Merge #739: descriptor.py: fix parsing tr() desc with single leaf TREE

4e8559b descriptor.py: fix parsing tr() desc with single leaf TREE (SomberNight)

Pull request description:

  `tr()` descriptors with a single leaf TREE expression cannot be parsed currently:
  ```py
  >>> from hwilib.descriptor import parse_descriptor
  >>> b = parse_descriptor("tr(a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bd,pk(669b8afcec803a0d323e9a17f3ea8e68e8abe5a278020a929adbec52421adbd0))")
  Traceback (most recent call last):
    File "/home/user/wspace/HWI/hwilib/descriptor.py", line 437, in _get_func_expr
      end = s.rindex(")")
  ValueError: substring not found

  During handling of the above exception, another exception occurred:

  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
    File "/home/user/wspace/HWI/hwilib/descriptor.py", line 637, in parse_descriptor
      return _parse_descriptor(desc, _ParseDescriptorContext.TOP)
    File "/home/user/wspace/HWI/hwilib/descriptor.py", line 600, in _parse_descriptor
      subscripts.append(_parse_descriptor(sarg, _ParseDescriptorContext.P2TR))
    File "/home/user/wspace/HWI/hwilib/descriptor.py", line 527, in _parse_descriptor
      func, expr = _get_func_expr(desc)
    File "/home/user/wspace/HWI/hwilib/descriptor.py", line 440, in _get_func_expr
      raise ValueError("A matching pair of parentheses cannot be found")
  ValueError: A matching pair of parentheses cannot be found
  ```

ACKs for top commit:
  achow101:
    ACK 4e8559b

Tree-SHA512: cafc92d4c95b89dc209522a735f37edbc4f91c9abe62668d28c97306e36cf48d1c6d059ead1788462ddf657e0eb9e594d3478cb6f0e170f79b1928fc04523dd2

hwilib/descriptor.py

@@ -473,6 +473,8 @@ def _get_expr(s: str) -> Tuple[str, str]:
             level -= 1
         elif level == 0 and c in [")", "}", ","]:
             break
+    else:
+        return s, ""
     return s[0:i], s[i:]
 
 def parse_pubkey(expr: str) -> Tuple['PubkeyProvider', str]:

test/test_descriptor.py

@@ -205,5 +205,15 @@ def test_tr_descriptor(self):
         self.assertEqual(desc.depths, [1, 3, 3, 2])
         self.assertEqual(desc.to_string_no_checksum(), d)
 
+        d = "tr(a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bd,pk(669b8afcec803a0d323e9a17f3ea8e68e8abe5a278020a929adbec52421adbd0))"
+        desc = parse_descriptor(d)
+        self.assertTrue(isinstance(desc, TRDescriptor))
+        self.assertEqual(len(desc.subdescriptors), 1)
+        self.assertEqual(desc.pubkeys[0].pubkey, "a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bd")
+        self.assertEqual(desc.depths, [0])
+        self.assertEqual(desc.subdescriptors[0].pubkeys[0].pubkey, "669b8afcec803a0d323e9a17f3ea8e68e8abe5a278020a929adbec52421adbd0")
+        self.assertEqual(desc.to_string_no_checksum(), d)
+
+
 if __name__ == "__main__":
     unittest.main()