed_mount_point>, <mbed_id>), ..]
         @details Helper function
         """
-        return [m for m in self.get_mbeds() if os.path.exists(m[0])]
+        return [m for m in self.get_mbeds() if self.mount_point_ready(m[0])]
 
     def get_mbeds(self):
         """! Function filters devices' mount points for valid TargetID
@@ -198,3 +198,22 @@ def regbin2str(self, regbin):
         """! Decode registry binary to readable string
         """
         return filter(lambda ch: ch in string.printable, regbin)
+
+    def mount_point_ready(self, path):
+        """! Check if a mount point is ready for file operations
+        @return Returns True if the given path exists, False otherwise
+        @details Calling the Windows command `dir` instead of using the python
+        `os.path.exists`. The latter causes a Python error box to appear claiming
+        there is "No Disk" for some devices that are in the ejected state. Calling
+        `dir` prevents this since it uses the Windows API to determine if the
+        device is ready before accessing the file system.
+        """
+        stdout, stderr, retcode = self.run_cli_process('dir %s' % path)
+        result = True if retcode == 0 else False
+
+        if result:
+            self.debug(self.mount_point_ready.__name__, "Mount point %s is ready" % path)
+        else:
+            self.debug(self.mount_point_ready.__name__, "Mount point %s reported not ready with error '%s'" % (path, stderr.strip()))
+
+        return result