gh-95782: Fix io.BufferedReader.tell() etc. being able to return offsets < 0 (GH-99709)

lseek() always returns 0 for character pseudo-devices like
`/dev/urandom` (for other non-regular files, e.g. `/dev/stdin`, it
always returns -1, to which CPython reacts by raising appropriate
exceptions). They are thus technically seekable despite not having seek
semantics.

When calling read() on e.g. an instance of `io.BufferedReader` that
wraps such a file, `BufferedReader` reads ahead, filling its buffer,
creating a discrepancy between the number of bytes read and the internal
`tell()` always returning 0, which previously resulted in e.g.
`BufferedReader.tell()` or `BufferedReader.seek()` being able to return
positions < 0 even though these are supposed to be always >= 0.

Invariably keep the return value non-negative by returning
max(former_return_value, 0) instead, and add some corresponding tests.

Lib/test/test_io.py

@@ -257,6 +257,27 @@ class PyMockUnseekableIO(MockUnseekableIO, pyio.BytesIO):
     UnsupportedOperation = pyio.UnsupportedOperation
 
 
+class MockCharPseudoDevFileIO(MockFileIO):
+    # GH-95782
+    # ftruncate() does not work on these special files (and CPython then raises
+    # appropriate exceptions), so truncate() does not have to be accounted for
+    # here.
+    def __init__(self, data):
+        super().__init__(data)
+
+    def seek(self, *args):
+        return 0
+
+    def tell(self, *args):
+        return 0
+
+class CMockCharPseudoDevFileIO(MockCharPseudoDevFileIO, io.BytesIO):
+    pass
+
+class PyMockCharPseudoDevFileIO(MockCharPseudoDevFileIO, pyio.BytesIO):
+    pass
+
+
 class MockNonBlockWriterIO:
 
     def __init__(self):
@@ -1648,6 +1669,30 @@ class BufferedReaderTest(unittest.TestCase, CommonBufferedTests):
         self.assertRaises(self.UnsupportedOperation, bufio.truncate)
         self.assertRaises(self.UnsupportedOperation, bufio.truncate, 0)
 
+    def test_tell_character_device_file(self):
+        # GH-95782
+        # For the (former) bug in BufferedIO to manifest, the wrapped IO obj
+        # must be able to produce at least 2 bytes.
+        raw = self.MockCharPseudoDevFileIO(b"12")
+        buf = self.tp(raw)
+        self.assertEqual(buf.tell(), 0)
+        self.assertEqual(buf.read(1), b"1")
+        self.assertEqual(buf.tell(), 0)
+
+    def test_seek_character_device_file(self):
+        raw = self.MockCharPseudoDevFileIO(b"12")
+        buf = self.tp(raw)
+        self.assertEqual(buf.seek(0, io.SEEK_CUR), 0)
+        self.assertEqual(buf.seek(1, io.SEEK_SET), 0)
+        self.assertEqual(buf.seek(0, io.SEEK_CUR), 0)
+        self.assertEqual(buf.read(1), b"1")
+
+        # In the C implementation, tell() sets the BufferedIO's abs_pos to 0,
+        # which means that the next seek() could return a negative offset if it
+        # does not sanity-check:
+        self.assertEqual(buf.tell(), 0)
+        self.assertEqual(buf.seek(0, io.SEEK_CUR), 0)
+
 
 class CBufferedReaderTest(BufferedReaderTest, SizeofTest):
     tp = io.BufferedReader
@@ -4880,7 +4925,7 @@ def load_tests(loader, tests, pattern):
     # classes in the __dict__ of each test.
     mocks = (MockRawIO, MisbehavedRawIO, MockFileIO, CloseFailureIO,
              MockNonBlockWriterIO, MockUnseekableIO, MockRawIOWithoutRead,
-             SlowFlushRawIO)
+             SlowFlushRawIO, MockCharPseudoDevFileIO)
     all_members = io.__all__
     c_io_ns = {name : getattr(io, name) for name in all_members}
     py_io_ns = {name : getattr(pyio, name) for name in all_members}