Fix #1070: bytes_to_read buffer size for Python 3.14

invoke/terminals.py

@@ -243,6 +243,12 @@ def bytes_to_read(input_: IO) -> int:
     # it's not a tty but has a fileno, or vice versa; neither is typically
     # going to work re: ioctl().
     if not WINDOWS and isatty(input_) and has_fileno(input_):
-        fionread = fcntl.ioctl(input_, termios.FIONREAD, b"  ")
-        return int(struct.unpack("h", fionread)[0])
+        # FIONREAD writes a C int (typically 4 bytes). Pass a 4-byte buffer
+        # and unpack as a signed int. The previous 2-byte buffer ("h")
+        # happened to work on <=3.13 thanks to CPython's internal 1024-byte
+        # static copy buffer, but CPython 3.14 tightened buffer-size checks
+        # (cpython#144206) so undersized mutable buffers now raise
+        # SystemError: buffer overflow. See pyinvoke/invoke#1070.
+        fionread = fcntl.ioctl(input_, termios.FIONREAD, b"\x00\x00\x00\x00")
+        return int(struct.unpack("i", fionread)[0])
     return 1

tests/runners.py

@@ -1226,7 +1226,7 @@ def fake_ioctl(fd, cmd, buf):
                 # This works since each mocked attr will still be its own mock
                 # object with a distinct 'is' identity.
                 if cmd is termios.FIONREAD:
-                    return struct.pack("h", len(stdin_data))
+                    return struct.pack("i", len(stdin_data))
 
             ioctl.side_effect = fake_ioctl
             # Set up our runner as one w/ mocked stdin writing (simplest way to

tests/terminals.py

@@ -1,4 +1,5 @@
 import fcntl
+import struct
 import termios
 
 from unittest.mock import Mock, patch
@@ -71,6 +72,22 @@ def returns_1_when_stream_has_fileno_but_is_not_a_tty(self, fcntl):
             assert bytes_to_read(stream) == 1
             assert not fcntl.ioctl.called
 
+        @patch("invoke.terminals.fcntl")
+        def uses_4_byte_buffer_for_FIONREAD(self, fcntl):
+            # FIONREAD writes a C int (4 bytes); the buffer must match to
+            # avoid SystemError on Python 3.14+ (#1070)
+            fcntl.ioctl.return_value = struct.pack("i", 42)
+            stream = Mock(isatty=lambda: True, fileno=lambda: 7)
+            assert bytes_to_read(stream) == 42
+            buf = fcntl.ioctl.call_args[0][2]
+            assert len(buf) == 4
+
+        @patch("invoke.terminals.fcntl")
+        def returns_0_when_FIONREAD_is_zero(self, fcntl):
+            fcntl.ioctl.return_value = struct.pack("i", 0)
+            stream = Mock(isatty=lambda: True, fileno=lambda: 7)
+            assert bytes_to_read(stream) == 0
+
         def returns_FIONREAD_result_when_stream_is_a_tty(self):
             skip()