gh-119342: Fix a potential denial of service in plistlib (GH-119343)

Reading a specially prepared small Plist file could cause OOM because file's
read(n) preallocates a bytes object for reading the specified amount of
data. Now plistlib reads large data by chunks, therefore the upper limit of
consumed memory is proportional to the size of the input file.

Lib/test/test_plistlib.py

@@ -903,8 +903,7 @@ def test_dump_naive_datetime_with_aware_datetime_option(self):
 
 class TestBinaryPlistlib(unittest.TestCase):
 
-    @staticmethod
-    def decode(*objects, offset_size=1, ref_size=1):
+    def build(self, *objects, offset_size=1, ref_size=1):
         data = [b'bplist00']
         offset = 8
         offsets = []
@@ -916,7 +915,11 @@ def decode(*objects, offset_size=1, ref_size=1):
                            len(objects), 0, offset)
         data.extend(offsets)
         data.append(tail)
-        return plistlib.loads(b''.join(data), fmt=plistlib.FMT_BINARY)
+        return b''.join(data)
+
+    def decode(self, *objects, offset_size=1, ref_size=1):
+        data = self.build(*objects, offset_size=offset_size, ref_size=ref_size)
+        return plistlib.loads(data, fmt=plistlib.FMT_BINARY)
 
     def test_nonstandard_refs_size(self):
         # Issue #21538: Refs and offsets are 24-bit integers
@@ -1024,6 +1027,34 @@ def test_invalid_binary(self):
                 with self.assertRaises(plistlib.InvalidFileException):
                     plistlib.loads(b'bplist00' + data, fmt=plistlib.FMT_BINARY)
 
+    def test_truncated_large_data(self):
+        self.addCleanup(os_helper.unlink, os_helper.TESTFN)
+        def check(data):
+            with open(os_helper.TESTFN, 'wb') as f:
+                f.write(data)
+            # buffered file
+            with open(os_helper.TESTFN, 'rb') as f:
+                with self.assertRaises(plistlib.InvalidFileException):
+                    plistlib.load(f, fmt=plistlib.FMT_BINARY)
+            # unbuffered file
+            with open(os_helper.TESTFN, 'rb', buffering=0) as f:
+                with self.assertRaises(plistlib.InvalidFileException):
+                    plistlib.load(f, fmt=plistlib.FMT_BINARY)
+        for w in range(20, 64):
+            s = 1 << w
+            # data
+            check(self.build(b'\x4f\x13' + s.to_bytes(8, 'big')))
+            # ascii string
+            check(self.build(b'\x5f\x13' + s.to_bytes(8, 'big')))
+            # unicode string
+            check(self.build(b'\x6f\x13' + s.to_bytes(8, 'big')))
+            # array
+            check(self.build(b'\xaf\x13' + s.to_bytes(8, 'big')))
+            # dict
+            check(self.build(b'\xdf\x13' + s.to_bytes(8, 'big')))
+            # number of objects
+            check(b'bplist00' + struct.pack('>6xBBQQQ', 1, 1, s, 0, 8))
+
     def test_load_aware_datetime(self):
         data = (b'bplist003B\x04>\xd0d\x00\x00\x00\x08\x00\x00\x00\x00\x00\x00'
                 b'\x01\x01\x00\x00\x00\x00\x00\x00\x00\x01\x00\x00\x00\x00\x00'