Ensure that plistlib doesn't corrupt deeply nested datastructures

Without this changeset plistlib would write empty tags for plistlib.Data
objects in deeply nested datastructures.

Fixes python#17353

Lib/plistlib.py

@@ -228,8 +228,8 @@ def writeValue(self, value):
     def writeData(self, data):
         self.beginElement("data")
         self.indentLevel -= 1
-        maxlinelength = 76 - len(self.indent.replace(b"\t", b" " * 8) *
-                                 self.indentLevel)
+        maxlinelength = max(16, 76 - len(self.indent.replace(b"\t", b" " * 8) *
+                                 self.indentLevel))
         for line in data.asBase64(maxlinelength).split(b"\n"):
             if line:
                 self.writeln(line)

Lib/test/test_plistlib.py

@@ -141,6 +141,18 @@ def test_bytes(self):
         data2 = plistlib.writePlistToBytes(pl2)
         self.assertEqual(data, data2)
 
+    def test_indentation_array(self):
+        data = [[[[[[[[{'test': plistlib.Data(b'aaaaaa')}]]]]]]]]
+        self.assertEqual(plistlib.readPlistFromBytes(plistlib.writePlistToBytes(data)), data)
+
+    def test_indentation_dict(self):
+        data = {'1': {'2': {'3': {'4': {'5': {'6': {'7': {'8': {'9': plistlib.Data(b'aaaaaa')}}}}}}}}}
+        self.assertEqual(plistlib.readPlistFromBytes(plistlib.writePlistToBytes(data)), data)
+
+    def test_indentation_dict_mix(self):
+        data = {'1': {'2': [{'3': [[[[[{'test': plistlib.Data(b'aaaaaa')}]]]]]}]}}
+        self.assertEqual(plistlib.readPlistFromBytes(plistlib.writePlistToBytes(data)), data)
+
     def test_appleformatting(self):
         pl = plistlib.readPlistFromBytes(TESTDATA)
         data = plistlib.writePlistToBytes(pl)

Misc/NEWS

@@ -36,6 +36,8 @@ Core and Builtins
 Library
 -------
 
+- Issue #17353: Plistlib emitted empty data tags with deeply nested datastructures
+
 - Issue #11714: Use 'with' statements to assure a Semaphore releases a
   condition variable.  Original patch by Thomas Rachel.