dconf: Check for changes properly despite style of quotes used by user

GVariant strings can use either single or double quotes, and therefore
the user can use either single or double quotes when specifying a
string value in their task, and what they choose to use may not match
what is output by the dconf command, so for accurate comparison we
need to strip the quotes and just compare what's inside them.

plugins/modules/dconf.py

@@ -258,6 +258,48 @@ def __init__(self, module, check_mode=False):
         # Check if dconf binary exists
         self.dconf_bin = self.module.get_bin_path('dconf', required=True)
 
+    # GVariant strings can use either single or double quotes, and therefore
+    # the user can use either single or double quotes when specifying a string
+    # value in their task, and what they choose to use may not match what is
+    # output by the dconf command, so for accurate comparison we need to strip
+    # the quotes and just compare what's inside them.
+    #
+    # This doesn't give us 100% accurate GVariant string interpretation, but
+    # it solves many of the common use cases and makes us much more accurate
+    # than if we just compare the two strings directly.
+    @staticmethod
+    def gvariant_maybe_string(value):
+        """
+        Partially canonicalization value to avoid quotation mark inconsistencies
+
+        :returns: a massaged value of the same type as its input
+        """
+        if not isinstance(value, str):
+            return value
+        if len(value) < 2:
+            return value
+        if value[0] in ('"', "'") and value[-1] == value[0]:
+            # Whichever quotation mark the user has chosen to use will
+            # definitely need to be backslash-quoted in the string, so to
+            # improve our likelihood of getting this check right, remove those
+            # backslashes.
+            chars = list(value[1:-1])
+            i = 0
+            while i < len(chars) - 1:
+                if chars[i] == '\\':
+                    if chars[i+1] == value[0]:
+                        chars[i] = ''
+                    else:
+                        # Otherwise we'll mishandle a \ quoted with a \
+                        i += 1
+                i += 1
+            return ''.join(chars)
+        return value
+
+    def gvariant_equal(self, value1, value2):
+        return (self.gvariant_maybe_string(value1) ==
+                self.gvariant_maybe_string(value2))
+
     def read(self, key):
         """
         Retrieves current value associated with the dconf key.
@@ -298,7 +340,7 @@ def write(self, key, value):
         """
         # If no change is needed (or won't be done due to check_mode), notify
         # caller straight away.
-        if value == self.read(key):
+        if self.gvariant_equal(value, self.read(key)):
             return False
         elif self.check_mode:
             return True